Which of these is used to perform all input & output operations in Java?

In Java, all input and output operations are performed using the Java I/O (Input and Output) Streams. Java I/O Stream is a flow of data that you can either read from, or write to. There are two types of streams: Byte Stream (to perform input and output of 8-bit bytes) and Character Stream (to perform input and output for 16-bit unicode).

For example, to read data from a user, you can use the BufferedReader class, and to write data, you can use the PrintWriter class.

Here is a simple example of how to use these classes:

import java.io.*;

public class Test {
   public static void main(String args[]) throws IOException {
      BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
      String str;
      System.out.println("Enter lines of text.");
      System.out.println("Enter 'end' to quit.");
      do {
         str = br.readLine();
         System.out.println(str);
      } while(!str.equals("end"));
   }
}

In this example, we create a BufferedReader object 'br' that reads data from the standard input stream. The do-while loop continues to read lines of text until the user types 'end'.