Java - PushbackInputStream unread(byte[] b,int off,int len) method

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Description

The Java PushbackInputStream unread(byte[] b,int off,int len) method pushes back a portion of an array of bytes by copying it to the front of the pushback buffer. After this method returns, the next byte to be read will have the value b[off], the byte after that will have the value b[off+1], and so forth.

unread(byte[] b,int off,int len) method −

• Pushes back len bytes from the byte array b, starting at offset off.

• These bytes will be returned before any unread bytes still in the stream.

• Useful for partial backtracking in parsing situations.

Declaration

Following is the declaration for java.io.PushbackInputStream.unread(byte[] b,int off,int len) method.

 public void unread(byte[] b,int off,int len) 

Parameters

• b − The byte array to push back.

• off − The start offset of the data.

• len − The number of bytes to push back.

Return Value

This method does not return a value.

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.

Example - Usage of PushbackInputStream unread(byte[] b,int off,int len) method

The following example shows the usage of PushbackInputStream unread(byte[] b,int off,int len) method.

PushbackInputStreamDemo.java

 package com.tutorialspoint; import java.io.ByteArrayInputStream; import java.io.InputStream; import java.io.PushbackInputStream; public class PushbackInputStreamDemo { public static void main(String[] args) { // declare a buffer and initialize its size: byte[] arrByte = new byte[1024]; // create an array for our message byte[] byteArray = new byte[]{'H', 'e', 'l', 'l', 'o',}; // create object of PushbackInputStream class for specified stream InputStream is = new ByteArrayInputStream(byteArray); PushbackInputStream pis = new PushbackInputStream(is, 10); try { // read from the buffer one character at a time for (int i = 0; i < byteArray.length; i++) { // read a char into our array arrByte[i] = (byte) pis.read(); // display the read byte System.out.print((char) arrByte[i]); } // change line System.out.println(); // create a new byte array to be unread byte[] b = {'W', 'o', 'r', 'l', 'd'}; // unread the byte array pis.unread(b, 2, 3); // read again from the buffer one character at a time for (int i = 0; i < 3; i++) { // read a char into our array arrByte[i] = (byte) pis.read(); // display the read byte System.out.print((char) arrByte[i]); } } catch (Exception ex) { ex.printStackTrace(); } } } 

Output

Let us compile and run the above program, this will produce the following result −

 Hello rld 

Example - Unread a portion of a buffer

The following example shows the usage of PushbackInputStream markSupported() method.

PushbackInputStreamDemo.java

 package com.tutorialspoint; import java.io.ByteArrayInputStream; import java.io.PushbackInputStream; import java.io.IOException; public class PushbackInputStreamDemo { public static void main(String[] args) throws IOException { byte[] data = "ABCDEFG".getBytes(); // 7 bytes try (PushbackInputStream pbis = new PushbackInputStream(new ByteArrayInputStream(data), 10)) { byte[] buffer = new byte[4]; pbis.read(buffer); // Read A B C D System.out.println("Initial read: " + new String(buffer)); // Output: ABCD // Push back just 'C' and 'D' from buffer (offset 2, length 2) pbis.unread(buffer, 2, 2); // Now reading again should return C, D, then E byte[] nextRead = new byte[3]; pbis.read(nextRead); System.out.println("After unread: " + new String(nextRead)); // Output: CDE } } } 

Output

Let us compile and run the above program, this will produce the following result−

 Initial read: ABCD After unread: CDE 

Explanation

• Reads the first 4 bytes (ABCD) into a buffer.

• Pushes back only 'C' and 'D' using unread(buffer, 2, 2).

• Next read() fetches CD (pushed back) and the next byte from the stream (E).

Example - Look ahead, then unread a partial header

The following example shows the usage of PushbackInputStream unread(byte[] b,int off,int len) method.

PushbackInputStreamDemo.java

 package com.tutorialspoint; import java.io.ByteArrayInputStream; import java.io.PushbackInputStream; import java.io.IOException; public class PushbackInputStreamDemo { public static void main(String[] args) throws IOException { byte[] data = "TYPE:DataPayload".getBytes(); try (PushbackInputStream pbis = new PushbackInputStream(new ByteArrayInputStream(data), 20)) { byte[] header = new byte[5]; pbis.read(header); // Read T Y P E : // Check the header String head = new String(header); System.out.println("Header: " + head); // Output: TYPE: // Push back only last 3 chars (P E :) pbis.unread(header, 2, 3); // Now read again from pushed-back section byte[] reread = new byte[5]; pbis.read(reread); System.out.println("Re-read: " + new String(reread)); // Output: PE:Da } } } 

Output

Let us compile and run the above program, this will produce the following result−

 Header: TYPE: Re-read: PE:Da 

Explanation

• Reads TYPE: (5 bytes), simulating header processing.

• Pushes back 'P', 'E', ':' (bytes 2 to 4 of the header).

• The next read starts with PE: and continues into the next part of the stream.