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

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

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