java - java程序中的抽象

Question

为什么输出如下？

bike is created

running safely..

gear changed

因为我们没有在任何地方调用 Bike() 方法。

abstract class Bike {
    Bike() {
        System.out.println("bike is created");
    }

    abstract void run();

    void changeGear() {
        System.out.println("gear changed");
    }
}

//Creating a Child class which inherits Abstract class  
class Honda extends Bike {
    void run() {
        System.out.println("running safely..");
    }
}

//Creating a Test class which calls abstract and non-abstract methods  
class TestAbstraction2 {
    public static void main(String args[]) {
        Bike obj = new Honda();
        obj.run();
        obj.changeGear();
    }
}

Answer 1

本田类是用 Default Constructor 创建的

If a class contains no constructor declarations, then a default constructor with no formal parameters and no throws clause is implicitly declared.

 public class Point {
      int x, y;
 }

is equivalent to the declaration:

public class Point {
      int x, y;
      public Point() { super(); }
 }

因此每次调用 new Honda();

时都会调用 Bike()