JAVA伊奥。Java中的PushbackInputStream类

施工人员：

方法：

1. int available（）： 返回可从此输入流读取（或跳过）的字节数的估计值，而无需在下次调用此输入流的方法时阻塞。下一次调用可能是同一个线程或另一个线程。一次读取或跳过这么多字节不会阻塞，但可以读取或跳过更少的字节。

   Syntax: public int available()
   Returns: the number of bytes that can be read
    (or skipped over) from the input stream without blocking.
   Exception: IOException - if this input stream has 
   been closed by invoking its close() method, or an I/O error occurs.
   

2. void close（）： 关闭此输入流并释放与该流关联的所有系统资源。一旦关闭流，进一步的read（）、unread（）、available（）、reset（）或skip（）调用将抛出IOException。关闭以前关闭的流没有效果。

   Syntax: public void close()
   Returns: NA
   Exception: IOException - if an I/O error occurs.
   

3. 支持布尔标记（） 测试此输入流是否支持标记和重置方法，但它不支持。

   Syntax: public boolean markSupported()
   Returns: false, since this class does not support the mark and reset methods.
   Exception: NA
   

   JAVA

   // Java code illustrating available(), close() // and markSupported() methods import java.io.ByteArrayInputStream; import java.io.IOException; import java.io.PrintWriter; import java.io.PushbackInputStream; public class PushbackInputStreamDemo { public static void main(String arg[]) throws IOException { PrintWriter pw = new PrintWriter(System.out, true ); String str = "Hey geeks " ; byte b[] = str.getBytes(); ByteArrayInputStream bout = new ByteArrayInputStream(b); PushbackInputStream push = new PushbackInputStream(bout); // checking no. of bytes available pw.println( "available bytes: " + push.available()); // checking if mark is supported pw.println( "mark supported? :" + push.markSupported()); pw.close(); } }

   输出：

   available bytes: 10
   mark supported? :false
   

4. int read（）： 从这个输入流中读取下一个字节的数据。字节值以0到255范围内的整数形式返回。如果由于到达流的结尾而没有字节可用，则返回值-1。此方法会一直阻塞，直到输入数据可用、检测到流结束或引发异常为止。

   Syntax: public int read()
   Returns: the next byte of data, or -1 if 
   the end of the stream has been reached.
   Exception: IOException - if this input stream 
   has been closed by invoking its close() method, or an I/O error occurs.
   

5. int read（字节[]b，int off，int len）： 从这个输入流中读取多达len字节的数据到一个字节数组中。此方法首先读取任何向后推送的字节；之后，如果读取的字节少于len字节，那么它将从底层输入流中读取。如果len不为零，该方法将阻塞，直到至少有1字节的输入可用；否则，不读取字节，返回0。

   Syntax: public int read(byte[] b, int off, int len).
   Returns: the total number of bytes read into 
   the buffer, or -1 if there is no more data because the end of 
   the stream has been reached.
   Exception:  NullPointerException - If b is null.
   IndexOutOfBoundsException - If off is negative, len is negative, or 
   len is greater than b.length - off
   IOException - if this input stream has been closed by invoking its 
   close() method, or an I/O error occurs.
   

   JAVA

   // Java code illustrating read() method import java.io.ByteArrayInputStream; import java.io.IOException; import java.io.PrintWriter; import java.io.PushbackInputStream; public class PushbackInputStreamDemo { public static void main(String arg[]) throws IOException { PrintWriter pw = new PrintWriter(System.out, true ); String str = "GeeksforGeeks a computer science portal " ; byte b[] = str.getBytes(); ByteArrayInputStream bout = new ByteArrayInputStream(b); PushbackInputStream push = new PushbackInputStream(bout); int c; while ((c=push.read())!=- 1 ) { pw.print(( char )c); } pw.println(); push.read(b, 0 , 13 ); for ( int i= 0 ; i< 13 ; i++) { pw.print(( char )b[i]); } pw.println(); pw.close(); } }

   输出：

   GeeksforGeeks a computer science portal 
   GeeksforGeeks
   

6. 无效标记（int readlimit）： 标记此输入流中的当前位置。 PushbackInputStream的标记方法不起任何作用。

   Syntax: public void mark(int readlimit)
   Returns: NA
   Exception: NA
   

7. void reset（）： 将此流重新定位到上次在此输入流上调用mark方法时的位置。 类PushbackInputStream的方法reset除了抛出IOException之外，什么都不做。

   Syntax: public void reset()
   Returns: NA
   Exception: IOException - if this method is invoked.
   

   JAVA

   // Java code illustrating mark() and reset() method import java.io.ByteArrayInputStream; import java.io.IOException; import java.io.PrintWriter; import java.io.PushbackInputStream; public class PushbackInputStreamDemo { public static void main(String arg[]) throws Exception { PrintWriter pw = new PrintWriter(System.out, true ); String str = "GeeksforGeeks a computer science portal " ; byte b[] = str.getBytes(); ByteArrayInputStream bout = new ByteArrayInputStream(b); PushbackInputStream push = new PushbackInputStream(bout); int c; while ((c=push.read())!=- 1 ) { pw.print(( char )c); } pw.println(); // marking the position push.mark( 5 ); // reseting is not supported throw exception push.reset(); pw.close(); } }

   输出：

   GeeksforGeeks a computer science portal 
   Exception in thread "main" java.io.IOException: mark/reset not supported
       at java.io.PushbackInputStream.reset(PushbackInputStream.java:364)
       at PushbackInputStreamDemo.main(PushbackInputStreamDemo.java:29)
   

8. 无效未读（字节[]b）： 通过将字节数组复制到回推缓冲区的前端来回推字节数组。此方法返回后，下一个要读取的字节将具有值b[0]，之后的字节将具有值b[1]，以此类推。

   Syntax: public void unread(byte[] b)
   returns: NA
   Exception: IOException - If there is not enough room in 
   the pushback buffer for the specified number of bytes, or this input 
   stream has been closed by invoking its close() method.
   

9. 无效未读（字节[]b，整数关闭，整数长度）： 通过将字节数组复制到回推缓冲区的前端来回推字节数组。此方法返回后，下一个要读取的字节将具有值b[0]，之后的字节将具有值b[1]，以此类推。

   Syntax: public void unread(byte[] b,int off,int len)
   Returns: NA
   Exception: IOException - If there is not enough room 
   in the pushback buffer for the specified number of bytes, or this input 
   stream has been closed by invoking its close() method.
   

   JAVA

   // Java code illustrating unread() method import java.io.ByteArrayInputStream; import java.io.IOException; import java.io.PrintWriter; import java.io.PushbackInputStream; public class PushbackInputStreamDemo { public static void main(String arg[]) throws Exception { PrintWriter pw = new PrintWriter(System.out, true ); String str = "GeeksforGeeks a computer science portal " ; byte b[] = str.getBytes(); ByteArrayInputStream bout = new ByteArrayInputStream(b); PushbackInputStream push = new PushbackInputStream(bout); int c; while ((c=push.read())!=- 1 ) { pw.print(( char )c); } pw.println(); // unread method push.unread(b); push.unread(b, 0 , 6 ); while ((c=push.read())!=- 1 ) { pw.print(( char )c); } pw.println(); pw.close(); } }

   输出：

   GeeksforGeeks a computer science portal
   orGeeks a computer science portal
   

10. 无效未读（int b）： 通过将字节复制到回推缓冲区的前端来回推字节。此方法返回后，下一个要读取的字节将具有值（字节）b。

    Syntax: public void unread(int b)
    Returns: NA
    Exception: IOException - If there is not enough 
    room in the pushback buffer for the byte, or this input stream 
    has been closed by invoking its close() method.
    

    JAVA

    // java code illustrating unread() method import java.io.ByteArrayInputStream; import java.io.IOException; import java.io.PrintWriter; import java.io.PushbackInputStream; public class PushbackInputStreamDemo { public static void main(String arg[]) throws Exception { PrintWriter pw = new PrintWriter(System.out, true ); String str = "GeeksforGeeks a computer science portal " ; byte b[] = str.getBytes(); ByteArrayInputStream bout = new ByteArrayInputStream(b); PushbackInputStream push = new PushbackInputStream(bout); // unread method push.unread( 'A' ); b[ 1 ] = ( byte )push.read(); pw.println(( char )b[ 1 ]); } }

    输出：

    A
    

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