Java 日看一类（29）之IO包中的InputStreamReader类

该类继承自Reader类
引入了如下包：import java.nio.charset.Charset;
import java.nio.charset.CharsetDecoder;
import sun.nio.cs.StreamDecoder;


类头注释如下/**
* An InputStreamReader is a bridge from byte streams to character streams: It
* reads bytes and decodes them into characters using a specified {@link
* java.nio.charset.Charset charset}. The charset that it uses
* may be specified by name or may be given explicitly, or the platform's
* default charset may be accepted.
*
* <p> Each invocation of one of an InputStreamReader's read() methods may
* cause one or more bytes to be read from the underlying byte-input stream.
* To enable the efficient conversion of bytes to characters, more bytes may
* be read ahead from the underlying stream than are necessary to satisfy the
* current read operation.
*
* <p> For top efficiency, consider wrapping an InputStreamReader within a
* BufferedReader. For example:
*
* <pre>
* BufferedReader in
* = new BufferedReader(new InputStreamReader(System.in));
* </pre>
*
* @see BufferedReader
* @see InputStream
* @see java.nio.charset.Charset
*
* @author Mark Reinhold
* @since JDK1.1
*/大意如下：
该类是从byte流转换到字符流的桥梁
该类读取byte并编码为指定类型（Charset声明）的字符
使用字符集可由名称指定或者显式指定，或者使用平台默认字符集
每一次对该类read方法调用的时候都会从底层流读入一个或多个字节
要有效的将字节转化为字符格式，可能需要提前读入更多的字节来满足最低转化要求
为了达到最高效率，可以用BufferedReader来包装InputStreamReader

该类含有如下的成员变量：
流字节解码工具同时也是底层流private final StreamDecoder sd;

该类含有如下成员方法：
构造函数（使用默认字符集public InputStreamReader(InputStream in) {
super(in);//调用上层构造方法
try {
sd = StreamDecoder.forInputStreamReader(in, this, (String)null); // ## check lock object//调用为该类专门构造的读入操作，使用默认的字符集
} catch (UnsupportedEncodingException e) {//抛出不支持编码异常
// The default encoding should always be available
throw new Error(e);
}
}构造函数（使用指定字符集public InputStreamReader(InputStream in, String charsetName)
throws UnsupportedEncodingException
{
super(in);
if (charsetName == null)
throw new NullPointerException("charsetName");
sd = StreamDecoder.forInputStreamReader(in, this, charsetName);//调用该类专用方法，使用指定字符集名称
}构造函数（使用指定字符集public InputStreamReader(InputStream in, Charset cs) {
super(in);
if (cs == null)
throw new NullPointerException("charset");
sd = StreamDecoder.forInputStreamReader(in, this, cs);//使用指定字符集对象
}构造函数（使用指定字符解码器进行构造（实质还是指定字符集public InputStreamReader(InputStream in, CharsetDecoder dec) {
super(in);
if (dec == null)
throw new NullPointerException("charset decoder");
sd = StreamDecoder.forInputStreamReader(in, this, dec);
}获得编码字符集名public String getEncoding() {
return sd.getEncoding();
}读入一个字符public int read() throws IOException {
return sd.read();
}读入到数组，特定位置和长度public int read(char cbuf[], int offset, int length) throws IOException {
return sd.read(cbuf, offset, length);
}判定当前流是否可以读取数据（缓冲不为空public boolean ready() throws IOException {
return sd.ready();
}关闭当前流public void close() throws IOException {
sd.close();
}

该类实质是一个对StreamDecoder的包装组合。该类基本所有功能都通过调用其底层流完成，本身代码没有什么有效信息，对该类的学习还是需要对StreamDecoder进行学习作为铺垫（也需要Reader类的学习