Java Static Initialization vs. Instance Initialization - When to use which?

Static Initialization

Typically static initialization block is used to initialize static variables of a class. The block is called at the time of class initialization. It is called only once. You can initialize static variables inline. If more complicated logic is required for initialization, a static initialization block can be used. The static initialization blocks are called in the order in which they occur, and they are called before the constructors.

For example:

	class StaticTest {
	static int i;
	static {
	System.out.println("Static Initialization block is called");
	i = 10;
	}
	StaticTest() {
	System.out.println("Static Test Constructor is called");
	}
	}
	class Main {
	public static void main (String args[]){
	System.out.println(StaticTest.i);
	}
	}

view raw static_initalizer.java hosted with ❤ by GitHub

The output of the above code is as shown below.

	Static Initialization block is called
	10

view raw op1.txt hosted with ❤ by GitHub

Instance Initialization

Instance Initialization or Initializer Block is called whenever an instance of the class is created.
It can be used to execute code that is common to all constructors. This block is executed before the constructor is executed.

For example:

	class InitializerTest {
	int i;
	{
	System.out.println("Instance Initialization block is called");
	i = 5;
	}
	InitializerTest() {
	System.out.println("Initializer Test Default Constructor is called");
	}
	InitializerTest(int x) {
	System.out.println("Initializer Test Parametrized Constructor is called");
	i = x;
	}
	}
	class Main {
	public static void main (String args[]){
	InitializerTest obj1 = new InitializerTest();
	System.out.println(obj1.i);
	InitializerTest obj2 = new InitializerTest(10);;
	System.out.println(obj2.i);
	}
	}

view raw instance_initializer.java hosted with ❤ by GitHub

The output of the above code is as shown below.

	Instance Initialization block is called
	Initializer Test Default Constructor is called
	5
	Instance Initialization block is called
	Initializer Test Parametrized Constructor is called
	10

view raw op2.txt hosted with ❤ by GitHub

Ref: https://docs.oracle.com/javase/tutorial/java/javaOO/initial.html