What is an initialization block in Java?

Initialization block in Java is a block of code that executes when an object is created. There are two types:

1. Static initialization block:
   • Declared with the keyword static {}.
   • Executes once when the class is loaded into JVM memory, before any objects of this class are created.
   • Used for initializing static variables or performing other static operations.

   // Static initialization block
   static {
       System.out.println("Static initializer block executed");
       staticVariable = 100;
   }
   

2. Non-static (instance) initialization block:
   • Declared without the static {} keyword.
   • Executes every time a new instance of the class is created, after memory allocation but before the constructor is called.
   • Used for initializing instance variables or performing operations specific to each object.

   // Non-static initialization block
   {
       System.out.println("Instance initializer block executed");
       instanceVariable = 10;
   }
   

Execution order:

1. Static initialization blocks (in the order of their declaration).
2. Non-static initialization blocks (in the order of their declaration).
3. Constructors.

Example:

class Example {
    static int staticVariable;
    int instanceVariable;

    static {
        System.out.println("Static block 1");
        staticVariable = 1;
    }

    {
        System.out.println("Instance block 1");
        instanceVariable = 11;
    }

    static {
        System.out.println("Static block 2");
        staticVariable = 2;
    }

    {
        System.out.println("Instance block 2");
        instanceVariable = 22;
    }

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

    public static void main(String[] args) {
        System.out.println("Creating object 1");
        new Example();
        System.out.println("Creating object 2");
        new Example();
    }
}

Output:

Static block 1
Static block 2
Creating object 1
Instance block 1
Instance block 2
Constructor
Creating object 2
Instance block 1
Instance block 2
Constructor

Main uses:

• Static blocks: Initialization of static fields, resource loading at class startup.
• Non-static blocks: Performing common initialization logic before any constructor (e.g., setting default values that can be changed in constructors).