java成员变量初始化顺序

最新推荐文章于 2022-02-04 12:01:02 发布

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本文深入探讨了Java中类加载过程中初始化顺序的重要性，从静态代码块、成员变量初始化、构造代码块到构造方法内部初始化的详细流程进行解析，并通过实例展示了初始化过程的实现方式。

1.在加载类时初始化静态代码块，且只初始化一次。静态代码块属于类。
2.在new一个对象时，先初始化成员变量区域。
3.按顺序运行构造代码块。
4.运行构造方法内部的代码初始化
（new 对象时，先加载父类）

package com.orderofinitialize;

public class SuperClass {
    private String name;
    private int x=3;
    private int y;
    public static String nameS;
    public SuperClass() {

    }
    public SuperClass(String name,int y) {
        this.name=name;
        this.y=y;
    }
    public String getName() {
        return name;
    }
    public int getX() {
        return x;
    }
    public int getY() {
        return y;
    }
    {
        y=4;
    }
    static {
        nameS="nameS";
        System.out.println("Superclass static code block");
    }
}



package com.orderofinitialize;

public class SubClass extends SuperClass{
    public SubClass() {
        super();
    }

    {
        System.out.println("subclass construct code block ");
    }
    static {
        System.out.println("subclass static code block");
    }

}



package com.orderofinitialize;

public class Test {
    public static void main(String[] args) {
         System.out.println("/* new subclass */");
         SubClass sb=new SubClass();
         sb.getClass();
         System.out.println("------static code block initialize only once \n");

        System.out.println("/* firstly,static code initialize  while class loading*/");
        System.out.println(SuperClass.nameS);

        SuperClass sc=new SuperClass();

        System.out.println("/* secondly,member variable initialize */");
        System.out.println(sc.getName());
        System.out.println(sc.getX());

        System.out.println("/* thirdly,construct code block initialize */");
        System.out.println(sc.getY());

        SuperClass sc1=new SuperClass("sc1",5);
        System.out.println("/* fourthly,  constructor code  */");
        System.out.println(sc1.getName());
        System.out.println(sc1.getX());
        System.out.println(sc1.getY());




    }
}

output:
/* new subclass */
Superclass static code block
subclass static code block
subclass construct code block 
------static code block initialize only once 

/* firstly,static code initialize  while class loading*/
nameS
/* secondly,member variable initialize */
null
3
/* thirdly,construct code block initialize */
4
/* fourthly,  constructor code  */
sc1
3
5