当我定义一个抽象类而不是声明时会发生什么.

Question

public class Temp {
public static void main(String args[]) {
    A ref;
    B objB = new B();
    C objC = new C();
    ref = objB;
    ref.show();
    ref = objC;
    ref.show();

}

}

abstract class A {

abstract void show();

 {
    System.out.println("In Class A");
 }
}

class B extends A {

void show() {
    System.out.println("In class B");
   }
 } 
class C extends B {

void show() {
    System.out.println("In Class C");
 }
}

在上面的代码中抽象方法包含了方法的定义.但是在书中说明抽象方法应该只包含该方法的声明而非定义.当我执行此程序时,它会导致以下输出没有任何错误.请解释我如何显示以下输出.

In Class A
In Class A
In class B
In Class C

Answer 1

您的抽象类不包含方法的定义.您的抽象类包含一个抽象方法和一个初始化块.

abstract class A {
    // Abstract method
    abstract void show();

    // Initializer block, completely unrelated to show
    {
        System.out.println("In Class A");
    }
}

初始化程序块在构造对象时运行 - 与构造函数类似(但是您可以有多个,并且它们不能有参数).这跟你写的完全一样:

abstract class A {
    // Abstract method
    abstract void show();

    // Constructor
    public A()
    {
        System.out.println("In Class A");
    }
}

输出来自:

A ref;
B objB = new B(); // calls B's constructor, then A's constructor which prints "In Class A"
C objC = new C(); // calls C's constructor, then A's constructor which prints "In Class A"
ref = objB;
ref.show(); // prints "In Class B"
ref = objC;
ref.show(); // prints "In Class C"