策略模式把所研究的对象的算法包装在不同的策略对象里，策略对象改变计算的结果也会发生改变。
    可以减少if else

    1. public interface Math {
    2.     double calculate(double a, double b);
    3. }
    5. public class Add implements Math {
    6.     @Override
    7.     public double calculate(double a, double b) {
    8.         return a + b;
    9.     }
    10. }
    12. public class Multiply implements Math {
    13.     @Override
    14.     public double calculate(double a, double b) {
    15.         return a * b;
    16.     }
    17. }
    19. // 创建context
    20. public class StrategyContext {
    22.     private Math math;
    24.     public void setMath(Math math) {
    25.         this.math = math;
    26.     }
    28.     public double getResult(double a, double b) {
    29.         return math.calculate(a, b);
    30.     }
    31. }
    1. public class Main {
    2.     public static void main(String[] args) {
    3.         StrategyContext strategyContext = new StrategyContext();
    4.         strategyContext.setMath(new Add());
    5.         System.out.println(strategyContext.getResult(1,2));
    7.         strategyContext.setMath(new Multiply());
    8.         System.out.println(strategyContext.getResult(1,2));
    9.     }
    10. }