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1. Overview

Simply put, before we can work with an object on the JVM, it has to be initialized.

In the following sections, we’ll take a look at various ways we can initialize primitive types and objects.

2. Declaration vs. Initialization

Let’s start by making sure that we’re on the same page.

Declaration is the process of defining the variable along with its type and name.

Here, we’re declaring the id variable:

int id;

Initialization, on the other hand, is all about assigning a value; for example:

id = 1;

To demonstrate, we’ll create a User class with a name and id properties:

public class User implements {
    private String name;
    private int id;
    
    // standard constructor, getters, setters,
}

Next, we’ll see that initialization works differently depending on the type of field we’re initializing.

3. Objects vs. Primitives

Java provides two types of data representation: primitive types and reference types. In this section, we’ll discuss the differences between the two with regards to initialization.

Java has eight built-in data types, referred to as Java primitive types; variables of this type hold their values directly.

Reference types hold references to objects (instances of classes). Unlike primitive types that hold their values in the memory where the variable is allocated, references don’t hold the value of the object they refer to.

Instead, a reference points to an object by storing the memory address where the object is located.

Note that Java doesn’t allow us to discover what the physical memory address is. Rather, we can only use the reference to refer to the object.

Let’s take a look at an example that declares and initializes a reference type out of our User class:

@Test
public void whenIntializedWithNew_thenInstanceIsNotNull() {
    User user = new User();
 
    assertThat(user).isNotNull();
}

As we can see here, a reference can be assigned to a new by using the keyword new, which is responsible for creating the new User object.

Let’s continue with learning more about object creation.

5. Creating Objects

Unlike with primitives, objects creation is a bit more complex. This is because we’re not just adding the value to the field; instead, we trigger the initialization using the new keyword. This, in return, invokes a constructor and initializes the object in memory.

Let’s discuss constructors and the new keyword in further detail.

The new keyword is responsible for allocating memory for the new object through a constructor.

A constructor is typically used to initialize instance variables representing the main properties of the created object.

If we don’t supply a constructor explicitly, the compiler will create a default constructor which has no arguments and just allocates memory for the object.

A class can have many constructors as long as their parameters lists are different (overload). Every constructor that doesn’t call another constructor in the same class has a call to its parent constructor whether it was written explicitly or inserted by the compiler through super().

Let’s add a constructor to our User class:

public User(String name, int id) {
    this.name = name;
    this.id = id;
}

Now we can use our constructor to create a User object with initial values for its properties:

User user = new User("Alice", 1);

6. Variable Scope

In the following sections, we’ll take a look at the different types of scopes that a variable in Java can exist within and how this affects the initialization process.

6.1. Instance and Class Variables

Instance and class variables don’t require us to initialize them. As soon as we declare these variables, they are given a default value as follows:

Now, let’s try to define some instance and class-related variables and test whether they have a default value or not:

@Test
public void whenValuesAreNotInitialized_thenUserNameAndIdReturnDefault() {
    User user = new User();
 
    assertThat(user.getName()).isNull();
    assertThat(user.getId() == 0);
}

6.2. Local Variables

Local variables must be initialized before use, as they don’t have a default value and the compiler won’t let us use an uninitialized value.

For example, the following code generates a compiler error:

public void print(){
    int i;
    System.out.println(i);
}

7. The Final Keyword

The final keyword applied to a field means that the field’s value can no longer be changed after initialization. In this way, we can define constants in Java.

Let’s add a constant to our User class:

private static final int YEAR = 2000;

Constants must be initialized either when they’re declared or in a constructor.

8. Initializers

In Java, an initializer is a block of code that has no associated name or data type and is placed outside of any method, constructor, or another block of code.

Java offers two types of initializers, static and instance initializers. Let’s see how we can use each of them.

8.1. Instance Initializers

We can use these to initialize instance variables.

To demonstrate, let’s provide a value for a user id using an instance initializer in our User class:

{
    id = 0;
}

8.2. Static Initialization Block

A static initializer or static block – is a block of code which is used to initialize static fields. In other words, it’s a simple initializer marked with the keyword static:

private static String forum;
static {
    forum = "Java";
}

9. Order of Initialization

When writing code that initializes different types of fields, of course, we have to keep an eye on the order of initialization.

In Java, the order for initialization statements is as follows:

  • static variables and static initializers in order
  • instance variables and instance initializers in order
  • constructors

10. Object Life Cycle

Now that we’ve learned how to declare and initialize objects, let’s discover what happens to objects when they’re not in use.

Unlike other languages where we have to worry about object destruction, Java takes care of obsolete objects through its garbage collector.

All objects in Java are stored in our program’s heap memory. In fact, the heap represents a large pool of unused memory, allocated for our Java application.

On the other hand, the garbage collector is a Java program that takes care of automatic memory management by deleting objects that are no longer reachable.

For a Java object to become unreachable, it has to encounter one of the following situations:

  • The object no longer has any references pointing to it
  • All reference pointing to the object are out of scope

In conclusion, an object is first created from a class, usually using the keyword new. Then the object lives its life and provides us with access to its methods and fields.

Finally, when it’s no longer needed, the garbage collector destroys it.

11. Other Methods for Creating Objects

In this section, we’ll take a brief look at methods other than new keyword to create objects and how to apply them, specifically reflection, cloning, and serialization.

Reflection is a mechanism we can use to inspect classes, fields, and methods at run-time. Here’s an example of creating our User object using reflection:

@Test
public void whenInitializedWithReflection_thenInstanceIsNotNull() 
  throws Exception {
    User user = User.class.getConstructor(String.class, int.class)
      .newInstance("Alice", 2);
 
    assertThat(user).isNotNull();
}

In this case, we’re using reflection to find and invoke a constructor of the User class.

The next method, cloning, is a way to create an exact copy of an object. For this, our User class must implement the Cloneable interface:

public class User implements Cloneable { //... }

Now we can use the clone() method to create a new clonedUser object which has the same property values as the user object:

@Test
public void whenCopiedWithClone_thenExactMatchIsCreated() 
  throws CloneNotSupportedException {
    User user = new User("Alice", 3);
    User clonedUser = (User) user.clone();
 
    assertThat(clonedUser).isEqualTo(user);
}

We can also use the sun.misc.Unsafe class to allocate memory for an object without calling a constructor:

User u = (User) unsafeInstance.allocateInstance(User.class);

12. Conclusion

In this tutorial, we covered initialization of fields in Java. We discovered different data types in Java and how to use them. We also took an in-depth on several ways of creating objects in Java.

The full implementation of this tutorial can be found over on Github.

I just announced the new Spring 5 modules in REST With Spring:

>> CHECK OUT THE LESSONS