解决的问题

为一个类添加更多功能。简单的情形可以使用继承。有些复杂的情况使用继承就比较麻烦了。例如要开发一个保存数据到云端的功能（CloudStream），有些数据可以要先加密再保存，有些数据可能要先压缩再保存，有些数据可能要压缩并且加密再保存，诸如此类。这时用Decorator Pattern（装饰模式）会比较好。

代码

Stream:

package com.cong.designpattern.decorator;

public interface Stream {
    public void write(String data);
}

CloudStream:

package com.cong.designpattern.decorator;

public class CloudStream  implements Stream{
    @Override
    public void write(String data) {
        System.out.println("Write " + data);
    }
}

EncryptedStream:

package com.cong.designpattern.decorator;

public class EncryptedStream implements Stream{
    private Stream stream;
    public EncryptedStream(Stream stream) {
        this.stream = stream;
    }
    @Override
    public void write(String data) {
        String encrypted = data + "[encrypted]";
        this.stream.write(encrypted);
    }
}

CompressedStream:

package com.cong.designpattern.decorator;

public class CompressedStream implements Stream{
    private Stream stream;
    public CompressedStream(Stream stream) {
        this.stream = stream;
    }
    @Override
    public void write(String data) {
        String compressed = data + "[compressed]";
        this.stream.write(compressed);
    }
}

Test code:

CloudStream cloudStream = new CloudStream();
EncryptedStream encryptedStream = new EncryptedStream(cloudStream);
CompressedStream compressedStream = new CompressedStream(encryptedStream);
compressedStream.write("1234567890");

UML

Decorator Pattern UML