unread(byte[], int, int): This method is available in the java.io.PushbackInputStream class of Java.
Syntax:
void java.io.PushbackInputStream.unread(byte[] b, int off, int len) throws IOException
This method takes three arguments. This method pushes back a portion of an array of bytes by copying it to the front of the pushback buffer.
Note: 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.
Parameters: Three parameters are required for this method.
b: the byte array to push back.
off: the start offset of the data.
len: the number of bytes to push back.
Throws:
1. NullPointerException - If b is null.
2. 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.
Approach 1: When no exception
Java
import java.io.IOException;import java.io.InputStream;import java.io.PushbackInputStream;public class PushbackInputStreamunread3 {public static void main(String[] args) throws IOException {InputStream in = new InputStream() {@Overridepublic int read() throws IOException {return 10;}};int size = 10;PushbackInputStream pushbackInputStream =new PushbackInputStream(in, size);byte b[] = { 'a', 'b', 'c', 'd', 'e' };int off = 0, len = 3;pushbackInputStream.unread(b, off, len);System.out.println("Successfully unread");pushbackInputStream.close();}}
Output:
Successfully unread
Approach 2: NullPointerException
Java
package com.PushbackInputStream;import java.io.IOException;import java.io.InputStream;import java.io.PushbackInputStream;public class PushbackInputStreamunread3 {public static void main(String[] args) throws IOException {InputStream in = new InputStream() {@Overridepublic int read() throws IOException {return 10;}};int size = 10;PushbackInputStream pushbackInputStream =new PushbackInputStream(in, size);byte b[] = null;int off = 0, len = 3;pushbackInputStream.unread(b, off, len);System.out.println("Successfully unread");pushbackInputStream.close();}}
Output:
Exception in thread "main" java.lang.NullPointerException at java.base/java.lang.System.arraycopy(Native Method) at java.base/java.io.PushbackInputStream.unread(PushbackInputStream.java:232) at com.PushbackInputStream.PushbackInputStreamunread3.main(PushbackInputStreamunread3.java:22)
Approach 3: IOException
Java
package com.PushbackInputStream;import java.io.IOException;import java.io.InputStream;import java.io.PushbackInputStream;public class PushbackInputStreamunread3 {public static void main(String[] args) throws IOException {InputStream in = new InputStream() {@Overridepublic int read() throws IOException {return 10;}};int size = 10;PushbackInputStream pushbackInputStream =new PushbackInputStream(in, size);byte b[] = { 'a', 'b', 'c', 'd', 'e' };int off = 0, len = 13;pushbackInputStream.unread(b, off, len);System.out.println("Successfully unread");pushbackInputStream.close();}}
Output:
Exception in thread "main" java.io.IOException: Push back buffer is full at java.base/java.io.PushbackInputStream.unread(PushbackInputStream.java:229) at com.PushbackInputStream.PushbackInputStreamunread3.main(PushbackInputStreamunread3.java:22)
Some other methods of PushbackInputStream
available(): This method returns an estimate of the number of bytes that can be read (or skipped over) from this input stream without blocking the next invocation of a method for this input stream.
mark(int): This method marks the current position in this input stream.
markSupported(): This method tests if this input stream supports the mark and reset methods, which it does not.
PushbackInputStream(InputStream): This method creates a PushbackInputStream with a 1-byte pushback buffer and saves its argument, the input stream, for later use.
PushbackInputStream(InputStream, int): This method creates a PushbackInputStream with a pushback buffer of the specified size, and saves its argument, the input stream, for later use.
read(): This method reads the next byte of data from this input stream.
read(byte[], int, int): This method reads up to len bytes of data from this input stream into an array of bytes.
reset(): This method repositions this stream to the position at the time the mark method was last called on this input stream.
skip(long): This method skips over and discards n bytes of data from this input stream.
unread(byte[]): This method pushes back an array of bytes by copying it to the front of the pushback buffer.
unread(int): This method pushes back a byte by copying it to the front of the push-back buffer.
unread(byte[], int, int): This method pushes back a portion of an array of bytes by copying it to the front of the pushback buffer.
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