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How to Deal with null or Absent Data in Java

Representing something as blank or absent of something is always a problem in programming. An absence of items, say, in a bag, simply means the bag is empty or the bag does not contain anything. But how do you represent an absence of something in computer memory? For example, an object declared in memory contains some value (it does not matter if the variable is initialized or not) even if it may not make any sense in the context – this is known as garbage values . Programmers can, at best, discard it but the point is that the object declared is not empty. We can initialize it by a value or put a null . However, this still represents some value; even null is something that represents nothing. In this programming tutorial, we analyze the absence of data and null see what Java offers in regards to dealing with this issue.

Before we begin, however, we wanted to point out an article we published recently highlighting some of the Best Online Courses to Learn Java that may be of interest to you.

What is null in Java?

Absence of data in computers is just a conceptual idea; the internal representation is actually contrary to it. A similar idea we can relate it to is set theory , which defines an empty set whose cardinality is 0 . But, in actual representation, it uses a symbol called null to mean emptiness. So, if we ask the question, “What does an empty set contain?”, one possible answer would be null , meaning nothing or empty. But, in software development, we know null is also a value.

Typically, the value 0 or all bits at 0 in memory is used to denote a constant, and the name given to it is null . Unlike other variables or constants, null means there is no value associated with the name and it is denoted as a built-in constant with a 0 value in it.

A piece of data is actually represented as a reference pointing to it. Therefore, to represent something in the absence of data, developers must make something up that represents nothing. So null (in Go it is called nil – maybe because they found nil is one less character than null and the lesser the better) is the chosen one. This is what we mean by a null pointer . Thus, we can see that null is both a pointer and a value. In Java, some objects (static and instance variables) are created with null by default, but later they can be changed to point to values.

It is worth mentioning here that null , as a reference in programming, was invented in 1965 by Tony Hoare while designing ALGOL . In his later years he regretted it as a billion-dollar mistake , stating:

I call it my billion-dollar mistake. It was the invention of the null reference in 1965. At that time, I was designing the first comprehensive type system for references in an object oriented language (ALGOL W). My goal was to ensure that all use of references should be absolutely safe, with checking performed automatically by the compiler. But I couldn’t resist the temptation to put in a null reference, simply because it was so easy to implement. This has led to innumerable errors, vulnerabilities, and system crashes, which have probably caused a billion dollars of pain and damage in the last forty years.

This harmless looking thing called null has caused some serious trouble throughout the years. But, perhaps the importance of null cannot totally be discarded in programming. This is the reason many later compiler creators thought it wise to keep the legacy alive. However, Java 8 and later versions tried to provide a type called Optional that directly deals with some of the problems related to the use of null .

Read: Best Tools for Remote Developers

Problems with the null Pointer in Java

The NullPointerException is a common bug frequently encountered by every programmer in Java. This error is raised when we try to dereference an identifier that points to nothing – this simply means that we are expecting to reach some data but the data is missing. The identifier we are trying to reach is pointing to null .

Here is a code example of how we can raise the NullPointerException error in Java:

Running this code in your integrated development environment (IDE) or code editor would produce the following output:

Often in programming, the best way to avoid a problem is to know how to create one. Now, although it is well known that null references must be avoided, the Java API is replete with using null as a valid reference. One such example is as follows. The documentation of the Socket class constructor from the java.net package states the following:

This Java code:

• Creates a socket and connects it to the specified remote address on the specified remote port. The Socket will also bind() to the local address and port supplied.
• If the specified local address is null , it is the equivalent of specifying the address as the AnyLocal address (see InetAddress.isAnyLocalAddress() ).
• A local port number of zero will let the system pick up a free port in the bind operation.
• If there is a security manager, its checkConnect method is called with the host address and port as its arguments. This could result in a SecurityException .

According to Java documentation, the highlighted point clearly means that the null reference is used as a valid parameter. This null is harmless here and used as a sentinel value to mean absence of something (here in case of an absence of a port value in a socket). Therefore, we can see that null is not altogether avoided, although it is dangerous at times. There are many such examples in Java.

How to Handle Absence of Data in Java

A perfect programmer, who always writes perfect code, cannot actually have any problem with null . But, for those of us who are prone to mistakes and need some sort of a safer alternative to represent an absence of something without resorting to innovative uses of null , we need some support. Therefore, Java introduced a type – a class called Optional – that deals with absence values, occurring not due to an error, in a more decent manner.

Now, before getting into any code examples, let’s look at the following excerpt derived from the Java API documentation:

This excerpt showcases:

• A container object which may, or may not, contain a non-null value. If a value is present, isPresent() will return true and get() will return the value.
• Additional methods that depend on the presence or absence of a contained value are provided, such as orElse() (returns a default value if value not present) and ifPresent() (executes a block of code if the value is present).
• This is a value-based class; use of identity-sensitive operations (including reference equality ( == ), identity hash code, or synchronization) on instances of Optional may have unpredictable results and should be avoided by developers.

In fact, there are a host of optional classes in Java, such as Optional , OptionalDouble , OptionalInt , and OptionalLong – all dealing with a situation where developers are unsure whether a value may be present or not. Before Java 8 introduced these classes, programmers used to use the value null to indicate an absence of value. Because of this, the bug known as NullPointerException was a frequent phenomenon, as we intentionally (or unintentionally) made an attempt to dereference a null reference; a way out was to frequently check for null values to avoid generating exceptions.

These classes provide a better strategy to cope with the situation. Note that all the optional classes are value-based, therefore they are immutable and have various restrictions, such as not using instances for synchronization and avoiding any use of reference equality. In this next section, we will focus on the Optional class specifically. Other optional classes function in a similar manner.

The T in the Optional class represents the type of value stored, which can be any value of type T . It may also be empty. The Optional class, despite defining several methods, does not define any constructor. Developers can determine if a value is present or not, obtain the value if it is present, obtain a default value when the value is not present, or construct an Optional value. Check out Java documentation for details on available functions of these classes.

Read: Best Project Management Tools for Developers

How to use Optional in Java

The code example below shows how we can gracefully deal with objects that return null or an absence of element in Java. The Optional class acts as a wrapper for the object that may not be present:

Some of the key functions of the Optional class are isPresent() and get() . The isPresent() function determines whether the value is present or not. This function returns a boolean true value if the value is present – otherwise it returns a false value.

A value that is present can be obtained using the get() function. However, if the get() function is called and it does not have a value then a NoSuchElementException is thrown. Ideally, the presence of a value is always checked using the ifPresent() function before calling the get() function.

You can learn more about using the Optional class in Java in our tutorial: How to Use Optional in Java .

Final Thoughts on null Values in Java

If there is something that programming cannot do away with, yet caution everyone in using, is null . In databases, while storing values, the advice is to avoid storing null values in the tables. A not properly normalized database table can have too many null values. In general, there is not a very clear definition about what an absence of value means in computing. In any event, the problem associated with null values can be handled, to some extent, using the Optional class in Java.

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A quick and thorough guide to ‘ null ’: what it is, and how you should use it

What is the meaning of null ? How is null implemented? When should you use null in your source code, and when should you not use it?

Introduction

null is a fundamental concept in many programming languages. It is ubiquitous in all kinds of source code written in these languages. So it is essential to fully grasp the idea of null . We have to understand its semantics and implementation, and we need to know how to use null in our source code.

Comments in programmer forums sometimes reveal a bit of confusion with null . Some programmers even try to completely avoid null . Because they think of it as the 'million-dollar mistake', a term coined by Tony Hoare, the inventor of null .

Here is a simple example: Suppose that Alice’s email_address points to null . What does this mean? Does it mean that Alice doesn't have an email address? Or that her email address is unknown? Or that it is secret? Or does it simply mean that email_address is 'undefined' or 'uninitialized'? Let's see. After reading this article, everybody should be able to answer such questions without hesitation.

Note: This article is programming-language-neutral — as far as possible. Explanations are general and not tied to a specific language. Please consult your programming language manuals for specific advice on null . However, this article contains some simple source code examples shown in Java. But it’s not difficult to translate them into your favorite language.

Run-time Implementation

Before discussing the meaning of null , we need to understand how null is implemented in memory at run-time.

Note: We will have a look at a typical implementation of null . The actual implementation in a given environment depends on the programming language and target environment, and might differ from the implementation shown here.

Suppose we have the following source code instruction:

Here we declare a variable of type String and with the identifier name that points to the string "Bob" .

Saying “points to” is important in this context, because we are assuming that we work with reference types (and not with value types ). More on this later.

To keep things simple, we will make the following assumptions:

• The above instruction is executed on a 16-bits CPU with a 16-bits address space.
• Strings are encoded as UTF-16. They are terminated with 0 (as in C or C++).

The following picture shows an excerpt of the memory after executing the above instruction:

The memory addresses in the above picture are chosen arbitrarily and are irrelevant for our discussion.

As we can see, the string "Bob" is stored at address B000 and occupies 4 memory cells.

Variable name is located at address A0A1. The content of A0A1 is B000, which is the starting memory location of the string "Bob" . That's why we say: The variable name points to "Bob" .

So far so good.

Now suppose that, after executing the above instruction, you execute the following:

Now name points to null .

And this is the new state in memory:

We can see that nothing has changed for the string "Bob" which is still stored in memory.

Note: The memory needed to store the string "Bob" might later be released if there is a garbage collector and no other reference points to "Bob" , but this is irrelevant in our discussion.

What’s important is that the content of A0A1 (which represents the value of variable name ) is now 0000. So, variable name doesn't point to "Bob" anymore. The value 0 (all bits at zero) is a typical value used in memory to denote null . It means that there is no value associated with name . You can also think of it as the absence of data or simply no data .

Note: The actual memory value used to denote null is implementation-specific. For example the Java Virtual Machine Specification states at the end of section 2.4. “ Reference Types and Values:”

The Java Virtual Machine specification does not mandate a concrete value encoding null .

If a reference points to null , it simply means that there is no value associated with it .

Technically speaking, the memory location assigned to the reference contains the value 0 (all bits at zero), or any other value that denotes null in the given environment.

Performance

As we learned in the previous section, operations involving null are extremely fast and easy to perform at run-time.

There are only two kinds of operations:

• Initialize or set a reference to null (e.g. name = null ): The only thing to do is to change the content of one memory cell (e.g. setting it to 0).
• Check if a reference points to null (e.g. if name == null ): The only thing to do is to check if the memory cell of the reference holds the value 0.

Operations on null are exceedingly fast and cheap.

Reference vs Value Types

So far we assumed working with reference types . The reason for this is simple: null doesn't exist for value types .

As we have seen previously, a reference is a pointer to a memory-address that stores a value (e.g. a string, a date, a customer, whatever). If a reference points to null , then no value is associated with it.

On the other hand, a value is, by definition, the value itself. There is no pointer involved. A value type is stored as the value itself. Therefore the concept of null doesn't exist for value types.

The following picture demonstrates the difference. On the left side you can see again the memory in case of variable name being a reference pointing to "Bob". The right side shows the memory in case of variable name being a value type.

As we can see, in case of a value type, the value itself is directly stored at the address A0A1 which is associated with variable name .

There would be much more to say about reference versus value types, but this is out of the scope of this article. Please note also that some programming languages support only reference types, others support only value types, and some (e.g. C# and Java) support both of them.

The concept of null exists only for reference types. It doesn't exist for value types .

Suppose we have a type person with a field emailAddress . Suppose also that, for a given person which we will call Alice, emailAddress points to null .

What does this mean? Does it mean that Alice doesn’t have an email address? Not necessarily.

As we have seen already, what we can assert is that no value is associated with emailAddress .

But why is there no value? What is the reason of emailAddress pointing to null ? If we don't know the context and history, then we can only speculate. The reason for null could be:

Alice doesn’t have an email address. Or…

Alice has an email address, but:

• it has not yet been entered in the database
• it is secret (unrevealed for security reasons)
• there is a bug in a routine that creates a person object without setting field emailAddress

In practice we often know the application and context. We intuitively associate a precise meaning to null . In a simple and flawless world, null would simply mean that Alice actually doesn't have an email address.

When we write code, the reason why a reference points to null is often irrelevant. We just check for null and take appropriate actions. For example, suppose that we have to write a loop that sends emails for a list of persons. The code (in Java) could look like this:

In the above loop we don’t care about the reason for null . We just acknowledge the fact that there is no email address, log a warning, and continue.

If a reference points to null then it always means that there is no value associated with it .

In most cases, null has a more specific meaning that depends on the context .

Why is it null ?

Sometimes it is important to know why a reference points to null .

Consider the following function signature in a medical application:

In this case, returning null (or an empty list) is ambiguous. Does it mean that the patient doesn't have allergies, or does it mean that an allergy test has not yet been performed? These are two semantically very different cases that must be handled differently. Or else the outcome might be life-threatening.

Just suppose that the patient has allergies, but an allergy test has not yet been done and the software tells the doctor that 'there are no allergies'. Hence we need additional information. We need to know why the function returns null .

It would be tempting to say: Well, to differentiate, we return null if an allergy test has not yet been performed, and we return an empty list if there are no allergies.

DON’T DO THIS!

This is bad data design for multiple reasons.

The different semantics for returning null versus returning an empty list would need to be well documented. And as we all know, comments can be wrong (i.e. inconsistent with the code), outdated, or they might even be inaccessible.

There is no protection for misuses in client code that calls the function. For example, the following code is wrong, but it compiles without errors. Moreover, the error is difficult to spot for a human reader. We can’t see the error by just looking at the code without considering the comment of getAllergiesOfPatient:

The following code would be wrong too:

If the null /empty-logic of getAllergiesOfPatient changes in the future, then the comment needs to be updated, as well as all client code. And there is no protection against forgetting any one of these changes.

If, later on, there is another case to be distinguished (e.g. an allergy test is pending — the results are not yet available), or if we want to add specific data for each case, then we are stuck.

So the function needs to return more information than just a list.

There are different ways to do this, depending on the programming language we use. Let’s have a look at a possible solution in Java.

In order to differentiate the cases, we define a parent type AllergyTestResult , as well as three sub-types that represent the three cases ( NotDone , Pending , and Done ):

As we can see, for each case we can have specific data associated with it.

Instead of simply returning a list, getAllergiesOfPatient now returns an AllergyTestResult object:

Client code is now less error-prone and looks like this:

Note: If you think that the above code is quite verbose and a bit hard to write, then you are not alone. Some modern languages allow us to write conceptually similar code much more succinctly. And null-safe languages distinguish between nullable and non-nullable values in a reliable way at compile-time — there is no need to comment the nullability of a reference or to check whether a reference declared to be non-null has accidentally been set to null .

If we need to know why there is no value associated with a reference, then additional data must be provided to differentiate the possible cases .

Initialization

Consider the following instructions:

The first instruction declares a String variable s1 and assigns it the value "foo" .

The second instruction assigns null to s2 .

The more interesting instruction is the last one. No value is explicitly assigned to s3 . Hence, it is reasonable to ask: What is the state of s3 after its declaration? What will happen if we write s3 to the OS output device?

It turns out that the state of a variable (or class field) declared without assigning a value depends on the programming language. Moreover, each programming language might have specific rules for different cases. For example, different rules apply for reference types and value types, static and non-static members of a class, global and local variables, and so on.

As far as I know, the following rules are typical variations encountered:

• It is illegal to declare a variable without also assigning a value
• There is an arbitrary value stored in s3 , depending on the memory content at the time of execution - there is no default value
• A default value is automatically assigned to s3. In case of a reference type, the default value is null. In case of a value type, the default value depends on the variable’s type. For example 0 for integer numbers, false for a boolean, and so on.
• the state of s3 is 'undefined'
• the state of s3 is 'uninitialized', and any attempt to use s3 results in a compile-time error.

The best option is the last one. All other options are error-prone and/or impractical — for reasons we will not discuss here, because this article focuses on null .

As an example, Java applies the last option for local variables. Hence, the following code results in a compile-time error at the second line:

Compiler output:

If a variable is declared, but no explicit value is assigned to it, then it’s state depends on several factors which are different in different programming languages.

In some languages, null is the default value for reference types.

When to Use null (And When Not to Use It)

The basic rule is simple: null should only be allowed when it makes sense for an object reference to have 'no value associated with it'. (Note: an object reference can be a variable, constant, property (class field), input/output argument, and so on.)

For example, suppose type person with fields name and dateOfFirstMarriage :

Every person has a name. Hence it doesn’t make sense for field name to have 'no value associated with it'. Field name is non-nullable . It is illegal to assign null to it.

On the other hand, field dateOfFirstMarriage doesn't represent a required value. Not everyone is married. Hence it makes sense for dateOfFirstMarriage to have 'no value associated with it'. Therefore dateOfFirstMarriage is a nullable field. If a person's dateOfFirstMarriage field points to null then it simply means that this person has never been married.

Note: Unfortunately most popular programming languages don’t distinguish between nullable and non-nullable types. There is no way to reliably state that null can never be assigned to a given object reference. In some languages it is possible to use annotations, such as the non-standard annotations @Nullable and @NonNullable in Java. Here is an example:

However, such annotations are not used by the compiler to ensure null-safety. Still, they are useful for the human reader, and they can be used by IDEs and tools such as static code analyzers.

It is important to note that null should not be used to denote error conditions.

Consider a function that reads configuration data from a file. If the file doesn’t exist or is empty, then a default configuration should be returned. Here is the function’s signature:

What should happen in case of a file read error?

Simply return null ?

Each language has it’s own standard way to signal error conditions and provide data about the error, such as a description, type, stack trace, and so on. Many languages (C#, Java, etc.) use an exception mechanism, and exceptions should be used in these languages to signal run-time errors. readConfigFromFile should not return null to denote an error. Instead, the function's signature should be changed in order to make it clear that the function might fail:

• Allow null only if it makes sense for an object reference to have 'no value associated with it'.
• Don’t use null to signal error conditions.

Null-safety

Consider the following code:

At run-time, the above code results in the infamous null pointer error , because we try to execute a method of a reference that points to null . In C#, for example, a NullReferenceException is thrown, in Java it is a NullPointerException .

The null pointer error is nasty.

It is the most frequent bug in many software applications, and has been the cause for countless troubles in the history of software development. Tony Hoare, the inventor of null , calls it the 'billion-dollar mistake'.

But Tony Hoare (Turing Award winner in 1980 and inventor of the Quicksort algorithm), also gives a hint to a solution in his speech :

More recent programming languages … have introduced declarations for non-null references. This is the solution, which I rejected in 1965.

Contrary to some common belief, the culprit is not null per se. The problem is the lack of support for null handling in many programming languages. For example, at the time of writing (May 2018), none of the top ten languages in the Tiobe index natively differentiates between nullable and non-nullable types.

Therefore, some new languages provide compile-time null-safety and specific syntax for conveniently handling null in source code. In these languages, the above code would result in a compile-time error. Software quality and reliability increases considerably, because the null pointer error delightfully disappears.

Null-safety is a fascinating topic that deserves its own article.

Whenever possible, use a language that supports compile-time null-safety.

Note: Some programming languages (mostly functional programming languages like Haskell) don’t support the concept of null . Instead, they use the Maybe/Optional Pattern to represent the ‘absence of a value’. The compiler ensures that the ‘no value’ case is handled explicitly. Hence, null pointer errors cannot occur.

Here is a summary of key points to remember:

• If a reference points to null , it always means that there is no value associated with it .
• In most cases, null has a more specific meaning that depends on the context.
• If we need to know why there is no value associated with a reference, then additional data must be provided to differentiate the possible cases.
• The concept of null exists only for reference types. It doesn't exist for value types.
• In some languages null is the default value for reference types.
• null operations are exceedingly fast and cheap.
• Whenever possible, use a language that supports compile-time-null-safety.

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What Is Null In Java?

The concept of “null” can be confusing when you first start off in Java.

Does it mean that something has gone wrong? Perhaps a code line is broken? Or maybe the language is telling you that nothing is there?

We will walk you through the basics of “null” and straighten out any confusion you may have around the subject.

Table of Contents

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The Basics – Explaining Null

In short, “null” is the reserved keyword you use to suggest that there is no value.

A lot of new starters confuse “null” as a type or an object, but it should be thought of as a value (or lack thereof).

You would use “null” to indicate a lack of something or the non-existence of something. 

As a reserved keyword, “null” basically acts as a true or false statement. If the value is false or doesn’t exist, the outcome will be “null”.

Behind The Scenes – What Null Does

For “null” to work properly, the script must know what it should be looking for. Without a variable for it to point towards, the “null” won’t know what to expect an absence of. 

You can consider variables in two main ways, primitives and references. We will explain them both to show you how “null” can interact with them.

Primitive Variables

Primitive variables are a set of data pre-defined by the language programming, in this case, Java.

When you acknowledge or declare a primitive variable, it will contain the expected value for “null” to consider. Examples of primitive variables are “float” and “int”.

Reference Variables

References are considered a type of pointer. They show you the value being represented.

Declaring a reference means you’ve stored the address, directing toward the value in question. It doesn’t point toward the value directly. For example, strings, arrays, and classes are all reference types.

Null And Its Variables

Primitive variables aren’t able to use “null” values. Attempting such will create an error message. Reference variables, however, can have “null” assigned to whatever type you choose.

Null And String

An example of “null” with a string reference could look like this – String myStr = null;

Null And Integers

Although integers are a primitive variable, you can assign null to them, however, you would need to assign it through a reference variable instead of directly.

An example of “null” with an integer reference could look like this – Integer a = null;

You would not write it as – int myInt = null;

If you try to use “null” to point to a primitive variable, you will receive an error message similar to this – incompatible types : required int found null

The Easiest Mistake To Make With Null

Now you understand what “null” is, it’s time to understand it with Java.

As with any programming language, each one will have its own spin on central processing. For “null” the nuance of Java isn’t complicated.

“Null”, just like every keyword in Java, is sensitive to case or capitalization usage. If you write “Null” or “NULL”, you will be given errors. Instead, you need to write “null” in all lowercase.

For a visual representation, look at these examples. 

• With this line, you will be given a null value. 
• With this line, you will be given the error – compile-time error : can’t find symbol ‘NULL’

The Easiest Way To Use Null

The easiest way to use “null” correctly, is to recognize it as the default. When looking at primitive variables, the default is often 0 or false, depending on if you are using integer, booleans, etc). 

When looking at reference variables, the value is “null”. This is the same for static variables and instance variables too. 

Null As A False Return

If you ever get confused about if an object can be considered as an interface, subclass, or class, you can use the “instanceOf” operator to check. 

However, if you use this hint with a reference variable, you will be given a false return. Be aware of this difference.

Explaining NullPointException 

If you forget to or simply don’t initialize the variables, you may be shown this error message – java.lang.NullPointerException

This mistake can happen easily, as “null” is the expected default for any uninitialized reference variable.

However, there is more than one way to receive this message. You may fall into this trap if you do the following:

• By modifying or accessing a member or data field with “null”.
• Treating “null” as an array in Java .
• Using a “null” in a method as an argument.
• Attempting to synchronize a “null”.
• Trying to involve a method using a “null”.

In simple terms, all variables need to be initialized correctly, which means that all objects need to have reference points leading to values considered valid. 

Here are some ways to avoid the Null PointEr exception error.

Choosing A Return Of Returning Empty Collections

If you hate the idea of null, and would rather something else instead, we suggest trying empty collections. To do this, follow this method:

Ternary Operators

Perhaps you would like an “empty when not in use” function instead. In that case, follow this method:

//If str’s reference is null, myStr will be empty. Else, the first 20 characters will be retrieved.

Understanding how “null” works and why it is important, will help you navigate through the language of Java with fewer error messages.

“Null” is a powerful tool, and shouldn’t be disregarded as a nuisance to get rid of, as it is a central part of the Java programming language.

Use our examples above directly or modify them to create a more accurate creation in your script. 

This information is based solely on the computer language Java, so keep that in mind when considering other languages.

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Nullish coalescing assignment (??=)

The nullish coalescing assignment ( ??= ) operator, also known as the logical nullish assignment operator, only evaluates the right operand and assigns to the left if the left operand is nullish ( null or undefined ).

Description

Nullish coalescing assignment short-circuits , meaning that x ??= y is equivalent to x ?? (x = y) , except that the expression x is only evaluated once.

No assignment is performed if the left-hand side is not nullish, due to short-circuiting of the nullish coalescing operator. For example, the following does not throw an error, despite x being const :

Neither would the following trigger the setter:

In fact, if x is not nullish, y is not evaluated at all.

Using nullish coalescing assignment

You can use the nullish coalescing assignment operator to apply default values to object properties. Compared to using destructuring and default values , ??= also applies the default value if the property has value null .

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• Nullish coalescing operator ( ?? )

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Operators constitute the basic building block of any programming language. Java too provides many types of operators which can be used according to the need to perform various calculations and functions, be it logical, arithmetic, relational, etc. They are classified based on the functionality they provide.

Types of Operators: 

• Arithmetic Operators
• Unary Operators
• Assignment Operator
• Relational Operators
• Logical Operators
• Ternary Operator
• Bitwise Operators
• Shift Operators

This article explains all that one needs to know regarding Assignment Operators. 

Assignment Operators

These operators are used to assign values to a variable. The left side operand of the assignment operator is a variable, and the right side operand of the assignment operator is a value. The value on the right side must be of the same data type of the operand on the left side. Otherwise, the compiler will raise an error. This means that the assignment operators have right to left associativity, i.e., the value given on the right-hand side of the operator is assigned to the variable on the left. Therefore, the right-hand side value must be declared before using it or should be a constant. The general format of the assignment operator is, 

Types of Assignment Operators in Java

The Assignment Operator is generally of two types. They are:

1. Simple Assignment Operator: The Simple Assignment Operator is used with the “=” sign where the left side consists of the operand and the right side consists of a value. The value of the right side must be of the same data type that has been defined on the left side.

2. Compound Assignment Operator: The Compound Operator is used where +,-,*, and / is used along with the = operator.

Let’s look at each of the assignment operators and how they operate: 

1. (=) operator: 

This is the most straightforward assignment operator, which is used to assign the value on the right to the variable on the left. This is the basic definition of an assignment operator and how it functions. 

Syntax:  

Example:  

2. (+=) operator: 

This operator is a compound of ‘+’ and ‘=’ operators. It operates by adding the current value of the variable on the left to the value on the right and then assigning the result to the operand on the left. 

Note: The compound assignment operator in Java performs implicit type casting. Let’s consider a scenario where x is an int variable with a value of 5. int x = 5; If you want to add the double value 4.5 to the integer variable x and print its value, there are two methods to achieve this: Method 1: x = x + 4.5 Method 2: x += 4.5 As per the previous example, you might think both of them are equal. But in reality, Method 1 will throw a runtime error stating the “i ncompatible types: possible lossy conversion from double to int “, Method 2 will run without any error and prints 9 as output.

Reason for the Above Calculation

Method 1 will result in a runtime error stating “incompatible types: possible lossy conversion from double to int.” The reason is that the addition of an int and a double results in a double value. Assigning this double value back to the int variable x requires an explicit type casting because it may result in a loss of precision. Without the explicit cast, the compiler throws an error. Method 2 will run without any error and print the value 9 as output. The compound assignment operator += performs an implicit type conversion, also known as an automatic narrowing primitive conversion from double to int . It is equivalent to x = (int) (x + 4.5) , where the result of the addition is explicitly cast to an int . The fractional part of the double value is truncated, and the resulting int value is assigned back to x . It is advisable to use Method 2 ( x += 4.5 ) to avoid runtime errors and to obtain the desired output.

Same automatic narrowing primitive conversion is applicable for other compound assignment operators as well, including -= , *= , /= , and %= .

3. (-=) operator: 

This operator is a compound of ‘-‘ and ‘=’ operators. It operates by subtracting the variable’s value on the right from the current value of the variable on the left and then assigning the result to the operand on the left. 

4. (*=) operator:

 This operator is a compound of ‘*’ and ‘=’ operators. It operates by multiplying the current value of the variable on the left to the value on the right and then assigning the result to the operand on the left. 

5. (/=) operator: 

This operator is a compound of ‘/’ and ‘=’ operators. It operates by dividing the current value of the variable on the left by the value on the right and then assigning the quotient to the operand on the left. 

6. (%=) operator: 

This operator is a compound of ‘%’ and ‘=’ operators. It operates by dividing the current value of the variable on the left by the value on the right and then assigning the remainder to the operand on the left. 

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Java 8 interview questions, download pdf.

Java, originally evolved from the Oak language, was born in early 1996 with its major version as Java 1 or JDK 1.0. Java was initially designed and developed by Sir James Gosling at Sun Microsystems. Java 8 or JDK 8.0 is one of the major releases of the Java programming language in 2014. It is also known by the codename Spider. Java is an open-source project and is currently managed by Oracle Corporation.

This article would walk you through the Java 8 interview questions for freshers and experienced, scope, and opportunities.

Java 8 Interview Questions for Freshers

1. what is the lambda expression in java and how does a lambda expression relate to a functional interface.

Lambda expression is a type of function without a name. It may or may not have results and parameters. It is known as an anonymous function as it does not have type information by itself. It is executed on-demand. It is beneficial in iterating, filtering, and extracting data from a collection.

As lambda expressions are similar to anonymous functions, they can only be applied to the single abstract method of Functional Interface. It will infer the return type, type, and several arguments from the signature of the abstract method of functional interface.

2. What are the various categories of pre-defined function interfaces?

Function: To transform arguments in returnable value.

Predicate: To perform a test and return a Boolean value.

Consumer: Accept arguments but do not return any values.

Supplier: Do not accept any arguments but return a value. 

Operator: Perform a reduction type operation that accepts the same input types.

3. What are some standard Java pre-defined functional interfaces?

Some of the famous pre-defined functional interfaces from previous Java versions are Runnable, Callable, Comparator, and Comparable. While Java 8 introduces functional interfaces like Supplier, Consumer, Predicate, etc. Please refer to the java.util.function doc for other predefined functional interfaces and its description introduced in Java 8.

Runnable: use to execute the instances of a class over another thread with no arguments and no return value. 

Callable: use to execute the instances of a class over another thread with no arguments and it either returns a value or throws an exception.

Comparator: use to sort different objects in a user-defined order

Comparable: use to sort objects in the natural sort order

4. What are static methods in Interfaces?

Static methods, which contains method implementation is owned by the interface and is invoked using the name of the interface, it is suitable for defining the utility methods and cannot be overridden.

5. What is the default method, and why is it required?

A method in the interface that has a predefined body is known as the default method. It uses the keyword default. default methods were introduced in Java 8 to have 'Backward Compatibility in case JDK modifies any interfaces. In case a new abstract method is added to the interface, all classes implementing the interface will break and will have to implement the new method. With default methods, there will not be any impact on the interface implementing classes. default methods can be overridden if needed in the implementation. Also, it does not qualify as synchronized or final.

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6. Can a functional interface extend/inherit another interface?

A functional interface cannot extend another interface with abstract methods as it will void the rule of one abstract method per functional interface. E.g:

It can extend other interfaces which do not have any abstract method and only have the default, static, another class is overridden, and normal methods. For eg:

7. What are functional or SAM interfaces?

Functional Interfaces are an interface with only one abstract method. Due to which it is also known as the Single Abstract Method (SAM) interface. It is known as a functional interface because it wraps a function as an interface or in other words a function is represented by a single abstract method of the interface.

Functional interfaces can have any number of default, static, and overridden methods. For declaring Functional Interfaces @FunctionalInterface annotation is optional to use. If this annotation is used for interfaces with more than one abstract method, it will generate a compiler error.

8. What is MetaSpace? How does it differ from PermGen?

PremGen: MetaData information of classes was stored in PremGen (Permanent-Generation) memory type before Java 8. PremGen is fixed in size and cannot be dynamically resized. It was a contiguous Java Heap Memory.

MetaSpace: Java 8 stores the MetaData of classes in native memory called 'MetaSpace'. It is not a contiguous Heap Memory and hence can be grown dynamically which helps to overcome the size constraints. This improves the garbage collection, auto-tuning, and de-allocation of metadata.

9. What are the significant advantages of Java 8?

• Compact, readable, and reusable code.
• Less boilerplate code.
• Parallel operations and execution.
• Can be ported across operating systems.
• High stability.
• Stable environment.
• Adequate support

10. In which programming paradigm Java 8 falls?

• Object-oriented programming language.
• Functional programming language.
• Procedural programming language.
• Logic programming language

11. Describe the newly added features in Java 8?

Here are the newly added features of Java 8:

Java 8 Interview Questions for Experienced

1. what is the basic structure/syntax of a lambda expression.

Lambda expression can be divided into three distinct parts as below:

1. List of Arguments/Params:

(String name) 

A list of params is passed in () round brackets. It can have zero or more params. Declaring the type of parameter is optional and can be inferred for the context. 

2. Arrow Token:

->  Arrow token is known as the lambda arrow operator. It is used to separate the parameters from the body, or it points the list of arguments to the body. 3. Expression/Body:

A body can have expressions or statements. {} curly braces are only required when there is more than one line. In one statement, the return type is the same as the return type of the statement. In other cases, the return type is either inferred by the return keyword or void if nothing is returned.

2. What are the features of a lambda expression?

Below are the two significant features of the methods that are defined as the lambda expressions: 

• Lambda expressions can be passed as a parameter to another method. 
• Lambda expressions can be standalone without belonging to any class.

3. What is a type interface?

Type interface is available even in earlier versions of Java. It is used to infer the type of argument by the compiler at the compile time by looking at method invocation and corresponding declaration.

4. What are the types and common ways to use lambda expressions?

A lambda expression does not have any specific type by itself. A lambda expression receives type once it is assigned to a functional interface. That same lambda expression can be assigned to different functional interface types and can have a different type.

For eg consider expression s -> s.isEmpty() :

Predicate<String> stringPredicate = s -> s.isEmpty();   Predicate<List> listPredicate = s -> s.isEmpty(); Function<String, Boolean> func = s -> s.isEmpty(); Consumer<String> stringConsumer = s -> s.isEmpty();

Common ways to use the expression

Assignment to a functional Interface —> Predicate<String> stringPredicate = s -> s.isEmpty(); Can be passed as a parameter that has a functional type —> stream.filter(s -> s.isEmpty()) Returning it from a function —> return s -> s.isEmpty() Casting it to a functional type —> (Predicate<String>) s -> s.isEmpty()

5. In Java 8, what is Method Reference?

Method reference is a compact way of referring to a method of functional interface. It is used to refer to a method without invoking it. :: (double colon) is used for describing the method reference. The syntax is class::methodName

Integer::parseInt(str) \\ method reference

str -> Integer.ParseInt(str); \\ equivalent lambda

6. What does the String::ValueOf expression mean?

It is a static method reference to method Valueof() of class String. It will return the string representation of the argument passed.

7. What is an Optional class?

Optional is a container type which may or may not contain value i.e. zero(null) or one(not-null) value. It is part of java.util package. There are pre-defined methods like isPresent(), which returns true if the value is present or else false and the method get(), which will return the value if it is present.

8. What are the advantages of using the Optional class?

Below are the main advantage of using the Optional class: 

It encapsulates optional values, i.e., null or not-null values, which helps in avoiding null checks, which results in better, readable, and robust code It acts as a wrapper around the object and returns an object instead of a value, which can be used to avoid run-time NullPointerExceptions.

9. What are Java 8 streams?

A stream is an abstraction to express data processing queries in a declarative way. 

A Stream, which represents a sequence of data objects & series of operations on that data is a data pipeline that is not related to Java I/O Streams does not hold any data permanently. The key interface is java.util.stream.Stream<T> . It accepts Functional Interfaces so that lambdas can be passed. Streams support a fluent interface or chaining. Below is the basic stream timeline marble diagram:

10. What are the main components of a Stream?

Components of the stream are:

• A data source
• Set of Intermediate Operations to process the data source
• Single Terminal Operation that produces the result

11. What are the sources of data objects a Stream can process?

A Stream can process the following data:

• A collection of an Array.
• An I/O channel or an input device.
• A reactive source (e.g., comments in social media or tweets/re-tweets) 
• A stream generator function or a static factory.

12. What are Intermediate and Terminal operations?

Intermediate Operations:

• Process the stream elements.
• Typically transforms a stream into another stream.
• Are lazy, i.e., not executed till a terminal operation is invoked.
• Does internal iteration of all source elements.
• Any number of operations can be chained in the processing pipeline.
• Operations are applied as per the defined order.
• Intermediate operations are mostly lambda functions.

Terminal Operations:

• Kick-starts the Stream pipeline.
• used to collect the processed Stream data.

13. What are the most commonly used Intermediate operations?

Filter(Predicate<T>) - Allows selective processing of Stream elements. It returns elements that are satisfying the supplied condition by the predicate.

map(Funtion<T, R>) - Returns a new Stream, transforming each of the elements by applying the supplied mapper function.= sorted() - Sorts the input elements and then passes them to the next stage.

distinct() - Only pass on elements to the next stage, not passed yet.

limit(long maxsize) - Limit the stream size to maxsize.

skip(long start) - Skip the initial elements till the start.

peek(Consumer) - Apply a consumer without modification to the stream.

flatMap(mapper) - Transform each element to a stream of its constituent elements and flatten all the streams into a single stream.

14. What is the stateful intermediate operation? Give some examples of stateful intermediate operations.

To complete some of the intermediate operations, some state is to be maintained, and such intermediate operations are called stateful intermediate operations. Parallel execution of these types of operations is complex.

For Eg: sorted() , distinct() , limit() , skip() etc. 

Sending data elements to further steps in the pipeline stops till all the data is sorted for sorted() and stream data elements are stored in temporary data structures.

15. What is the most common type of Terminal operations?

• collect() - Collects single result from all elements of the stream sequence.
• count() - Returns the number of elements on the stream.
• min() - Returns the min element from the stream.
• max() - Returns the max element from the stream.
• anyMatch() , noneMatch() , allMatch() , ... - Short-circuiting operations.
• Takes a Predicate as input for the match condition.
• Stream processing will be stopped, as and when the result can be determined.
• forEach() - Useful to do something with each of the Stream elements. It accepts a consumer.
• forEachOrdered() - It is helpful to maintain order in parallel streams.

16. What is the difference between findFirst() and findAny()?

17. How are Collections different from Stream?

Collections are the source for the Stream. Java 8 collection API is enhanced with the default methods returning Stream<T> from the collections.

18. What is the feature of the new Date and Time API in Java 8?

• Immutable classes and Thread-safe 
• Timezone support
• Fluent methods for object creation and arithmetic
• Addresses I18N issue for earlier APIs
• Influenced by popular joda-time package
• All packages are based on the ISO-8601 calendar system

19. What are the important packages for the new Data and Time API?

• time-zones 
• Java.time.format
• Java.time.temporal
• java.time.zone

20. Explain with example, LocalDate, LocalTime, and LocalDateTime APIs.

• Date with no time component
• Default format - yyyy-MM-dd (2020-02-20)
• LocalDate today = LocalDate.now();  // gives today’s date
• LocalDate aDate = LocalDate.of(2011, 12, 30); //(year, month, date)
• Time with no date with nanosecond precision
• Default format - hh:mm:ss:zzz (12:06:03.015) nanosecond is optional
• LocalTime now = LocalTime.now();  // gives time now
• LocalTime aTime2 = LocalTime.of(18, 20, 30); // (hours, min, sec)

LocalDateTime

• Holds both Date and Time
• Default format - yyyy-MM-dd-HH-mm-ss.zzz (2020-02-20T12:06:03.015)
• LocalDateTime timestamp = LocalDateTime.now(); // gives timestamp now
• //(year, month, date, hours, min, sec)
• LocalDateTime dt1 = LocalDateTime.of(2011, 12, 30, 18, 20, 30);

21. Define Nashorn in Java 8

Nashorn is a JavaScript processing engine that is bundled with Java 8. It provides better compliance with ECMA (European Computer Manufacturers Association) normalized JavaScript specifications and better performance at run-time than older versions.

22. What is the use of JJS in Java 8?

As part of Java 8, JJS is a command-line tool that helps to execute the JavaScript code in the console. Below is the example of CLI commands:

JAVA>jjs jjs> print("Hello, Java 8 - I am the new JJS!") Hello, Java 8 - I am the new JJS! jjs> quit() >>

All in all, Java is a prevalent programming language securing the second rank in popularity in both TIOBE and PYPL programming language ranking. The world's leading tech giants like Twitter, LinkedIn, Amazon, PayPal, etc., use Java to build their web apps and backend web systems. Java is also one of the primary languages used to develop Android apps; an operating system backed and promoted by Google.

As of today, there are 1,751,661 questions around Java on StackOverflow and 123,776 Java public repositories on GitHub and continuously increasing. Considering Java 8 to be one of the most stable versions, there are immense career opportunities and scope in the same. Just understand the concepts, implement them and get ready for the interviews!

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Of course, Azure Container Apps has really solid support for our ecosystem, from a number of build options, managed Java components, native metrics, dynamic logger, and quite a bit more.

To learn more about Java features on Azure Container Apps, you can get started over on the documentation page .

And, you can also ask questions and leave feedback on the Azure Container Apps GitHub page .

Whether you're just starting out or have years of experience, Spring Boot is obviously a great choice for building a web application.

Jmix builds on this highly powerful and mature Boot stack, allowing devs to build and deliver full-stack web applications without having to code the frontend. Quite flexibly as well, from simple web GUI CRUD applications to complex enterprise solutions.

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DbSchema is a super-flexible database designer, which can take you from designing the DB with your team all the way to safely deploying the schema .

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Slow MySQL query performance is all too common. Of course it is. A good way to go is, naturally, a dedicated profiler that actually understands the ins and outs of MySQL.

The Jet Profiler was built for MySQL only , so it can do things like real-time query performance, focus on most used tables or most frequent queries, quickly identify performance issues and basically help you optimize your queries.

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Get the most out of the Apache HTTP Client

Get Started with Apache Maven:

Working on getting your persistence layer right with Spring?

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Building a REST API with Spring?

Explore Spring Boot 3 and Spring 6 in-depth through building a full REST API with the framework:

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Get started with Spring and Spring Boot, through the reference Learn Spring course:

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Yes, Spring Security can be complex, from the more advanced functionality within the Core to the deep OAuth support in the framework.

I built the security material as two full courses - Core and OAuth , to get practical with these more complex scenarios. We explore when and how to use each feature and code through it on the backing project .

You can explore the course here:

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Spring Data JPA is a great way to handle the complexity of JPA with the powerful simplicity of Spring Boot .

Get started with Spring Data JPA through the guided reference course:

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Get started with Spring Boot and with core Spring, through the Learn Spring course:

1. Overview

Sometimes, when we compile our Java source, the compiler may print a warning message “unchecked conversion” or “ The expression of type List needs unchecked conversion .”

In this tutorial, we’re going to take a deeper look at the warning message. We’ll discuss what this warning means, what problem it can lead to, and how to solve the potential problem.

2. Enabling the Unchecked Warning Option

Before we look into the “ unchecked conversion ” warning, let’s make sure that the Java compiler option to print this warning has been enabled.

If we’re using the Eclipse JDT Compiler , this warning is enabled by default.

When we’re using the Oracle or OpenJDK javac compiler, we can enable this warning by adding the compiler option -Xlint:unchecked.

Usually, we write and build our Java program in an IDE. We can add this option in the IDE’s compiler settings.

For example, the screenshot below shows how this warning is enabled in JetBrains IntelliJ :

Apache Maven is a widely used tool for building Java applications. We can configure maven-compiler-plugin ‘s compilerArguments  to enable this option:

Now that we’ve confirmed that our Java compiler has this warning option enabled, let’s take a closer look at this warning.

3. When Will the Compiler Warn Us: “unchecked conversion”?

In the previous section, we’ve learned how to enable the warning by setting the Java compiler option. Therefore, it’s not hard to imagine that “unchecked conversion” is a compile-time warning. Usually, we’ll see this warning when assigning a raw type to a parameterized type without type checking.

This assignment is allowed by the compiler because the compiler has to allow this assignment to preserve backward compatibility with older Java versions that do not support generics .

An example will explain it quickly. Let’s say we have a simple method to return a raw type List :

Next, let’s create a test method that calls the method and assigns the result to a variable with the type List<String> :

Now, if we compile our test above, we’ll see the warning from the Java compiler.

Let’s build and test our program using Maven:

As the output above shows, we’ve reproduced the compiler warning.

A typical example in the real world is when we use Java Persistence API ‘s Query.getResultList() method. The method returns a raw type List object.

However, when we try to assign the raw type list to a list with a parameterized type, we’ll see this warning at compile-time:

Moreover, we know that if the compiler warns us of something, it means there are potential risks. If we review the Maven output above, we’ll see that although we get the “ unchecked conversion ” warning, our test method works without any problem.

Naturally, we may want to ask why the compiler warns us with this message and what potential problem we might have?

Next, let’s figure it out.

4. Why Does the Java Compiler Warn Us?

Our test method works well in the previous section, even if we get the “ unchecked conversion ” warning. This is because the getRawList()  method only adds String s into the returned list.

Now, let’s change the method a little bit:

In the new getRawListWithMixedTypes() method, we add a  Date object to the returned list. It’s allowed since we’re returning a raw type list that can contain any types.

Next, let’s create a new test method to call the getRawListWithMixedTypes() method and test the return value:

If we run the test method above, we’ll see the “ unchecked conversion ” warning again, and the test will pass.

This means a ClassCastException has been thrown when we get the Date object by calling get(3) and attempt to cast its type to String.

In the real world, depending on the requirements, sometimes the exception is thrown too late.

For example, we assign  List<String> strList = getRawListWithMixedTypes(). For each String object in strList, suppose that we use it in a pretty complex or expensive process such as external API calls or transactional database operations.

When we encounter the ClassCastException on an element in the strList , some elements have been processed. Thus, the ClassCastException comes too late and may lead to some extra restore or data cleanup processes.

So far, we’ve understood the potential risk behind the  “unchecked conversion” warning. Next, let’s see what we can do to avoid the risk.

5. What Shall We Do With the Warning?

If we’re allowed to change the method that returns raw type collections, we should consider converting it into a generic method. In this way, type safety will be ensured.

However, it’s likely that when we encounter the “ unchecked conversion ” warning, we’re working with a method from an external library. Let’s see what we can do in this case.

5.1. Suppressing the Warning

We can use the annotation SuppressWarnings(“unchecked”) to suppress the warning.

However, we should use the @SuppressWarnings(“unchecked”) annotation only if we’re sure the typecast is safe because it merely suppresses the warning message without any type checking.

Let’s see an example:

As we’ve mentioned earlier, JPA’s Query.getResultList() method returns a raw typed  List object. Based on our query, we’re sure the raw type list can be cast to List<Object[]> . Therefore, we can add the  @SuppressWarnings above the assignment statement to suppress the “ unchecked conversion ” warning.

5.2. Checking Type Conversion Before Using the Raw Type Collection

The warning message “ unchecked conversion ” implies that we should check the conversion before the assignment.

To check the type conversion, we can go through the raw type collection and cast every element to our parameterized type. In this way, if there are some elements with the wrong types, we can get ClassCastException before we really use the element.

We can build a generic method to do the type conversion. Depending on the specific requirement, we can handle ClassCastException in different ways.

First, let’s say we’ll filter out the elements that have the wrong types:

Let’s test the  castList() method above by a unit test method:

When we build and execute the test method, the “ unchecked conversion ” warning is gone, and the test passes.

Of course, if it’s required, we can change our  castList()  method to break out of the type conversion and throw ClassCastException immediately once a wrong type is detected:

As usual, let’s create a unit test method to test the castList2() method:

The test method above will pass if we give it a run. It means that once there’s an element with the wrong type in rawList , the castList2() method will stop the type conversion and throw ClassCastException.

6. Conclusion

In this article, we’ve learned what the “ unchecked conversion ” compiler warning is. Further, we’ve discussed the cause of this warning and how to avoid the potential risk.

As always, the code in this write-up is all available over on GitHub .

Slow MySQL query performance is all too common. Of course it is.

The Jet Profiler was built entirely for MySQL , so it's fine-tuned for it and does advanced everything with relaly minimal impact and no server changes.

Just published a new writeup on how to run a standard Java/Boot application as a Docker container, using the Liberica JDK on top of Alpaquita Linux:

>> Spring Boot Application on Liberica Runtime Container.

Explore the secure, reliable, and high-performance Test Execution Cloud built for scale. Right in your IDE:

Basically, write code that works the way you meant it to.

AI is all the rage these days, but for very good reason. The highly practical coding companion, you'll get the power of AI-assisted coding and automated unit test generation . Machinet's Unit Test AI Agent utilizes your own project context to create meaningful unit tests that intelligently aligns with the behavior of the code.

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Java, What is the difference between assigning null to object and just declaration

What is difference between :

• Object o = null ; and
• Object o; (just declaration)

Can anyone please answer me?

• declaration

2 Answers 2

It depends on the scope where you declare the variable. For instance, local variables don't have default values in which case you will have to assign null manually, where as in case of instance variables assigning null is redundant since instance variables get default values.

• 1 could u pls fix the code tho? Object localVariableObj1.getClass(); is invalid.. Same with Object localVariableObj2.getClass(); –  DarkAtra Commented Apr 9, 2018 at 18:39

As mentioned, object reference as instance variable need not be assigned null as those take null as default value. But local variables must be initialized otherwise you will get compilation error saying The local variable s may not have been initialized .

For more details you can refer this link

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