设计模式：策略模式（Strategy Pattern）

/**
 * 策略模式。
 * @author Bright Lee
 */
public class StrategyPattern {

	public static void main(String[] args) {
		int num1 = 1;
		int num2 = 2;
		
		int result = 0;
		Algorithm algorithm = null;
		
		// 传入加法策略，算法变成了加法：
		algorithm = new Algorithm(new AdditionStrategy());
		result = algorithm.calucate(num1, num2);
		System.out.println(result);
		
		// 传入减法策略，算法变成了减法：
		algorithm = new Algorithm(new SubtractionStrategy());
		result = algorithm.calucate(num1, num2);
		System.out.println(result);
	}

}

class Algorithm {
	
	private Strategy strategy;
	
	public Algorithm(Strategy strategy) {
		this.strategy = strategy;
	}
	
	public int calucate(int num1, int num2) {
		return strategy.calucate(num1, num2);
	}
	
}

/**
 * 策略接口。
 */
interface Strategy {
	
	int calucate(int num1, int num2);
	
}

/**
 * 加法策略。
 */
class AdditionStrategy implements Strategy {

	public int calucate(int num1, int num2) {
		return num1 + num2;
	}
	
}

/**
 * 减法策略。
 */
class SubtractionStrategy implements Strategy {

	public int calucate(int num1, int num2) {
		return num1 - num2;
	}
	
}

运行结果：
3
-1