Filter模式

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继承的问题

如果新功能的实现依赖于继承（每个新功能对应一个子类）会出现子类数目过多的情况

栗子

• 给FileInputStream添加解压、签名、加解密

问题的解决

分类是不可或缺的

• InputStream

  • FileInputStream
  • ByteArrayInputStream
  • ServletInputStream
  • BufferedInputStream
  • DigestInputStream
  • CipherInputStream

先new基础类，再以基础类为参数new额外功能类，但统筹调度统一用InputStream

编写FilterInputStream

package eternal.fire;

import java.io.FilterInputStream;
import java.io.IOException;
import java.io.InputStream;

public class CountInputStream extends FilterInputStream {
    private int count = 0;

    /**
     * Creates a {@code FilterInputStream}
     * by assigning the  argument {@code in}
     * to the field {@code this.in} so as
     * to remember it for later use.
     *
     * @param in the underlying input stream, or {@code null} if
     *           this instance is to be created without an underlying stream.
     */
    public CountInputStream(InputStream in) {
        super(in);
    }
    
    public int getBytesRead() {
        return this.count;
    }
    
    @Override
    public int read() throws IOException {
        int n = in.read();
        if (n != -1) {
            this.count++;
        }
        return n;
    }
    
    @Override
    public int read(byte[] b, int off, int len) throws IOException {
        int n = in.read(b, off, len);
        this.count++;
        return n;
    }

}