Java Java.io.Reader用法及代码示例

Java Reader类是一个用于读取字符流的抽象类。子类必须实现的唯一方法是 read(char[], int, int) 和 close()。然而，大多数子类将重写此处定义的一些方法，以提供更高的效率和/或附加函数。

Java Reader 类中的构造函数

Java Reader 类使用两个构造函数，如下所述：

• 受保护Reader()：创建一个新的字符流读取器，其关键部分将在读取器本身上同步。
• 受保护的读卡器(对象锁)：创建一个新的字符流读取器，其关键部分将在给定对象上同步。

Java Reader 类中的方法

1.抽象无效close()

关闭流并释放与其关联的所有系统资源。流关闭后，进一步的 read()、ready()、mark()、reset() 或 skip() 调用将抛出 IOException。关闭之前关闭的流没有任何效果。

Syntax: public abstract void close() throws IOException

Throws: 
IOException 

2. 无效标记(int readAheadLimit)

标记流中的当前位置。后续调用 reset() 将尝试将流重新定位到该点。并非所有character-input流都支持mark()操作。

Syntax: public void mark(int readAheadLimit) throws IOException

参数：
readAheadLimit - Limit on the number of characters that may be read
while still preserving the mark. After reading this many characters, 
attempting to reset the stream may fail.

Throws: 
IOException 

3.布尔值markSupported()

告知此流是否支持mark()操作。默认实现始终返回 false。子类应该重写此方法。

Syntax :public boolean markSupported()

返回：
true if and only if this stream supports the mark operation.

4.int read()

读取单个字符。此方法将阻塞，直到有字符可用、发生 I/O 错误或到达流末尾。想要支持高效单字符输入的子类应该重写此方法。

Syntax :public int read()
         throws IOException

返回：
The character read, as an integer in the range 0 to 65535 (0x00-0xffff), 
or -1 if the end of the stream has been reached

Throws: 
IOException 

5.int读取(char[] cbuf)

将字符读入数组。此方法将阻塞，直到某些输入可用、发生 I/O 错误或到达流末尾。

Syntax :public int read(char[] cbuf)
         throws IOException

参数：
cbuf - Destination buffer

返回：
The number of characters read, or -1 if the end of the stream has been reached

Throws: 
IOException 

6. 抽象 int 读取(char[] cbuf, int off, int len)

将字符读入数组的一部分。此方法将阻塞，直到某些输入可用、发生 I/O 错误或到达流末尾。

Syntax :public abstract int read(char[] cbuf,int off,int len) throws IOException

参数：
cbuf - Destination buffer
off - Offset at which to start storing characters
len - Maximum number of characters to read

返回：
The number of characters read, or -1 if the end of the stream has been reached

Throws: 
IOException 

7. int 读取(CharBuffer 目标)

尝试将字符读入指定的字符缓冲区。缓冲区按原样用作字符存储库：所做的唯一更改是 put 操作的结果。不执行缓冲区的翻转或倒带。

Syntax :public int read(CharBuffer target) throws IOException

参数：
target - the buffer to read characters into

返回：
The number of characters added to the buffer, 
or -1 if this source of characters is at its end

Throws:
IOException 
NullPointerException
ReadOnlyBufferException

8. 布尔值ready()

告知该流是否已准备好读取。

Syntax :public boolean ready() throws IOException

返回：
True if the next read() is guaranteed not to block for input, false otherwise. 
Note that returning false does not guarantee that the next read will block.

Throws:
IOException 

9.无效reset()

重置流。如果流已被标记，则尝试将其重新定位在标记处。如果流尚未被标记，则尝试以适合特定流的某种方式重置它，例如通过将其重新定位到其起始点。并非所有character-input流都支持reset()操作，有些流支持reset()而不支持mark()。

Syntax :public void reset() throws IOException

Throws:
IOException

10. 长跳过(long n)

跳过字符。此方法将阻塞，直到某些字符可用、发生 I/O 错误或到达流末尾。

Syntax :public long skip(long n) throws IOException
          
参数：
n - The number of characters to skip

返回：
The number of characters actually skipped

Throws:
IllegalArgumentException - If n is negative.
IOException

示例

// Java program demonstrating Reader methods 
import java.io.*; 
import java.nio.CharBuffer; 
import java.util.Arrays; 
  
// Driver Class 
class ReaderDemo { 
    // Main function 
    public static void main(String[] args) 
        throws IOException 
    { 
        Reader r = new FileReader(" file.txt & quot;); 
        PrintStream out = System.out; 
        char c[] = new char[10]; 
        CharBuffer cf = CharBuffer.wrap(c); 
  
        // illustrating markSupported() 
        if (r.markSupported()) { 
            // illustrating mark() 
            r.mark(100); 
            out.println(" 
                        mark method is supported & quot;); 
        } 
        // skipping 5 characters 
        r.skip(5); 
  
        // checking whether this stream is ready to be read. 
        if (r.ready()) { 
            // illustrating read(char[] cbuf,int off,int 
            // len) 
            r.read(c, 0, 10); 
            out.println(Arrays.toString(c)); 
  
            // illustrating read(CharBuffer target ) 
            r.read(cf); 
            out.println(Arrays.toString(cf.array())); 
  
            // illustrating read() 
            out.println((char)r.read()); 
        } 
        // closing the stream 
        r.close(); 
    } 
}

输出

[f, g, h, i, g, k, l, m, n, o]
[p, q, r, s, t, u, v, w, x, y]
z

Reader类的实现

下面提到了 Java 中 Reader 类的一些实现：

• Java.io.BufferedReader
• Java.io.CharArrayReader
• Java.io.FilterReader
• InputStreamReader
• Java.io.PipedReader
• Java.io.StringReader