Java构造器与初始化-优快云博客

Thinking in Java(2)

1.构造器初始化

1.1 初始化顺序

无法阻止自动初始化的进行，自动初始化发生在构造器被调用之前。

在类的内部，变量定义先后顺序决定了初始化顺序。即使变量定义散布于方法定义之间，它们也会在任何方法被调用之前初始化。

import java.util.*;

class Window {
    Window(int marker) {
        System.out.println("Windows("+marker+")");
    }
}

class House {
    Window w1 = new Window(1);
    House() {
        System.out.println("House");
        w3 = new Window(33);
    }
    void f() {
        System.out.println("f()");
    }
    Window w2 = new Window(2);
    Window w3 = new Window(3);
}

public class OrderOfInitialization {
    public static void main(String[] args) {
        House h = new House();
        h.f();
    }
}

//result:
//Windows(1)
//Windows(2)
//Windows(3)
//House
//Windows(33)
//f()

可以看出先初始化类内成员变量再调用构造函数初始化。

1.2 静态数据的初始化

初始化顺序是先静态对象再非静态对象,且静态初始化只有在必要(创建对象或者访问静态方法静态域)时刻才会进行。

import java.util.*;

class Bowl {
    Bowl(int i) {
        System.out.println("Bowl("+i+")");
    }
}

public class StaticInitialization {
    Bowl b1 = new Bowl(1);
    static Bowl b2 = new Bowl(2);
    static Bowl b3 = new Bowl(3);
    public static void main(String[] args) {
        new StaticInitialization();
    }
}
//result
//Bowl(2)
//Bowl(3)
//Bowl(1)

1.3 显式静态初始化

将多个静态初始化动作组织成一个特殊的静态子句

import java.util.*;

class Cup {
    Cup(int i) {
        System.out.println("Cup("+i+")");
    }   
}

public class ExplicitStatic {
    Cup cups3 = new Cup(3);
    static Cup cups1;
    static Cup cups2;
    //静态块
    static {
        cups1 = new Cup(1);
        cups2 = new Cup(2);
    }
    public static void main(String[] args) {
        new ExplicitStatic();
    }
}
//result
//Cup(1)
//Cup(2)
//Cup(3)

1.4非静态实例初始化

类似于静态块，非静态实例也可以进行同样的初始化

import java.util.*;

class Cup {
    Cup(int i) {
        System.out.println("Cup("+i+")");
    }   
}

public class Normal {
    Cup c1;
    Cup c2;
    {
        c1 = new Cup(1);
        c2 = new Cup(2);
    }   
    static Cup c3 = new Cup(3);
    public static void main(String[] args) {
        new Normal();
    }
}
//result
//Cup(3)
//Cup(1)
//Cup(2)