设计模式之组织行为(一)

分类: DESIGN PATTERN 发布于:

如果对每一个类型为 T1的对象 o1，都有类型为 T2 的对象o2，使得以 T1定义的所有程序 P 在所有的对象 o1 都代换成 o2 时，程序 P 的行为没有发生变化，那么类型 T2 是类型 T1 的子类型。

画图不易，请勿转载。

对象行为组织类

Chain of repository(责任链)

import java.util.*;

abstract class Logger 
{
    public static int ERR = 3;
    public static int NOTICE = 5;
    public static int DEBUG = 7;
    protected int mask;

    // The next element in the chain of responsibility
    protected Logger next;
    public Logger setNext( Logger l)
    {
        next = l;
        return this;
    }

    public final void message( String msg, int priority )
    {
        if ( priority <= mask ) 
        {
            writeMessage( msg );
            if ( next != null )
            {
                next.message( msg, priority );
            }
        }
    }
    
    protected abstract void writeMessage( String msg );

}

class StdoutLogger extends Logger 
{

    public StdoutLogger( int mask ) { this.mask = mask; }

    protected void writeMessage( String msg )
    {
        System.out.println( "Writting to stdout: " + msg );
    }
}


class EmailLogger extends Logger 
{

    public EmailLogger( int mask ) { this.mask = mask; }

    protected void writeMessage( String msg )
    {
        System.out.println( "Sending via email: " + msg );
    }
}

class StderrLogger extends Logger 
{

    public StderrLogger( int mask ) { this.mask = mask; }

    protected void writeMessage( String msg )
    {
        System.out.println( "Sending to stderr: " + msg );
    }
}

public class ChainOfResponsibilityExample
{
    public static void main( String[] args )
    {
        // Build the chain of responsibility
        Logger l = new StdoutLogger( Logger.DEBUG).setNext(
                            new EmailLogger( Logger.NOTICE ).setNext(
                            new StderrLogger( Logger.ERR ) ) );

        // Handled by StdoutLogger
        l.message( "Entering function y.", Logger.DEBUG );

        // Handled by StdoutLogger and EmailLogger
        l.message( "Step1 completed.", Logger.NOTICE );

        // Handled by all three loggers
        l.message( "An error has occurred.", Logger.ERR );
    }
}

命令模式(command)

命令模式是比较深刻的设计模式。它抽象了方法«调用过程»,把方法请求和受到影响的类之间的关联解耦。

与这种模式相反的情形就是一个’big function‘，各种情况放在一个函数当中。

源码示例

import java.util.List;
import java.util.ArrayList;

/* The Command interface */
public interface Command {
   void execute();
}

/* The Invoker class */
public class Switch {
   private List<Command> history = new ArrayList<Command>();

   public Switch() {
   }

   public void storeAndExecute(Command cmd) {
      this.history.add(cmd); // optional 
      cmd.execute();        
   }
}

/* The Receiver class */
public class Light {
   public Light() {
   }

   public void turnOn() {
      System.out.println("The light is on");
   }

   public void turnOff() {
      System.out.println("The light is off");
   }
}

/* The Command for turning on the light - ConcreteCommand #1 */
public class FlipUpCommand implements Command {
   private Light theLight;

   public FlipUpCommand(Light light) {
      this.theLight = light;
   }

   public void execute(){
      theLight.turnOn();
   }
}

/* The Command for turning off the light - ConcreteCommand #2 */
public class FlipDownCommand implements Command {
   private Light theLight;

   public FlipDownCommand(Light light) {
      this.theLight = light;
   }

   public void execute() {
      theLight.turnOff();
   }
}

/* The test class or client */
public class PressSwitch {
   public static void main(String[] args){
      Light lamp = new Light();
      Command switchUp = new FlipUpCommand(lamp);
      Command switchDown = new FlipDownCommand(lamp);

      Switch mySwitch = new Switch();

      try {
         if ("ON".equalsIgnoreCase(args[0])) {
            mySwitch.storeAndExecute(switchUp);
         }
         else if ("OFF".equalsIgnoreCase(args[0])) {
            mySwitch.storeAndExecute(switchDown);
         }
         else {
            System.out.println("Argument \"ON\" or \"OFF\" is required.");
         }
      } catch (Exception e) {
         System.out.println("Arguments required.");
      }
   }
}

解释器(Interpreter)

源码示例

import java.util.Map;

interface Expression {
    public int interpret(final Map<String, Expression> variables);
}

class Number implements Expression {
    private int number;
    public Number(final int number)       { this.number = number; }
    public int interpret(final Map<String, Expression> variables)  { return number; }
}

class Plus implements Expression {
    Expression leftOperand;
    Expression rightOperand;
    public Plus(final Expression left, final Expression right) {
        leftOperand = left;
        rightOperand = right;
    }
		
    public int interpret(final Map<String, Expression> variables) {
        return leftOperand.interpret(variables) + rightOperand.interpret(variables);
    }
}

class Minus implements Expression {
    Expression leftOperand;
    Expression rightOperand;
    public Minus(final Expression left, final Expression right) {
        leftOperand = left;
        rightOperand = right;
    }
		
    public int interpret(final Map<String, Expression> variables) {
        return leftOperand.interpret(variables) - rightOperand.interpret(variables);
    }
}

class Variable implements Expression {
    private String name;
    public Variable(final String name)       { this.name = name; }
    public int interpret(final Map<String, Expression> variables) {
        if (null == variables.get(name)) return 0; // Either return new Number(0).
        return variables.get(name).interpret(variables);
    }
}

//
import java.util.Map;
import java.util.Stack;

class Evaluator implements Expression {
    private Expression syntaxTree;

    public Evaluator(final String expression) {
        final Stack<Expression> expressionStack = new Stack<Expression>();
        for (final String token : expression.split(" ")) {
            if (token.equals("+")) {
                final Expression subExpression = new Plus(expressionStack.pop(), expressionStack.pop());
                expressionStack.push(subExpression);
            } else if (token.equals("-")) {
                // it's necessary to remove first the right operand from the stack
                final Expression right = expressionStack.pop();
                // ..and then the left one
                final Expression left = expressionStack.pop();
                final Expression subExpression = new Minus(left, right);
                expressionStack.push(subExpression);
            } else
                expressionStack.push(new Variable(token));
        }
        syntaxTree = expressionStack.pop();
    }

    public int interpret(final Map<String, Expression> context) {
        return syntaxTree.interpret(context);
    }
}
//

import java.util.Map;
import java.util.HashMap;

public class InterpreterExample {
    public static void main(final String[] args) {
        final String expression = "w x z - +";
        final Evaluator sentence = new Evaluator(expression);
        final Map<String, Expression> variables = new HashMap<String, Expression>();
        variables.put("w", new Number(5));
        variables.put("x", new Number(10));
        variables.put("z", new Number(42));
        final int result = sentence.interpret(variables);
        System.out.println(result);
    }
}