Java Java.io.Reader用法及代碼示例

Java Reader類是一個用於讀取字符流的抽象類。子類必須實現的唯一方法是 read(char[], int, int) 和 close()。然而，大多數子類將重寫此處定義的一些方法，以提供更高的效率和/或附加函數。

Java Reader 類中的構造函數

Java Reader 類使用兩個構造函數，如下所述：

• 受保護Reader()：創建一個新的字符流讀取器，其關鍵部分將在讀取器本身上同步。
• 受保護的讀卡器(對象鎖)：創建一個新的字符流讀取器，其關鍵部分將在給定對象上同步。

Java Reader 類中的方法

1.抽象無效close()

關閉流並釋放與其關聯的所有係統資源。流關閉後，進一步的 read()、ready()、mark()、reset() 或 skip() 調用將拋出 IOException。關閉之前關閉的流沒有任何效果。

Syntax: public abstract void close() throws IOException

Throws: 
IOException 

2. 無效標記(int readAheadLimit)

標記流中的當前位置。後續調用 reset() 將嘗試將流重新定位到該點。並非所有character-input流都支持mark()操作。

Syntax: public void mark(int readAheadLimit) throws IOException

參數：
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.布爾值markSupported()

告知此流是否支持mark()操作。默認實現始終返回 false。子類應該重寫此方法。

Syntax :public boolean markSupported()

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

4.int read()

讀取單個字符。此方法將阻塞，直到有字符可用、發生 I/O 錯誤或到達流末尾。想要支持高效單字符輸入的子類應該重寫此方法。

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讀取(char[] cbuf)

將字符讀入數組。此方法將阻塞，直到某些輸入可用、發生 I/O 錯誤或到達流末尾。

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

參數：
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 讀取(char[] cbuf, int off, int len)

將字符讀入數組的一部分。此方法將阻塞，直到某些輸入可用、發生 I/O 錯誤或到達流末尾。

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

參數：
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 讀取(CharBuffer 目標)

嘗試將字符讀入指定的字符緩衝區。緩衝區按原樣用作字符存儲庫：所做的唯一更改是 put 操作的結果。不執行緩衝區的翻轉或倒帶。

Syntax :public int read(CharBuffer target) throws IOException

參數：
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. 布爾值ready()

告知該流是否已準備好讀取。

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.無效reset()

重置流。如果流已被標記，則嘗試將其重新定位在標記處。如果流尚未被標記，則嘗試以適合特定流的某種方式重置它，例如通過將其重新定位到其起始點。並非所有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. 長跳過(long n)

跳過字符。此方法將阻塞，直到某些字符可用、發生 I/O 錯誤或到達流末尾。

Syntax :public long skip(long n) throws IOException
          
參數：
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(); 
    } 
}

輸出

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

Reader類的實現

下麵提到了 Java 中 Reader 類的一些實現：

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