我们来看看下面的
Java代码片段.
interface Sum
{
abstract public void showSum();
}
interface Mul
{
abstract public void showMul();
}
abstract class Super implements Sum
{
protected int x;
protected int y;
public Super(int x,int y)
{
this.x=x;
this.y=y;
}
//No error,though the method showSum() of the implementing iterface Sum is commented. Why?
/*public void showSum()
{
System.out.println("Sum = "+(x+y));
}*/
}
final class Calculation extends Super implements Mul
{
public Calculation(int x,int y)
{
super(x,y);
}
public void showSum()
{
System.out.println("Summation = "+(x+y));
}
//If showMul() is commented,it would issue a compile-time error. Why?
public void showMul()
{
System.out.println("Multiplication = "+(x*y));
}
}
final public class Main
{
public static void main(String[] args) throws IOException
{
Scanner s=new Scanner(system.in);
System.out.print("\nEnter a number : ");
int x=s.nextInt();
System.out.print("\nEnter another number : ");
int y=s.nextInt();
Calculation c=new Calculation(x,y);
c.showSum();
c.showMul();
}
}
由于接口Sum只包含一个方法showSum();正在由抽象类Super实现,应该有必要对抽象类Super来实现该方法showSum();.编译器根本就没有抱怨,程序运行良好,根本没有问题.为什么？
类似地,非抽象最终类Calculate正在实现Mul接口,并包含方法showMul()的实际实现;在其实现界面中呈现.在这种情况下,如果该方法在类Calculate中显示Mul(),则会发出编译时错误.为什么同样的事情不适用于那个抽象类Super？
解决方法
抽象类不是真正的实现类.它可能包含抽象方法,并且不需要从接口实现方法. REAL实现类的关注是定义抽象/接口方法.
看到抽象类和接口之间的区别
Interface:
public interface InterfaceClass {
void interfaceMethod();
//no method deFinition
}
//Abstract Class implementing InterfaceClass
abstract class AbsClass implements InterfaceClass{
abstract void abstractMethod();
public void doSomethingCommon() {
System.out.println("Abstract class may contain method deFinition");
}
//no need to implement methods of InterfaceClass because AbsClass is abstract
}
而这里是扩展AbsClass的真正类：它的RealClass的义务来定义所有的抽象方法和接口方法.另外,它也可以覆盖抽象类中定义的方法.
public class RealClass extends AbsClass{
@Override
public void interfaceMethod() {
//implement interface method here
}
@Override
void abstractMethod() {
}
// you may override the doSomethingCommon() of AbsClass too
@Override
public void doSomethingCommon() {
// Todo Auto-generated method stub
super.doSomethingCommon();
}
}
为什么AbsClass上没有编译时错误：我们不能创建抽象类的实例.这就是为什么在编译时没有显示错误的意义.