解決的問題

為一個類添加更多功能。簡單的情形可以使用繼承。有些復雜的情況使用繼承就比較麻煩了。例如要開發(fā)一個保存數(shù)據(jù)到云端的功能（CloudStream），有些數(shù)據(jù)可以要先加密再保存，有些數(shù)據(jù)可能要先壓縮再保存，有些數(shù)據(jù)可能要壓縮并且加密再保存，諸如此類。這時用Decorator Pattern（裝飾模式）會比較好。

代碼

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