11.22总结

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

package test;
public class Adder {
private int num=0;
public int add(int value) {
num += value;
return num;
}
}

package test;

public class CalculatorForm {
private AbstractCommand command;

public void setCommand(AbstractCommand command) {    
    this.command = command;    
}    
    
public void compute(int value) {    
    int i = command.execute(value);    
    System.out.println("执行结果为" + i);    
}    

public void undo() {    
    int i = command.undo();    
    System.out.println("撤销：" + i);    
}    

}

package test;

public class Client {
public static void main(String args[]) {
CalculatorForm form = new CalculatorForm();
AbstractCommand command;
command = new ConcreteCommand();
form.setCommand(command);

    form.compute(3);    
    form.compute(4);    
    form.compute(5);    
    form.undo();  
    form.undo();
    form.undo();
}    

}

package test;
class ConcreteCommand extends AbstractCommand {
private Adder adder = new Adder();
private int value;
public int execute(int value) {
this.value=value;
return adder.add(value);
}
public int undo() {
return adder.add(-value);
}
}