An InputStreamReader is a bridge from byte streams to character streams: It reads bytes and decodes them into characters using a specified charset.

Constructor

InputStreamReader(InputStream in) : Creates an InputStreamReader that uses the default charset. It wraps the given input stream in as it acts as a bridge.

InputStreamReader(InputStream in, String charsetName) : Creates an InputStreamReader that uses the named charset.

InputStreamReader(InputStream in, Charset cs) : Creates an InputStreamReader that uses the given charset.

InputStreamReader(InputStream in, CharsetDecoder dec) : Creates an InputStreamReader that uses the given charset decoder.

import java.io.*;

public class InputStreamReaderExample {
    
    public static void main(String[] args) {
        readFileExample();
        readByteArrayExample();
        readFileWithDifferentCharset();
    }
    
    // 1. Reading from a File
    public static void readFileExample() {
        try (FileInputStream fileInputStream = new FileInputStream("example.txt");
             InputStreamReader inputStreamReader = new InputStreamReader(fileInputStream, "UTF-8")) {
             
            System.out.println("Reading from file example.txt:");
            int data = inputStreamReader.read();
            while (data != -1) {
                System.out.print((char) data);
                data = inputStreamReader.read();
            }
            System.out.println("n");
        } catch (IOException e) {
            e.printStackTrace();
        }
    }
    
    // 2. Reading from a Byte Array
    public static void readByteArrayExample() {
        String input = "This is a sample string to be read as byte array.";
        byte[] byteArray = input.getBytes();
        
        try (ByteArrayInputStream byteArrayInputStream = new ByteArrayInputStream(byteArray);
             InputStreamReader inputStreamReader = new InputStreamReader(byteArrayInputStream, "UTF-8")) {
             
            System.out.println("Reading from byte array:");
            int data = inputStreamReader.read();
            while (data != -1) {
                System.out.print((char) data);
                data = inputStreamReader.read();
            }
            System.out.println("n");
        } catch (IOException e) {
            e.printStackTrace();
        }
    }
    
    // 3. Specifying Different Charsets
    public static void readFileWithDifferentCharset() {
        try (FileInputStream fileInputStream = new FileInputStream("example.txt");
             InputStreamReader inputStreamReader = new InputStreamReader(fileInputStream, "ISO-8859-1")) {
             
            System.out.println("Reading from file example.txt with ISO-8859-1 charset:");
            char[] buffer = new char[100];
            int charsRead = inputStreamReader.read(buffer, 0, buffer.length);
            while (charsRead != -1) {
                System.out.print(new String(buffer, 0, charsRead));
                charsRead = inputStreamReader.read(buffer, 0, buffer.length);
            }
            System.out.println("n");
        } catch (IOException e) {
            e.printStackTrace();
        }
    }
}

Common Methods

read() : Reads a single character.

read(char[] cbuf, int offset, int length) : Reads characters into a portion of an array. cbuf: buffer array into which the characters will be read. offset: starting position in the buffer array where the read characters will be stored. length: maximum number of characters to read from the stream.

getEncoding: Returns the name of the character encoding being used by this stream.

ready() : return true if this stream is ready to be read.

Reading character by character from keyword

import java.io.*;

public class InputStreamReaderExample {
    public static void main(String[] args) {
        // Create an InputStreamReader using System.in (standard input - keyboard)
        InputStreamReader isr = new InputStreamReader(System.in);
        
        // Buffer to store the read character
        char[] buffer = new char[1];
        
        try {
            System.out.println("Enter characters, 'q' to quit:");
            
            while (true) {
                // Read a single character into the buffer
                int numCharsRead = isr.read(buffer, 0, 1);
                
                // Check if the end of the stream has been reached
                if (numCharsRead == -1) {
                    break;
                }
                
                // Get the character from the buffer
                char ch = buffer[0];
                
                // Print the read character
                System.out.print("You entered: ");
                System.out.println(ch);
                
                // Exit the loop if 'q' is entered
                if (ch == 'q') {
                    break;
                }
            }
        } catch (IOException e) {
            e.printStackTrace();
        } finally {
            try {
                // Close the InputStreamReader
                isr.close();
            } catch (IOException e) {
                e.printStackTrace();
            }
        }
    }
}

import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;

public class LineReadingExample {
    public static void main(String[] args) {
        // Creating an InputStreamReader that reads from standard input (keyboard)
        InputStreamReader inputStreamReader = new InputStreamReader(System.in);
        // Wrapping the InputStreamReader with a BufferedReader for efficient reading
        BufferedReader bufferedReader = new BufferedReader(inputStreamReader);

        System.out.println("Enter text (type 'exit' to quit):");

        String inputLine;
        try {
            // Continuously read lines of text from the keyboard
            while ((inputLine = bufferedReader.readLine()) != null) {
                // Print the entered line
                System.out.println("You entered: " + inputLine);
                
                // Break the loop if the user types 'exit'
                if ("exit".equalsIgnoreCase(inputLine)) {
                    break;
                }
            }
        } catch (IOException e) {
            e.printStackTrace();
        } finally {
            // Close the BufferedReader and InputStreamReader
            try {
                bufferedReader.close();
                inputStreamReader.close();
            } catch (IOException e) {
                e.printStackTrace();
            }
        }
    }
}