What is an initialization block?

Question

We can put code in a constructor or a method or an initialization block. What is the use of initialization block? Is it necessary that every java program must have it?

Answer 1

public class StaticInitializationBlock {

    static int staticVariable;
    int instanceVariable;

    // Static Initialization Block
    static { 
        System.out.println("Static block");
        staticVariable = 5;

    }

    // Instance Initialization Block
    { 

        instanceVariable = 7;
        System.out.println("Instance Block");
        System.out.println(staticVariable);
        System.out.println(instanceVariable);

        staticVariable = 10;
    }


    public StaticInitializationBlock() { 

        System.out.println("Constructor");
    }

    public static void main(String[] args) {
        new StaticInitializationBlock();
        new StaticInitializationBlock();
    }

}

Output:

Static block
Instance Block
5
7
Constructor
Instance Block
10
7
Constructor