Blocks

A block is a pair of { } braces.
It is used to group a set of statements.

A block can be declared:

Inside a class
Inside a method
Inside a constructor
Inside another block

Types of Blocks

Instance Block (Non-Static Block)
Static Block
Local Block

1. Instance Block (Non-Static Block)

A block declared inside a class without static keyword
Also called Non-Static Block

Executes when object is created
Runs before constructor
Can have multiple instance blocks
Execution order → top to bottom (as declared)
Can access:
- Instance variables
- Static variables

java

class A {

    int a = 10;        // instance variable
    static int b = 20; // static variable

    {
        System.out.println("Instance Block 1");
        System.out.println("a: " + a);
        System.out.println("b: " + b);
    }

    {
        System.out.println("Instance Block 2");
    }
}

class Main {
    public static void main(String[] args) {
        new A();
        new A();
    }
}

Output :

java

Instance Block 1
a: 10
b: 20
Instance Block 2

Runs every time object is created

2. Static Block

A block declared with static keyword
Executes when class is loaded

Runs only once in program life cycle
Executes before main() method
Can directly access:
- Static variables ✅
- Instance variables ❌ (need object)
Used for:
- Initialization logic
- Database connection setup
- Configuration loading

java

class A {
    static int b = 20;

    static {
        System.out.println("Static Block in A");
        System.out.println("b: " + b);
    }

    int a = 10;

    {
        System.out.println("Instance Block in A");
    }
}

class Main {
    public static void main(String[] args) {
        new A();
    }
}

Output:

java

Static Block in A
b: 20
Instance Block in A

Static block runs first, then instance block

Important Note (Execution Order)

java

class A {
    static {
        System.out.println("SB-1");
    }

    static int a = 10;

    static {
        System.out.println("SB-2");
        System.out.println(a);
    }
}

Output:

plaintext

SB-1
SB-2
10

Execution depends on declaration order

3. Local Block

A block defined inside a method or local context

Executes when method is called
Not static or instance block
Used for:
- Code grouping
- Block-level operations
- Synchronization (advanced use)

java

class A {

    void m1() {

        System.out.println("Before Local Block");

        {
            System.out.println("Inside Local Block");
        }

        System.out.println("After Local Block");
    }
}

class Main {
    public static void main(String[] args) {
        new A().m1();
    }
}

Output:

java

Before Local Block
Inside Local Block
After Local Block

Combined Example

java

class A {

    int a = 10;
    static int b = 20;

    static {
        System.out.println("Static Block in A");
    }

    {
        System.out.println("Instance Block in A");
    }
}

class Main {

    static {
        System.out.println("Static Block in Main");
    }


    public static void main(String[] args) {
        A a1 = new A();
    }
}

Output:

java

Static Block in Main
Static Block in A
Instance Block in A