Java PushbackReader Class

The PushbackReader class in Java is part of the java.io.package, and is used for reading characters from a stream. This class lets us push characters back into the stream.

Features of PushbackReader Class:

• This class uses a buffer that allows us to push characters back into the stream.
• This class is great for reading text and checking characters without using them right away. If the character isn’t needed, it can be pushed back.

Example:

// Demonstrating the working 
// of PushbackReader
import java.io.*;

public class Geeks {
    
    public static void main(String[] args) {
        String s = "Hello, World!";
        StringReader sr = new StringReader(s);

        // Create a PushbackReader with the StringReader
        PushbackReader pr = new PushbackReader(sr);

        try {
            // Read the first character
            int char1 = pr.read();
            System.out.println("Read character: " + (char) char1);

            // Push this character back into the stream
            pr.unread(char1);

            // Read the same character again (since we pushed it back)
            int char2 = pr.read();
            System.out.println("Read character again: " + (char) char2); 

            // Continue reading the next character
            int char3 = pr.read();
            System.out.println("Next character: " + (char) char3); 

            // Close the reader
            pr.close();

        } catch (IOException e) {
            e.printStackTrace();
        }
    }
}

Read character: H
Read character again: H
Next character: e

Declaration of PushbackReader Class

The Declaration of PushbackReader class is listed below:

public class PushbackReader extends FilterReader

All Implemented Interfaces:

• Closeable: This interface is used to release resources
• AutoCloseable: This interface allows automatic closing in a try-with-resources statement.
• Readable: This interface is used to read data from the object.

Constructors in PushbackReader

This class consists of two constructors with the help of which we can create object of this class in different ways. The following are the constructors available in this class:

1. PushbackReader(Reader push): This constructor is used to create a new PushbackReader with a default pushback buffer.

Syntax:

public PushbackReader(Reader push)

Example:

// Demonstrating the working 
// of PushbackReader(Reader push)
import java.io.*;

public class Geeks {
    public static void main(String[] args) {
        
        // String to be read
        String s = "Hello, World!";
        
        // Create a StringReader to read from the string
        StringReader sr = new StringReader(s);

        // Create a PushbackReader with the StringReader
        PushbackReader pr = new PushbackReader(sr);

        try {
            // Read and print characters
            int charRead;
            while ((charRead = pr.read()) != -1) {
                System.out.print((char) charRead);
            }

            // Close the PushbackReader
            pr.close();
        } catch (IOException e) {
            e.printStackTrace();
        }
    }
}

Hello, World!

2. PushbackReader(Reader push, int size): This constructor allows us to specify the size of the pushback buffer

Syntax:

public PushbackReader(Reader push, int size)

Example:

// Demonstrating the working
// of PushbackReader(Reader push, int size)
import java.io.*;

public class Geeks{
    public static void main(String[] args) {
        
        // String to be read
        String s = "Hello, World!";
        
        // Create a StringReader to read from the string
        StringReader sr = new StringReader(s);

        // Create a PushbackReader with the StringReader 
        // and a specified buffer size of 20
        PushbackReader pr = new PushbackReader(sr, 20);

        try {
            // Read and print characters
            int charRead;
            while ((charRead = pr.read()) != -1) {
                System.out.print((char) charRead);
            }

            // Close the PushbackReader
            pr.close();
        } catch (IOException e) {
            e.printStackTrace();
        }
    }
}

Hello, World!

Java PushbackReader Methods

The image below demonstrates the methods of CharArrayWriter class.

Now, we are going to discuss about each method one by one in detail:

1. read(): This method is used to read one character at a time from the stream

Syntax:

public int read()

• Parameter: This method does not take any parameter.
• Return Type: This method returns the character read as an integer or return -1 when the end of the stream is reached.

2. read(char[] carray, int offset, int maxlen): This method is used to read a chunk of characters.

Syntax:

public int read(char[] carray, int offset, int maxlen)

• Parameter: This method includes three parameters which are listed below
  • char[] carray: It is a character array where read characters will be stored.
  • int offset: It is the starting position in the array
  • int maxlen: It is the maximum number of characters to read.
• Return Type: This method returns the number of character read

3. close(): This method is used to close the reader

Syntax:

public void close()

• Parameter: This method does not take any parameter
• Return Type: This method does not return anything.

4. mark(int readAheadLimit): This method is used to mark the current position in the stream

Syntax:

public void mark(int readAheadLimit)

• Parameter: This method includes one parameter, readAheadLimit, which specifies the maximum number of characters that can be read before the mark position becomes invalid.
• Return Type: This method does not return anything

5. skip(long n): This method is used to skip certain number of characters.

Syntax:

public long skip(long n)

• Parameter: This method includes one parameter, long n, which is the number of characters to skip in the stream.
• Return Type: This method returns the number of characters that is skipped

6. reset(): This method is used to go back to the position where mark() was called.

Syntax:

public void reset()

• Parameter: This method does not take any parameter
• Return Type: This method does not return anything

7. markSupported(): This method tells whether the stream supports marking or not

Syntax:

public boolean markSupported()

• Parameter: This method does not take any parameter
• Return Type: This method returns false, because marking is not supported.

8. ready(): This method checks whether the stream is ready to read or not.

Syntax:

public boolean ready()

• Parameter: This method does not take any parameter
• Return Type: This method returns true, if the stream is ready.

9. unread(int c): This method is used to push character back into the stream.

Syntax:

public void unread(int c)

• Parameter: This method takes a single parameter that is c, which is the character to be pushed back into the stream.
• Return Type: This method does not return anything.

10. unread(char[] carray): This method is used to push array of characters back into the stream.

Syntax:

public void unread(char[] carray)

• Parameter: This method takes an array of characters to be pushed back into the stream.
• Return Type: This method does not return anything.

Java Program to Demonstrate the PushbackReader Methods

Example:

// Demonstrating the working
// of methods of PushbackReader class
import java.io.*;

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

        String s = "GeeksForGeeks";
        StringReader sr = new StringReader(s);
        PushbackReader pr = new PushbackReader(sr);

        // Check if the stream is ready
        System.out.println("Is the stream ready? " + pr.ready());

        // Use read() and skip()
        System.out.print("Using read() and skip(): ");
        for (int i = 0; i < 3; i++) {
            char c = (char) pr.read(); // Read 1 char
            System.out.print(c);
            pr.skip(1); // Skip next char
        }

        char c = (char) pr.read(); // read next char after 3 skip cycles
        System.out.print(c); 

        // Now read() with array – only 1 char expected
        char[] arr = new char[1];
        pr.read(arr);
        System.out.println("\nUsing read() with an array: " + new String(arr));

        // Unread and read back
        pr.unread('F');
        char nextChar = (char) pr.read();
        System.out.println("After unread(), the next character: " + nextChar);

        // Close the reader
        pr.close();
    }
}

Is the stream ready? true
Using read() and skip(): Geso
Using read() with an array: r
After unread(), the next character: F