Using Instance Blocks in Java

An instance block (also called an instance initialization block) is a nameless block of code defined inside a class. It executes every time an object of the class is created, before the constructor is executed.

Declared inside a class but outside all methods
Executed once per object creation
Common logic shared across all constructors
Executed before the constructor
Cannot accept parameters

Syntax

class ClassName {
{
// Instance block code
}
}

Example 1: Instance Block with Multiple Constructors

Java

class GFG {

    GFG() {
        System.out.println("1st argument constructor");
    }
    GFG(String a) {
        System.out.println("2nd argument constructor");
    }
    GFG(int a, int b) {
        System.out.println("3rd arguments constructor");
    }

    {
        System.out.println("Instance block");
    }
}
class GFGJava {
    public static void main(String[] args) {

        new GFG();
        new GFG("I like Java");
        new GFG(10, 20);
    }
}

Output

Instance block
1st argument constructor
Instance block
2nd argument constructor
Instance block
3rd arguments constructor

Explanation:

The instance block executes before every constructor call.
Regardless of which constructor is used, the instance block runs first.
This ensures shared logic executes uniformly for all object types.

Note: The sequence of execution of instance blocks follows the order- Static block, Instance block, and Constructor.

Example 2: Demonstrating Execution Order

Java

class GFG {

    static {
        System.out.println("I am Static block!");
    }

    {
        System.out.println("I am Instance block!");
    }
    GFG() {
        System.out.println("I am Constructor!");
    }
    public static void main(String[] args) {
        new GFG();
    }
}

Output

I am Static block!
I am Instance block!
I am Constructor!

Explanation:

Static block executes once when the class is loaded.
Instance block executes each time an object is created.
Constructor executes last to complete object initialization.

Why Use Instance Blocks?

Advantages

Avoids code duplication across multiple constructors
Ensures common initialization logic runs for every object
Useful when multiple constructors require the same setup logic

Limitations

Cannot accept parameters
Not suitable for object-specific initialization
All objects are initialized with the same logic and values

When to Use Instance Blocks

When common logic must run for all constructors.
When initialization does not depend on constructor parameters.
When code reuse across constructors is required.

When to Avoid Instance Blocks

When initialization requires parameters
When constructor-specific logic is sufficient
When readability may be affected due to hidden execution flow

Using Instance Blocks in Java

Syntax

Why Use Instance Blocks?

When to Use Instance Blocks

When to Avoid Instance Blocks

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