Instance initializer block

Instance Initializer block is used to initialize the instance data member. It run each time when object of the class is created.

The initialization of the instance variable can be done directly but there can be performed extra operations while initializing the instance variable in the instance initializer block.

Que) What is the use of instance initializer block while we can directly assign a value in instance data member? For example:

Example

snippet

class Bike{
    int speed=100;
}

Usage of instance initializer block

Suppose I have to perform some operations while assigning value to instance data member e.g. a for loop to fill a complex array or error handling etc.

Example

Let's see the simple example of instance initializer block that performs initialization.

snippet

class Bike7{
    int speed;
    
    Bike7(){System.out.println("speed is "+speed);}
 
    {speed=100;}
     
    public static void main(String args[]){
    Bike7 b1=new Bike7();
    Bike7 b2=new Bike7();
    }    
}

Output

Output:speed is 100 speed is 100

There are three places in java where you can perform operations:

method
constructor
block

What is invoked first, instance initializer block or constructor?

Example

snippet

class Bike8{
    int speed;
    
    Bike8(){System.out.println("constructor is invoked");}
 
    {System.out.println("instance initializer block invoked");}
     
    public static void main(String args[]){
    Bike8 b1=new Bike8();
    Bike8 b2=new Bike8();
    }    
}

Output

Output:instance initializer block invoked constructor is invoked instance initializer block invoked constructor is invoked

In the above example, it seems that instance initializer block is firstly invoked but NO. Instance intializer block is invoked at the time of object creation. The java compiler copies the instance initializer block in the constructor after the first statement super(). So firstly, constructor is invoked. Let's understand it by the figure given below:

Note: The java compiler copies the code of instance initializer block in every constructor.

Rules for instance initializer block :

There are mainly three rules for the instance initializer block. They are as follows:

The instance initializer block is created when instance of the class is created.
The instance initializer block is invoked after the parent class constructor is invoked (i.e. after super() constructor call).
The instance initializer block comes in the order in which they appear.

Program of instance initializer block that is invoked after super()

Example #1

snippet

class A{
A(){
System.out.println("parent class constructor invoked");
}
}
class B2 extends A{
B2(){
super();
System.out.println("child class constructor invoked");
}

{System.out.println("instance initializer block is invoked");}

public static void main(String args[]){
B2 b=new B2();
}
}

Output

Output:parent class constructor invoked instance initializer block is invoked child class constructor invoked

Another example of instance block

Example #2

snippet

class A{
A(){
System.out.println("parent class constructor invoked");
}
}

class B3 extends A{
B3(){
super();
System.out.println("child class constructor invoked");
}

B3(int a){
super();
System.out.println("child class constructor invoked "+a);
}

{System.out.println("instance initializer block is invoked");}

public static void main(String args[]){
B3 b1=new B3();
B3 b2=new B3(10);
}
}

Output

Output:parent class constructor invoked instance initializer block is invoked child class constructor invoked parent class constructor invoked instance initializer block is invoked child class constructor invoked 10