Java设计模式－－装饰模式

定义

• 装饰者模式动态的将功能加到对象上，若要扩展功能，装饰者提供了比继承更有弹性的替代方案。

几个要点

• 装饰者和被装饰者有相同的父类。
• 你可以用一个或多个装饰者装饰一个对象。
• 既然装饰者和被装饰者有相同的父类，所以在任何需要原始对象（被包装对象）的场合，可以用装饰过的对象代替它。
• 装饰者可以在所委托被装饰者的行为之前与／或之后，加上自己的行为，以达到特定的目的
• 对象可以在任何时候被装饰，所以可以在运行时动态地、不限量地用你喜欢的装饰者装饰对象

看看Java I/O中的InputStream怎么用的装饰者模式(io包中应用大量的装饰者模式，以InputStream为例)

InputStream

/**
 * This abstract class is the superclass of all classes representing
 * an input stream of bytes.
 *
 * 
 Applications that need to define a subclass of InputStream
 * must always provide a method that returns the next byte of input.
public abstract class InputStream implements Closeable {
        ...
}

InputStream 是所有字节输入流的父类，继承InputStream的子类都要实现返回下一个输入字节的方法（read()）.

FileInputStream

/**
 * A FileInputStream obtains input bytes
 * from a file in a file system. What files
 * are  available depends on the host environment.
 *
 * 
FileInputStream is meant for reading streams of raw bytes
 * such as image data. For reading streams of characters, consider using
 * FileReader.
public class FileInputStream extends InputStream {
    ...
}

FileInputStream主要功能是从文件中读入字节

BufferedInputStream

/**
 * A BufferedInputStream adds
 * functionality to another input stream-namely,
 * the ability to buffer the input and to
 * support the mark and reset
 * methods. When  the BufferedInputStream
 * is created, an internal buffer array is
 * created. As bytes  from the stream are read
 * or skipped, the internal buffer is refilled
 * as necessary  from the contained input stream,
 * many bytes at a time. The mark
 * operation  remembers a point in the input
 * stream and the reset operation
 * causes all the  bytes read since the most
 * recent mark operation to be
 * reread before new bytes are  taken from
 * the contained input stream.
 */
public
class BufferedInputStream extends FilterInputStream {
    ...
}

BufferedInputStream给InputStream增加一个功能：利用缓存高效读入字节，支持标记和重置。

• FileInputStream是InputStream的子类，继承之后针对文件读取添加功能，BufferedInputStream是FilterInputStream的子类，FilterInputStream继承了InputStream，所以BufferedInputStream同样是InputStream的子类，构造方法中需要传入一个InputStream。所以如何高效的读入文件就是这样了

public class IOTest {
    public static void main(String[] args) {
        File file = new File("test.txt");
        try {
            BufferedInputStream bi = new BufferedInputStream(new FileInputStream(file));
        } catch (FileNotFoundException e) {
            e.printStackTrace();
        }
    }

}

• 创建出来的FileInputStream作为被包装者传给BufferedInputStream，FileInputStream只负责读入文件，当想要提高效率的时候并没有在FileInputStream内部添加缓冲的方法，而是将起作为被装饰者传给BufferedInputStream，BufferedInputStream负责添加高效读入的功能，实现高效率读入文件.单一原则、开闭原则（对扩展开放，对修改关闭）体现的淋漓尽致！

结束语

仔细品味装饰者模式的要点，结合Java的IO操作就可以很好的理解装饰者模式了，最后附上一篇结合装饰者模式 优雅的为RecyclerView添加Header以及Footer的博文，加深理解。