抽象类，并且有3 个抽象方法

abstract class Math{
 public abstract void f(int x);
 public abstract void g(int x,int y);
 public abstract double h(double x);
}
class  Cheng extends Math{
 int x=10;
 int y=10;
 public void f(int x){
  System.out.println("我爱乘法");
  System.out.printf("x=%d",x);
  }
  public void g(int x,int y){
   
   
   System.out.println("我不爱乘法");
                 System.out.printf("x=%d,y=%d",x,y);
  }
  public double h(double x){
   
   System.out.printf("x=%f",x);
   System.out.println("你喜欢乘法吗?");
                      return x*x;                
                      }
  }
class Jia extends Math{
 int x=20;
 int y=20;
 public void f(int x){ 
  System.out.println("我爱加法");
                 System.out.printf("x=%d",x);
}
 public void g(int x,int y){
  System.out.println("我不爱加法");
                 System.out.printf("x=%d,y=%d",x,y);
  
 }
 public double h(double x){
  return x+x;
 }
}
class  Jian extends Math{
 int x=30;
 int y=30;
 public void f(int x){
  System.out.println("我爱减法");
                 System.out.printf("x=%d",x);
  }
 public void g(int x,int y){
  System.out.println("我不爱减法");
                 System.out.printf("x=%d,y=%d",x,y);
  }
 public double h(double x){
  return x-x;
 }
 
}
public class Example1_1{
 public static void main(String arg[]){
  Math m;
  m=new Cheng();
  m.f(100);
        m.g(100,100);
  m.h(100.00);
 }
}

过程中学到的另一个问题：局部变量，成员变量的区别。

abstract class Math{
 public abstract void f(int x);
 public abstract void g(int x,int y);
 public abstract double h(double x);
}
class  Cheng extends Math{
 public void f(int x){
  x=0;
  System.out.println("我爱乘法");
  System.out.printf("x=%d",x);
  }
  public void g(int x,int y){
   x=0;
   y=0;
   System.out.println("我不爱乘法");
                 System.out.printf("x=%d,y=%d",x,y);
  }
  public double h(double x){
   x=0.0;
   System.out.printf("x=%f",x);
   System.out.println("你喜欢乘法吗?");
                      return x*x;                
                      }
  }
class Jia extends Math{
 int x=20;
 int y=20;
 public void f(int x){ 
  System.out.println("我爱加法");
                 System.out.printf("x=%d",x);
}
 public void g(int x,int y){
  System.out.println("我不爱加法");
                 System.out.printf("x=%d,y=%d",x,y);
  
 }
 public double h(double x){
  return x+x;
 }
}
class  Jian extends Math{
 int x=30;
 int y=30;
 public void f(int x){
  System.out.println("我爱减法");
                 System.out.printf("x=%d",x);
  }
 public void g(int x,int y){
  System.out.println("我不爱减法");
                 System.out.printf("x=%d,y=%d",x,y);
  }
 public double h(double x){
  return x-x;
 }
 
}
public class Example1_1{
 public static void main(String arg[]){
  Math m;
  m=new Cheng();
  m.f(100);
        m.g(100,100);
  m.h(100.00);
 }
}