没有抽象方法的抽象类，存在什么样的意义呢？

都知道抽象类可以没有抽象方法,这时的抽象类和普通类的最大差别就是不能实例化.一直没找到在什么环境下用到这种特殊的抽象类.最近看了jdk的I/O包后,发现里面有个很好的例子,就是FilterReader类.以下是FilterReader的代码:

public abstract class FilterReader extends Reader ...{    /** *//**     * The underlying character-input stream.     */    protected Reader in;    /** *//**     * Create a new filtered reader.     *     * @param in  a Reader object providing the underlying stream.     * @throws NullPointerException if <code>in</code> is <code>null</code>     */    protected FilterReader(Reader in) ...{    super(in);    this.in = in;    }    /** *//**     * Read a single character.     *     * @exception  IOException  If an I/O error occurs     */    public int read() throws IOException ...{    return in.read();    }    /** *//**     * Read characters into a portion of an array.     *     * @exception  IOException  If an I/O error occurs     */    public int read(char cbuf[], int off, int len) throws IOException ...{    return in.read(cbuf, off, len);    }    /** *//**     * Skip characters.     *     * @exception  IOException  If an I/O error occurs     */    public long skip(long n) throws IOException ...{    return in.skip(n);    }    /** *//**     * Tell whether this stream is ready to be read.     *     * @exception  IOException  If an I/O error occurs     */    public boolean ready() throws IOException ...{    return in.ready();    }    /** *//**     * Tell whether this stream supports the mark() operation.     */    public boolean markSupported() ...{    return in.markSupported();    }    /** *//**     * Mark the present position in the stream.     *     * @exception  IOException  If an I/O error occurs     */    public void mark(int readAheadLimit) throws IOException ...{    in.mark(readAheadLimit);    }    /** *//**     * Reset the stream.     *     * @exception  IOException  If an I/O error occurs     */    public void reset() throws IOException ...{    in.reset();    }    /** *//**     * Close the stream.     *     * @exception  IOException  If an I/O error occurs     */    public void close() throws IOException ...{    in.close();    }}

可以看到FilterReader就是一个没有抽象方法的抽象类,里面的每个方法都是调用构造函数传入的Reader对象的方法.这种抽象类你不能实例化它,因为实例化它没意义,它还没实现任何Filter的功能.在extends具体子类时实现Filter功能,实例化相应的子类才有实际意义.

我们可以写FilterReader的一个具体子类如下:

 

/** * FilterReader是一个抽象类,不能实例化该类,FilterReader类的每个方法都是实现的, * 里面调用的都是FilterReader(Reader in)的传入参数in的方法.     */public class UppercaseConvertor extends FilterReader...{    // 构造方法由FilterReader的protected上升到public的    public UppercaseConvertor(Reader in) ...{        super(in);    }        @Override    public int read() throws IOException...{        int c = super.read();        return (-1==c?c:Character.toUpperCase((char)c));    }        @Override    public int read(char[] buf,int offset, int count) throws IOException...{        int nread = super.read(buf, offset, count);        int last = offset + nread;        for(int i=0;i<last;i++)            buf[i] = Character.toUpperCase(buf[i]);        return nread;    }    public static void main(String[] args) throws IOException ...{        UppercaseConvertor uc = new UppercaseConvertor(new FileReader("d:\log.txt"));        BufferedReader br = new BufferedReader(uc);        String r;        while(null!=(r=br.readLine()))...{            System.out.println(r);        }    }}