高飞实验16和17

package shiyan16;

public abstract class AbstractCommand {    
      public abstract int execute(int value);     
      public abstract int undo();    
    }    

package shiyan16;

 
public class Adder {    
private int num=0;   
    

public int add(int value) {    
    num += value;    
    return num;    
}    
}