Java 日看一类（55）之IO包中的StringReader类

该类的无引入包

继承自Reader类

该类的类头注释如下：

* A character stream whose source is a string.

大意如下：

读取对象是字符串的字符读入流

该类含有如下的成员变量：

需要读取的字符串

private String str;

字符串长度

private int length;

读取位置偏移量

private int next = 0;

标记回溯点

private int mark = 0;

该类含有如下的成员方法：

构造函数（传入需要读取的字符串

public StringReader(String s) {
    this.str = s;
    this.length = s.length();
}

确认流开启（确认目标字符串存在

private void ensureOpen() throws IOException {
    if (str == null)
        throw new IOException("Stream closed");
}

读取出字符串的单个字符（返回编码值

public int read() throws IOException {
    synchronized (lock) {
        ensureOpen();
        if (next >= length)//读取越界判定
            return -1;
        return str.charAt(next++);//读取完后修改指针
    }
}

读取出字符串中多个字符（写入传入的数组，返回实际读取值

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)) {//数组有效性检查
            throw new IndexOutOfBoundsException();
        } else if (len == 0) {
            return 0;
        }
        if (next >= length)
            return -1;
        int n = Math.min(length - next, len);//读取有效值
        str.getChars(next, next + n, cbuf, off);
        next += n;//修改读取指针
        return n;
    }
}

跳过字符串后多个字符读取（返回实际跳过值

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;
    }
}

检测当前流是否有效（调用上述的ensureOpen方法

public boolean ready() throws IOException {
    synchronized (lock) {
    ensureOpen();
    return true;
    }
}

是否支持标记（都设置了标记变量肯定支持啊

public boolean markSupported() {
    return true;
}

标记当前位置（传入最大回滚距离

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

回滚到标记位置

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

关闭该读取流

public void close() {
    str = null;
}

该类十分简单，主要通过调用String类中的一些方法和自己设置的回滚操作来完成，对于学习字符读入流可以从该类开始，可以尝试自己来写一个类似的面向字符串的字符读取流。