I.Class Initialization

In Java, we know a class can extend a father class. When we create a instance of a class in Java, how this instance has been initialized?

The sequence of initializing a class

  • Initialize member variables first (before constructor)
    • Default: null, 0, false; private int a;
    • Assign a value directly; private int a = 1;
    • Through constructor to assign the value
  • Multiple level initialization (initialize member variables)
    • Initialize father class first
    • Then children classes

II. Example

Example 1

Code:

class Father {
	static {
		System.out.println("Father: static code block");
	}

	{
		System.out.println("Father: construction code block");
	}

	public Father(){
		System.out.println("Father: constructor");
	}
}


class Son extends Father{
	static {
		System.out.println("Son: static code block");
	}

	{
		System.out.println("Son: construction code block");
	}

	public Son(){
		System.out.println("Son: Constructor");
	}
}

class Demo1 {
	public static void main(String[] args) {
		Son s = new Son();
	}
}

Result:

Father: static code block
Son: static code block
Father: construction code block
Father: constructor
Son: construction code block
Son: Constructor

Example2

Code:

package org.lovian.inheritance;

class X {
	Y y = new Y();
	X() {
		System.out.println("X");
	}
}

class Y {
	Y(){
		System.out.println("Y");
	}
}

public class Z extends X{
	Y y = new Y();

	Z() {
		//super();
		System.out.println("Z");
	}

	public static void main(String[] args) {
		new Z();
	}
}

Result:

Y
X
Y
Z

Note here: Even if there is a super() in the constructor of class Z, but the sequence of initialization is not as execute super() first, then print Z. Here is using multiple level initialization

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