The Initializer Block in Java

In order to perform any operations while assigning values to an instance data member, an initializer block is used. In simpler terms, the initializer block is used to declare/initialize the common part of various constructors of a class. It runs every time whenever the object is created. 

The initializer block contains the code that is always executed whenever an instance is created and it runs each time when an object of the class is created. There are 3 areas where we can use the initializer blocks:

• Constructors
• Methods
• Blocks 

Tip: If we want to execute some code once for all objects of a class then we will be using Static Block in Java

Example 1:

Java

// Java Program to Illustrate Initializer Block

// Class
class Car {

    // Class member variable
    int speed;

    // Constructor
    Car()
    {

        // Print statement when constructor is called
        System.out.println("Speed of Car: " + speed);
    }
    // Block
    {
        speed = 60;
    }
    // Class member method
    public static void main(String[] args)
    {

        Car obj = new Car();
    }
}

Output

Speed of Car: 60

Example 2:

Java

// Java Program to Illustrate Initializer Block

// Importing required classes
import java.io.*;

// Main class
public class GFG {

    // Initializer block starts..
    {
        // This code is executed before every constructor.
        System.out.println(
            "Common part of constructors invoked !!");
    }
    // Initializer block ends

    // Constructor 1
    // Default constructor
    public GFG()
    {

        // Print statement
        System.out.println("Default Constructor invoked");
    }

    // Constructor 2
    // Parameterized constructor
    public GFG(int x)
    {

        // Print statement
        System.out.println(
            "Parameterized constructor invoked");
    }

    // Main driver method
    public static void main(String arr[])
    {

        // Creating variables of class type
        GFG obj1, obj2;

        // Now these variables are
        // made to object and interpreted by compiler
        obj1 = new GFG();

        obj2 = new GFG(0);
    }
}

Output

Common part of constructors invoked !!
Default Constructor invoked
Common part of constructors invoked !!
Parameterized constructor invoked

Note: The contents of the initializer block are executed whenever any constructor is invoked (before the constructor’s contents)
The order of initialization constructors and initializer block doesn’t matter, the initializer block is always executed before the constructor.

For more details about the Instance Initialization in Java, refer to the article Instance Initialization Block (IIB) in Java

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