Java Java.io.PushbackInputStream用法及代码示例

推回用于输入流，以允许读取字节然后将其返回(即“pushed back”)到流。 PushbackInputStream 类实现了这个想法。它提供了一种机制“peek”来处理来自输入流的内容，而不会中断它。
它扩展了 FilterInputStream。
领域：

• 受保护字节[] buf：这是推回缓冲区。
  
• 受保护的 int 位置- 这是推回缓冲区内将从中读取下一个字节的位置。
  
• 受保护的InputStream- 这是要过滤的输入流。

构造函数：

• PushbackInputStream(输入流中)：这将创建一个流对象，允许将一个字节返回到输入流。
  
• PushbackInputStream(输入流，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()：返回可以从此输入流读取(或跳过)的字节数的估计值，而不会被该输入流的方法的下一次调用所阻塞。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.

• 无效close()：关闭此输入流并释放与该流关联的所有系统资源。流关闭后，进一步的 read()、unread()、available()、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 void close()
Returns: NA
Exception: IOException - if an I/O error occurs.

• 布尔值markSupported()：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 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：

available bytes: 10
mark supported? :false

• int read()：从此输入流读取下一个数据字节。值字节以 int 形式返回，范围为 0 到 255。如果由于已到达流末尾而没有可用字节，则返回值 -1。此方法会阻塞，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.

• int 读取(字节[] b，int 关闭，int len)：从此输入流中读取最多 len 个字节的数据到字节数组中。该方法首先读取任意pushed-back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，则该方法将阻塞，直到至少有 1 个字节的输入可用；否则，不会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.

输出：

GeeksforGeeks a computer science portal 
GeeksforGeeks

• 无效标记(int readlimit)：标记此输入流中的当前位置。
  PushbackInputStream 的标记方法不执行任何操作。

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

• 无效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 的重置方法除了抛出 IOException 之外什么也不做。

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

// 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)

• 无效未读(字节[] b)：通过将字节数组复制到推回缓冲区的前面来推回字节数组。该方法返回后，要读取的下一个字节的值为 b[0]，之后的字节的值为 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字节数组。该方法返回后，要读取的下一个字节的值为 b[0]，之后的字节的值为 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 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

• 无效未读(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。该方法返回后，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 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