都知道抽象類可以沒有抽象方法,這時的抽象類和普通類的最大差別就是不能實例化.一直沒找到在什么環(huán)境下用到這種特殊的抽象類.最近看了jdk的I/O包后,發(fā)現(xiàn)里面有個很好的例子,就是FilterReader類.以下是FilterReader的代碼:

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就是一個沒有抽象方法的抽象類,里面的每個方法都是調(diào)用構(gòu)造函數(shù)傳入的Reader對象的方法.這種抽象類你不能實例化它,因為實例化它沒意義,它還沒實現(xiàn)任何Filter的功能.在extends具體子類時實現(xiàn)Filter功能,實例化相應(yīng)的子類才有實際意義.

我們可以寫FilterReader的一個具體子類如下:

/** *//**
* FilterReader是一個抽象類,不能實例化該類,FilterReader類的每個方法都是實現(xiàn)的,
* 里面調(diào)用的都是FilterReader(Reader in)的傳入?yún)?shù)in的方法.
*/
public class UppercaseConvertor extends FilterReader...{
// 構(gòu)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);
}
}
}