What is an initializer?

A process that is executed once before using a class instance. Used when initializing class variables.

class initializer_demo {
	//Class initializer
	static {
		//processing
	}
	//Instance initializer
	{
		//processing
	}
}

What is the difference between an instance initializer and a constructor?

When you think of the process of instantiating, the constructor comes to mind, The difference is that the instance initializer is executed before the constructor and the construct is executed after instantiation. If the constructor is overloaded and there are multiple constructors, the initializer seems to use it by executing common processing first.

Reference code below

class User {
    private String name;
    public static int count;

    //Class initializer
    static {
       User.count = 0;
       System.out.println("Class initializer execution\n↓");
    }

    //Instance initializer
    {
        System.out.println("Instance initializer execution\n↓");
    }

    //Constructor
    public User(String name) {
        this.name = name;
        User.count++;
        System.out.println("Constructor execution\n↓");
    }

    //Class method
    public static void getInfo() {
        System.out.println(User.count + "Instantiated times\n↓");
    }
}

public class MyApp {
    public static void main(String[] args) {
        User.getInfo();
        User ryo = new User("ryo");
        User.getInfo();
        User yu = new User("yu");
        User.getInfo();
    }
}

Execution result

$ java Myapp

Class initializer execution
↓
Instantiated 0 times
↓
Instance initializer execution
↓
Constructor execution
↓
Instantiated once
↓
Instance initializer execution
↓
Constructor execution
↓
Instantiated twice
↓

Java initializer story

What is an initializer?

What is the difference between an instance initializer and a constructor?