策略模式Strategy

//Purpose.  Strategy design pattern

//1. Define the interface of an interchangeable family of algorithms
//2. Bury algorithm implementation details in derived classes
//3. Derived classes could be implemented using the Template Method pattern
//4. Clients of the algorithm couple themselves strictly to the interface

interface Strategy { public void solve(); }          // 1. Define the interface
                                                  //    of the algorithm
abstract class TemplateMethod1 implements Strategy { // 2. Bury implementation
public void solve() {                             // 3. Template Method 
   start();
   while (nextTry() && ! isSolution())
      ;
   stop();
}
protected abstract void    start();
protected abstract boolean nextTry();
protected abstract boolean isSolution();
protected abstract void    stop();
}

class Impl1 extends TemplateMethod1 {
private int state = 1;
protected void    start()   { System.out.print( "start  " ); }
protected void    stop()    { System.out.println( "stop" ); }
protected boolean nextTry() {
   System.out.print( "nextTry-" + state++ + "  " );
   return true; }
protected boolean isSolution() {
   System.out.print( "isSolution-" + (state == 3) + "  " );
   return (state == 3);
}  }

abstract class TemplateMethod2 implements Strategy { // 2. Bury implementation
public void solve() {                             // 3. Template Method
   while (true) {
      preProcess();
      if (search()) break;
      postProcess();
}  }
protected abstract void    preProcess();
protected abstract boolean search();
protected abstract void    postProcess();
}

class Impl2 extends TemplateMethod2 {
private int state = 1;
protected void    preProcess()  { System.out.print( "preProcess  " ); }
protected void    postProcess() { System.out.print( "postProcess  " ); }
protected boolean search() {
   System.out.print( "search-" + state++ + "  " );
   return state == 3 ? true : false;
}  }

public class StrategyDemo {    // 4. Clients couple strictly to the interface
	   public static void clientCode( Strategy strat ) { strat.solve(); }
	   public static void main( String[] args ) {
	      Strategy[] algorithms = { new Impl1(), new Impl2() };
	      for (int i=0; i < algorithms.length; i++)
	         clientCode( algorithms[i] );
	}  }

	// start  nextTry-1  isSolution-false  nextTry-2  isSolution-true  stop
	// preProcess  search-1  postProcess  preProcess  search-2