Java基础知识之StringReader流的使用

    //流对象
    private String str;
    
    //流的长度
    private int length;
    
    //流的当前位置，下个字符的索引
    private int next = 0;
    
    //流的标记位置
    private int mark = 0;

    /**
     * 利用字符串创建字符输入流
     */
    public StringReader(String s) {
        this.str = s;
        this.length = s.length();
    }

    /**
     * 从流中读取单个字符，若到文件末尾则返回-1
     */
    public int read() throws IOException {
        synchronized (lock) {
            ensureOpen();
            if (next >= length)
                return -1;
            return str.charAt(next++);
        }
    }

 /**
     * 读取最多len个字节到目标数组中，从目标数组的下标off开始存储，返回实际读取的字节数
     * @exception  IOException  If an I/O error occurs
     */
    public int read(char cbuf[], int off, int len) throws IOException {
        synchronized (lock) {
            ensureOpen();
            if ((off < 0) || (off > cbuf.length) || (len < 0) ||
                ((off + len) > cbuf.length) || ((off + len) < 0)) {
                throw new IndexOutOfBoundsException();
            } else if (len == 0) {
                return 0;
            }
            if (next >= length)
                return -1;
            int n = Math.min(length - next, len);
            str.getChars(next, next + n, cbuf, off);
            next += n;
            return n;
        }
    }

 
    /**
     * 关闭流无效，关闭后调用该类其它方法会报异常
     */
    public void close() {
        str = null;
    }

    private String str;
    private int length;
    private int next = 0;
    private int mark = 0;
    
    public StringReader(String s) {      //传入字符串
        this.str = s;
        this.length = s.length();
    }
    
    str    :指向这个字符串
    length :为字符串长度
    next   :为读取元素的下标索引
    mark   :为标记点
    

    //读取一个字符
    public int read() throws IOException {
        synchronized (lock) {
            ensureOpen();
            if (next >= length)
                return -1;
            return str.charAt(next++);
        }
    }

//批量读取
 public int read(char cbuf[], int off, int len) throws IOException {
        synchronized (lock) {
            ensureOpen();
            if ((off < 0) || (off > cbuf.length) || (len < 0) ||
                ((off + len) > cbuf.length) || ((off + len) < 0)) {
                throw new IndexOutOfBoundsException();
            } else if (len == 0) {
                return 0;
            }
            if (next >= length)
                return -1;
            int n = Math.min(length - next, len);
            str.getChars(next, next + n, cbuf, off);
            next += n;
            return n;
        }
    }

	//判断是否支持标记
    public boolean markSupported() {
        return true;
    }

 //标记
    public void mark(int readAheadLimit) throws IOException {
        if (readAheadLimit < 0){
            throw new IllegalArgumentException("Read-ahead limit < 0");
        }
        synchronized (lock) {
            ensureOpen();
            mark = next;
        }
    }

 //重置指针
 public void reset() throws IOException {
      synchronized (lock) {
          ensureOpen();
          next = mark;
      }
  }

//跳过流中指定数量的字符  返回跳过的字符数
    public long skip(long ns) throws IOException {
        synchronized (lock) {
            ensureOpen();
            if (next >= length)
                return 0;
            // Bound skip by beginning and end of the source
            long n = Math.min(length - next, ns);
            n = Math.max(-next, n);
            next += n;
            return n;
        }
    }

//访问字符串 StringReader
    import java.io.IOException;
    import java.io.StringReader;
    
public class StringReader_work {
    public static void StringReader() {
        String str = "今天外面真的冷啊果然应该点外的";
        try {
            //1. 构造方法  传入字符串
            StringReader stringReader = new StringReader(str);
            //2. int read() 读操作，读取一个字符   （将String字符串操作 适配成 Reader字符操作 对外提供服务）
//            int read=stringReader.read();
//            System.out.println((char) read);
            //int read(char cbuf[]) throws IOException   批量读
            char[] chars = new char[3];
            int read1=stringReader.read(chars);
            System.out.println(new String(chars,0,3));
            //int read(char cbuf[], int off, int len)   批量读
//            char[] chars1 = new char[3];
//            int read2=stringReader.read(chars1,0,3);
//            System.out.println(new String(chars1,0,3));
            // boolean ready()  判断数据源是否存在
            stringReader.ready();
            //3. markSupported()  是否支持标记    FileInputStream和FileReader 都不支持
            System.out.println("是否支持标记："+stringReader.markSupported());
            // void mark(int readAheadLimit) throws IOException  标记
   //TODO: 测试发现并未从标记位置读，依旧是接着原先next所指位置读数据
            stringReader.mark(5);
            char[] chars2=new char[3];
            int read3 = stringReader.read(chars2,0,3);
            System.out.println("mark标记后读三个数："+new String(chars2,0,3));
            //TODO：mark使用并不相当于指针 --> 和RandomAccessFile的seek操作不是一个性质
            // void reset() throws IOException   重置指针
            // 单独使用 reset() 方法
            stringReader.reset();
            char[] chars3=new char[3];
            int read4 = stringReader.read(chars3,0,3);
            System.out.println("单独使用reset()方法:"+new String(chars3,0,3));
            // mark()方法 与 reset()方法 结合使用
            stringReader.mark(2);
            stringReader.reset();
            char[] chars4=new char[3];
            int read5 = stringReader.read(chars4,0,3);
            System.out.println("mark()与reset()结合使用:"+new String(chars4,0,3));
          
            // long skip(long ns) throws IOException   跳过流中指定数量的字符 返回跳过的字符数
            stringReader.skip(2);     //正数往前跳
//            stringReader.skip(-2);    //负数往后跳
            char[] chars5=new char[3];
            int read6 = stringReader.read(chars5,0,3);
            System.out.println("向前跳两个字符："+new String(chars5,0,3));
            //关闭流
            stringReader.close();
        } catch (IOException e) {
            e.printStackTrace();
        }
    }
    
    public static void main(String[] args) {
        StringReader();
    }
}