Java Java类

Pushback 用于输入流以允许读取一个字节，然后将其返回（即“推回”）到流中。 PushbackInputStream 类实现了这个想法。它提供了一种机制“窥视”来自输入流的内容，而不会破坏它。
它扩展了 FilterInputStream。
领域：

• protected byte[] buf ：这是推回缓冲区。
  
• protected int pos – 这是下一个字节将被读取的推回缓冲区中的位置。
  
• protected InputStream in – 这是要过滤的输入流。

构造函数：

• PushbackInputStream(InputStream in)：这将创建一个流对象，允许将一个字节返回到输入流。
  
• PushbackInputStream(InputStream in, int numBytes)：这将创建一个具有 numBytes 长的推回缓冲区的流。这允许将多个字节返回到输入流。

方法：

• int available()：返回对可以从此输入流读取（或跳过）的字节数的估计值，而不会被下一次调用此输入流的方法阻塞。下一次调用可能是同一个线程或另一个线程。单次读取或跳过这么多字节不会阻塞，但可能会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.

• void close()：关闭此输入流并释放与该流关联的所有系统资源。关闭流后，进一步的 read()、unread()、available()、reset() 或 skip() 调用将引发 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.

• boolean markSupported()：测试此输入流是否支持 mark 和 reset 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

• int read()：从此输入流中读取数据的下一个字节。值字节作为 int 返回，范围为 0 到 255。如果由于到达流的末尾而没有可用的字节，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。

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.

• int read(byte[] b, int off, int len)：从此输入流中读取最多 len 个字节的数据到一个字节数组中。此方法首先读取任何推回的字节；之后，如果读取的字节数少于 len，则从底层输入流中读取。如果 len 不为零，则该方法会阻塞，直到至少有 1 个字节的输入可用；否则，不读取任何字节并返回 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

• void mark(int readlimit)：标记此输入流中的当前位置。
  PushbackInputStream 的 mark 方法什么都不做。

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

• void reset()：将此流重新定位到最后一次在此输入流上调用标记方法时的位置。
  PushbackInputStream 类的方法 reset 除了抛出一个 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);
         
        // resetting 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)

• void unread(byte[] b)：通过将字节数组复制到推回缓冲区的前面来推回它。此方法返回后，要读取的下一个字节将具有值 b[0]，之后的字节将具有值 b[1]，依此类推。

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.

• void unread(byte[] b,int off,int len)：通过将字节数组复制到推回缓冲区的前面来推回它。此方法返回后，要读取的下一个字节将具有值 b[0]，之后的字节将具有值 b[1]，依此类推。

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

• void unread(int b)：通过将字节复制到推回缓冲区的前面来推回一个字节。此方法返回后，下一个要读取的字节将具有值 (byte)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