Java Java.io.PushbackInputStream用法及代碼示例

推回用於輸入流，以允許讀取字節然後將其返回(即“pushed back”)到流。 PushbackInputStream 類實現了這個想法。它提供了一種機製“peek”來處理來自輸入流的內容，而不會中斷它。
它擴展了 FilterInputStream。
領域：

• 受保護字節[] buf：這是推回緩衝區。
  
• 受保護的 int 位置- 這是推回緩衝區內將從中讀取下一個字節的位置。
  
• 受保護的InputStream- 這是要過濾的輸入流。

構造函數：

• PushbackInputStream(輸入流中)：這將創建一個流對象，允許將一個字節返回到輸入流。
  
• PushbackInputStream(輸入流，int numBytes)：這將創建一個具有 numBytes 長的推回緩衝區的流。這允許將多個字節返回到輸入流。

方法：

• int available()：返回可以從此輸入流讀取(或跳過)的字節數的估計值，而不會被該輸入流的方法的下一次調用所阻塞。下一次調用可能是同一個線程或另一個線程。單次讀取或跳過這麽多字節不會阻塞，但可能會讀取或跳過更少的字節。
  

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.

• 無效close()：關閉此輸入流並釋放與該流關聯的所有係統資源。流關閉後，進一步的 read()、unread()、available()、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 void close()
Returns: NA
Exception: IOException - if an I/O error occurs.

• 布爾值markSupported()：測試此輸入流是否支持標記和重置方法，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]);
    }
}

輸出：

available bytes: 10
mark supported? :false

• int read()：從此輸入流讀取下一個數據字節。值字節以 int 形式返回，範圍為 0 到 255。如果由於已到達流末尾而沒有可用字節，則返回值 -1。此方法會阻塞，直到輸入數據可用、檢測到流結束或引發異常為止。

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 讀取(字節[] b，int 關閉，int len)：從此輸入流中讀取最多 len 個字節的數據到字節數組中。該方法首先讀取任意pushed-back字節；之後，如果讀取的字節數少於 len，則從底層輸入流中讀取。如果 len 不為零，則該方法將阻塞，直到至少有 1 個字節的輸入可用；否則，不會讀取任何字節並返回 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.

輸出：

GeeksforGeeks a computer science portal 
GeeksforGeeks

• 無效標記(int readlimit)：標記此輸入流中的當前位置。
  PushbackInputStream 的標記方法不執行任何操作。

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

• 無效reset()：將此流重新定位到上次在此輸入流上調用 mark 方法時的位置。
  類 PushbackInputStream 的重置方法除了拋出 IOException 之外什麽也不做。

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();
    }
}

輸出：

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)

• 無效未讀(字節[] b)：通過將字節數組複製到推回緩衝區的前麵來推回字節數組。該方法返回後，要讀取的下一個字節的值為 b[0]，之後的字節的值為 b[1]，依此類推。

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):通過將字節數組複製到推回緩衝區的前麵來推回字節數組。該方法返回後，要讀取的下一個字節的值為 b[0]，之後的字節的值為 b[1]，依此類推。

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();
    }
}

輸出：

GeeksforGeeks a computer science portal
orGeeks a computer science portal

• 無效未讀(int b)：通過將字節複製到推回緩衝區的前麵來推回一個字節。該方法返回後，下一個要讀取的字節將具有值(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]);
    }
}

輸出：

A