解决的问题

开发一个保存图片的功能，保存图片前，需要先对图片进行压缩、添加滤镜等处理。

代码

Compressor:

package com.cong.designpattern.strategy;

public interface Compressor {
    public void compress(String fileName);
}

JpegCompressor:

package com.cong.designpattern.strategy;

public class JpegCompressor implements Compressor{
    @Override
    public void compress(String fileName) {
        System.out.println("Compress to Jpeg");
    }
}

PngCompressor:

package com.cong.designpattern.strategy;

public class PngCompressor implements Compressor{
    @Override
    public void compress(String fileName) {
        System.out.println("Compress to PNG");
    }
}

Filter:

package com.cong.designpattern.strategy;

public interface Filter {
    public void filter(String fileName);
}

BlackAndWhiteFilter:

package com.cong.designpattern.strategy;

public class BlackAndWhiteFilter implements Filter{
    @Override
    public void filter(String fileName) {
        System.out.println("Filter with black and white");
    }
}

ImageStore:

package com.cong.designpattern.strategy;

public class ImageStore {
    public ImageStore(Compressor compressor, Filter filter) {
        this.compressor = compressor;
        this.filter = filter;
    }

    private Compressor compressor;
    private Filter filter;

    public void store(String fileName) {
        compressor.compress(fileName);
        filter.filter(fileName);
    }
}

Test code:

ImageStore imageStore = new ImageStore(new PngCompressor(), new BlackAndWhiteFilter());
imageStore.store("a");

UML

Strategy Pattern UML

好处

满足单一职责原则。ImageStore应该只有保存图片的职责，通过组合的方式把压缩图片、添加滤镜等操作封装到其他相应的类。
Strategy Pattern和State Pattern非常像，原理是一样的。可以理解为State Pattern是只有一种Strategy的Strategy Pattern。