Instance initializer block

Instance block: Instance block is used to write the logics and initialize the instance variables, both are done during object creation time.

Syntax:

{

//logics here

}

why do we need instance block?

• Instance block is executed during object creation before constructor execution. i.e. first instance block then constructor
• Constructor logics are specific to object (i.e. three object, three different constructors) , but instance block logics are common for all objects (instance block logic is common for all object)
• Inside the class, it is possible to declare multiple instance block
• Instance block execution depends on object creation not on constructors execution. Instance block is executed for every object creation.

Advantages of instance block

• Advantage # 1: Write the logics and those logics are executed during object creation:

Example I: During object creation time if any instance blocks are there, first instance block will be executed, once the instance block is executed then only constructor is executed

class Test1{

       Test1(){

             System.out.println("0-argument constructor");

       }

       {

             System.out.println("instance block");

       }

}

public class InstanceBlockDemo1 {

public static void main(String[] args) {

       //Test1 t = new Test1();  ;// named object creation

       new Test1();             // nameless object creation

  

}

}

Output:

instance block

0-argument constructor

Why do we need instance block?

Example 2: Instance block is executed for every object creation

class Test2{

       Test2(){

             System.out.println("0-argument constructor");

       }

       Test2(int a){

             System.out.println("1-arguement constructor");

       }

       Test2(int a, int b){

             System.out.println("2-argument constructor");

       }

       {

             System.out.println("instance block");

       }

}

public class InstanceBlockDemo2 {

public static void main(String[] args) {

       Test2 t1 = new Test2();                   // instance block then constructor

       Test2 t2 = new Test2(10);                // instance block then constructor

       Test2 t3 = new Test2(10, 20);           // instance block then constructor

}

}

Output:

instance block

0-argument constructor

instance block

1-arguement constructor

instance block

2-argument constructor

Example IV: Instance block execution depends on object creation not on constructor execution

class Test3{

Test3(){

       this(10);

       System.out.println("0-argument constructor");

}

Test3(int a){

       System.out.println("1-argument constructor");

}

Test3(int a, int b){

       System.out.println("2-argument constructor");

}

{

       System.out.println("instance block");

}

}

public class InstanceBlockDemo3 {

public static void main(String[] args) {

       new Test3();

}

}

Output:

instance block

1-argument constructor

0-argument constructor

Example V: Inside the class, we can write multiple instance block and execution order is from top to bottom

class Test4{

       Test4(){

             System.out.println("0-argument constructor");

       }

       Test4(int a){

             System.out.println("1-argument constructor");

       }

       {

             System.out.println("instance block-1");

       }

       {

             System.out.println("instance block-2");

       }

}

public class InstanceBlockDemo4 {

public static void main(String[] args) {

       new Test4();

       new Test4(10);

}

}

Output:

instance block-1

instance block-2

0-argument constructor

instance block-1

instance block-2

1-argument constructor

Advantage # 2: Initialize the variables:

class Test5{

       int eid;

void display(){

       System.out.println(eid);

}

//instance block initializing value to variable during object creation

{

eid = 111;

}

}

public class InstanceBlockDemo5 {

public static void main(String[] args) {

       new Test5().display();

}

}

Output:

111