多态和构造函数 [英] Polymorphism and Constructors

问题描述

我是AP Java学生，并且正在为考试做练习.我遇到了这个问题，但我不明白答案:

I am an AP Java Student and I am practicing for my exam. I came across this question and I don't understand the answer:

请考虑以下课程:

public class A
{
  public A() { methodOne(); }

  public void methodOne() { System.out.print("A"); }
}

public class B extends A
{
  public B() { System.out.print("*"); }

  public void methodOne() { System.out.print("B"); }
}

执行以下代码时输出是什么?

What is the output when the following code is executed:

A obj = new B();

正确答案是B *.有人可以告诉我方法调用的顺序吗?

The correct answer is B*. Can someone please explain to me the sequence of method calls?

推荐答案

调用B构造函数. B构造函数的第一条隐式指令是super()(调用超类的默认构造函数).因此，调用了A的构造函数. A的构造函数调用super()，后者调用java.lang.Object构造函数，该构造函数不输出任何内容.然后调用methodOne().由于对象的类型为B，因此将调用B的methodOne版本，并打印B.然后，B构造函数继续执行，并打印*.

The B constructor is called. The first implicit instruction of the B constructor is super() (call the default constructor of super class). So A's constructor is called. A's constructor calls super(), which invokes the java.lang.Object constructor, which doesn't print anything. Then methodOne() is called. Since the object is of type B, the B's version of methodOne is called, and B is printed. Then the B constructor continues executing, and * is printed.

必须注意，从构造函数(如A的构造函数)中调用可重写方法是非常不好的做法:它在尚未构造的对象上调用方法.

It must be noted that calling an overridable method from a constructor (like A's constructor does) is very bad practice: it calls a method on an object which is not constructed yet.

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