程序示例：

class Grap{	public Double getArea(){		return 0.0;	}}class Circle extends Grap{	private Integer r;   //半径	Circle(Integer r) {		this.r = r;	}	public Double getArea(){		return 3.14*r*r;	}}class Equals extends Grap{	private Integer x;   //长	private Integer y;   //宽	Equals(Integer x,Integer y) {		this.x = x;		this.y = y;	}	public Double getArea(){		return x*y.doubleValue();	}}public class UnAbstract {	public static void main(String[] args) {				System.out.println(new Circle(10).getArea());		System.out.println(new Equals(10,10).getArea());	}}

程序示例运行结果：

314.0100.0

如何解决上述问题？  抽象！

抽象方法 抽象类

程序示例：

abstract class Grap{	abstract public Double getArea();  //抽象方法的类一定是抽象类}class Circle extends Grap{	private Integer r;   //半径	Circle(Integer r) {		this.r = r;	}		public Double getArea(){		return 3.14*r*r;	}}// 抽象类的继承类为非抽象类时必须重新父类的抽象方法。abstract class Equals extends Grap{	private Integer x;   //长	private Integer y;   //宽	Equals(Integer x,Integer y) {		this.x = x;		this.y = y;	}	}public class UnAbstract {	public static void main(String[] args) {				System.out.println(new Circle(10).getArea());			}}

程序示例运行结果：

314.0