java - 使用构造函数创建新对象

Question

以下代码在编译和运行时给出的输出为“alpha subsub”。 SubSubAlpha(); 构造函数应将“subsub”添加到变量 s 中，这应该是输出。

为什么输出是“alpha subsub”？

class Alpha {
    static String s = " ";

    protected Alpha() {
       s += "alpha "; 
    }
}

public class SubSubAlpha extends Alpha {
    private SubSubAlpha() { 
       s += "subsub "; 
    }

    public static void main(String[] args) {
        new SubSubAlpha();
        System.out.println(s);
        // This prints as " alpha subsub".
        //Shouldn't this one be printed as " subsub"
        //Who made the call to Alpha(); ?
    }
}

Answer 1

当存在类层次结构时，子类的构造函数(在本例中为 SubSubAlpha)总是调用(首先)父类(super class)的构造函数，在本例中为 Alpha .

所以实际上发生的是:

private SubSubAlpha() { 
    super();
    s += "subsub ";
}

因此，这使得:

s += "alpha ";
s += "subsub ";

制作字符串“alpha subsub”