PushbackInputStream unread(int) in Java

unread(int): This method is available in the java.io.PushbackInputStream class of Java.

Syntax:

void java.io.PushbackInputStream.unread(int b) throws IOException

This method takes one argument. This method pushes back a byte by copying it to the front of the push-back buffer.

Note: After this method returns, the next byte to be read will have the value (byte)b.

Parameters: One parameter is required for this method.

b: the int value whose low-order byte is to be pushed back.

Returns: NA

Throws:

1. 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.

Approach 1: When no exception

Java

import java.io.IOException;
import java.io.InputStream;
import java.io.PushbackInputStream;

public class PushbackInputStreamunread2 {
    public static void main(String[] args) throws IOException {

        InputStream in = new InputStream() {

            @Override
            public int read() throws IOException {
                return 10;
            }
        };
        int size = 10;
        PushbackInputStream pushbackInputStream = 
                  new PushbackInputStream(in, size);

        int b = 'a';
        pushbackInputStream.unread(b);

        System.out.println("Successfully unread");
        pushbackInputStream.close();
    }
}

Output:

Successfully unread

Approach 2: IOException 

Java

package com.PushbackInputStream;

import java.io.IOException;
import java.io.InputStream;
import java.io.PushbackInputStream;

public class PushbackInputStreamunread2 {
    public static void main(String[] args) throws IOException {

        InputStream in = new InputStream() {

            @Override
            public int read() throws IOException {
                return 10;
            }
        };
        int size = 10;
        PushbackInputStream pushbackInputStream = 
                   new PushbackInputStream(in, size);

        int b = 'a';
        pushbackInputStream.close();
        pushbackInputStream.unread(b);

        System.out.println("Successfully unread");

    }
}

Output:

Exception in thread "main" java.io.IOException: Stream closed
	at java.base/java.io.PushbackInputStream.ensureOpen(PushbackInputStream.java:74)
	at java.base/java.io.PushbackInputStream.unread(PushbackInputStream.java:203)
	at com.PushbackInputStream.PushbackInputStreamunread2.main(PushbackInputStreamunread2.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.