Static And Instance Initialization Block In Java

 Static Initialization Block 

• The static initialization block is executed only once when a class is loaded the first time. 
• Multiple static initialization block is allowed.

Instance Initialization Block

• The instance initialization block is executed whenever a new instance is created.
• Multiple instance initialization block is allowed.

Order of executions

• The static initialization block is executed first.
• Instance initialization block is executed after all super constructor is executed and before the execution constructor of their own class.
• If we have multiple blocks in the same class then they will execute in the sequence it has been declared but first all static blocks and then all instance blocks.

Example

Class Animal

package com.demo; public class Animal { public Animal() { System.out.println("Animal constructor called."); } }

Class Dog subclass of Animal

package com.demo; public class Dog extends Animal{ Dog(){ System.out.println("Dog constructor called"); } static { System.out.println("1st dog static block called"); } { System.out.println("1st dog instance block called"); } static { System.out.println("2nd dog static block called"); } { System.out.println("2nd dog instance block called"); } }

Now we will create an instance of the Dog class in TestOrder and verify the order of execution of the constructor and blocks

package com.demo; public class TestOrder { public static void main(String[] args) { Dog d1=new Dog(); } }

Output 

1st dog static block called

2nd dog static block called

Animal constructor called.

1st dog instance block called

2nd dog instance block called

Dog constructor called

Another post you may like to refer

Final Keyword In Java

Method Overloading In Java + Compile Time Polymorphism In Java

Method Overriding In Java + Runtime Polymorphism In Java

How to remove duplicate elements from list + Java 8

For spring tutorial visit Spring Tutorial

Thanks

Happy Learning!