策略模式指一个类的行为可以在运行时更改，这种类型的设计模式属于行为型模式。

一、需求举例：

项目中需求开发应用场景：
1.简单的项目开发由初级工程师完成；
2.中等难的项目开发由中级工程师完成；
3.高难度的项目开发由高级工程师完成；
4.难度过高的项目默认拒绝。

二、类图

三、代码实战

1.接口：Engineer.java

package com.lance.strategy;

public interface Engineer {


    default void develop(String program) {

        System.out.println("Engineer default refuse " + program);
    }
}

2.初级工程师类：JuniorEngineer.java

package com.lance.strategy;

import com.lance.strategy.Engineer;

public class JuniorEngineer implements Engineer {

    @Override
    public void develop(String program) {
        System.out.println("Junior Engineer finish " + program);

    }

}

3.中级工程师类：IntermediateEngineer.java

package com.lance.strategy;

import com.lance.strategy.Engineer;

public class IntermediateEngineer implements Engineer {

    @Override
    public void develop(String program) {
        System.out.println("Intermediate Engineer finish " + program);
    }
}

4.高级工程师类：SeniorEngineer.java

package com.lance.strategy;

import com.lance.strategy.Engineer;

public class SeniorEngineer implements Engineer {

    @Override
    public void develop(String program) {
        System.out.println("Senior Engineer finish " + program);

    }
}

5.工程师上下文类：EngineerContext.java

package com.lance.strategy;

public class EngineerContext {
    private Engineer engineer;

    public EngineerContext(Engineer engineer) {
        this.engineer = engineer;
    }

    public void develop(String program) {
        engineer.develop(program);
    }


}

6.主类：StrategyPattern.java

package com.lance.strategy;

public class StrategyPattern {

    public static void main(String[] args) {


        System.out.println("==========start==========");
        EngineerContext context = new EngineerContext(new JuniorEngineer());
        context.develop("easy program");

        System.out.println("=======================");

        context = new EngineerContext(new IntermediateEngineer());
        context.develop("middle program");

        System.out.println("=======================");

        context = new EngineerContext(new SeniorEngineer());
        context.develop("difficult program");

        System.out.println("=======================");

        context = new EngineerContext(new Engineer() {
        });
        context.develop("too difficult program");

        System.out.println("==========end==========");


    }
}

输出结果：

==========start==========
Junior Engineer finish easy program
=======================
Intermediate Engineer finish middle program
=======================
Senior Engineer finish difficult program
=======================
Engineer default refuse too difficult program
==========end==========