Instance Initialization Block (IIB) in Java

In a Java program, operations can be performed on methods, constructors, and initialization blocks. Instance Initialization Blocks or IIBs are used to initialize instance variables. So firstly, the constructor is invoked and the java compiler copies the instance initializer block in the constructor after the first statement super(). They run each time when the object of the class is created. 
 

• Initialization blocks are executed whenever the class is initialized and before constructors are invoked.
• They are typically placed above the constructors within braces.
• It is not at all necessary to include them in your classes.

// Java program to illustrate
// Instance Initialization Block
class GfG {
    // Instance Initialization Block
    {
        System.out.println("IIB block");
    }

    // Constructor of GfG class
    GfG() { System.out.println("Constructor Called"); }
    public static void main(String[] args)
    {
        GfG a = new GfG();
    }
}

IIB block
Constructor Called

Multiple Instance Initialization Blocks in a Program

We can also have multiple IIBs in a single class. If the compiler finds multiple IIBs, then they all are executed from top to bottom i.e. the IIB which is written at the top will be executed first. 

// Java program to illustrate
// execution of multiple
// Instance Initialization Blocks
// in one program
class GfG {
    // Instance Initialization Block - 1
    {
        System.out.println("IIB1 block");
    }

    // Instance Initialization Block - 2
    {
        System.out.println("IIB2 block");
    }

    // Constructor of class GfG
    GfG() { System.out.println("Constructor Called"); }

    // Instance Initialization Block - 3
    {
        System.out.println("IIB3 block");
    }

    // main function
    public static void main(String[] args)
    {
        GfG a = new GfG();
    }
}

IIB1 block
IIB2 block
IIB3 block
Constructor Called

Instance Initialization Block with parent class

You can have IIBs in parent class also. Instance initialization block code runs immediately after the call to super() in a constructor. The compiler executes the parent's class's IIB before executing the current class's IIBs. 

Have a look at the following example.  

// Java program to illustrate
// Instance Initialization Block
// with super()

// Parent Class
class B {
    B() { System.out.println("B-Constructor Called"); }

    {
        System.out.println("B-IIB block");
    }
}

// Child class
class A extends B {
    A()
    {
        super();
        System.out.println("A-Constructor Called");
    }
    {
        System.out.println("A-IIB block");
    }

    // main function
    public static void main(String[] args)
    {
        A a = new A();
    }
}

B-IIB block
B-Constructor Called
A-IIB block
A-Constructor Called

In the above example, the compiler tries to execute the class A constructor, when the object of class A is created. But it finds super() statement and goes to the parent class constructor first to be executed. The order of execution, in this case, will be as follows: 

1. Instance Initialization Block of the superclass. 
2. Constructors of the superclass. 
3. Instance Initialization Blocks of the subclass. 
4. Constructors of the subclass.

Important points:  

• Instance Initialization Blocks run every time a new instance is created.
• Initialization Blocks run in the order they appear in the program
• The Instance Initialization Block is invoked after the parent class constructor is invoked (i.e. after super() constructor call)

Related Article : 
The Initializer Block in Java