Standard Input/Output (System.out, System.in)

System.out.println("Hello, World!");
System.out.print("Java Programming");
System.out.printf("Name: %s, Age: %d", "Mahesh", 25);Code language: CSS (css)

Example: Printing Messages

public class StandardOutputExample {
    public static void main(String[] args) {
        System.out.println("Welcome to Java!");
        System.out.print("Learning ");
        System.out.print("Standard Output.");
        System.out.printf("\nYour age is %d years.", 9);
    }
}
/*
Welcome to Java!
Learning Standard Output.
Your age is 9 years.
*/

2. Standard Input (System.in)

The System.in object is an InputStream that reads bytes from the keyboard. To convert bytes into characters, we often use the Scanner class.

Using Scanner for Input

To read different types of data:

• nextInt(): Reads an integer.
• nextDouble(): Reads a double.
• nextLine(): Reads a line of text.
• next(): Reads a single word.

Syntax:

Scanner sc = new Scanner(System.in);
int number = sc.nextInt();
String name = sc.nextLine();Code language: JavaScript (javascript)

Example: Reading User Input

import java.util.Scanner;

public class StandardInputExample {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);

        System.out.println("Enter your name:");
        String name = sc.nextLine();

        System.out.println("Enter your age:");
        int age = sc.nextInt();

        System.out.printf("Name: %s, Age: %d%n", name, age);
        sc.close();
    }
}
/*
Enter your name:
Mahesh
Enter your age:
25
Name: Mahesh, Age: 25
*/

3. Handling Different Data Types

The Scanner class also supports reading various primitive types:

• nextByte()
• nextShort()
• nextLong()
• nextFloat()
• nextDouble()
• nextBoolean()

Example: Different Data Types

import java.util.Scanner;

public class MultiInputExample {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);

        System.out.println("Enter an integer:");
        int num = sc.nextInt();

        System.out.println("Enter a floating-point number:");
        float fnum = sc.nextFloat();

        System.out.println("Enter a word:");
        String word = sc.next();

        System.out.printf("Integer: %d, Float: %.2f, Word: %s%n", num, fnum, word);
        sc.close();
    }
}

4. Reading Multiple Words

Sometimes, we need to read a complete line rather than a single word. In such cases, use nextLine().

Example:

Scanner sc = new Scanner(System.in);
System.out.println("Enter a sentence:");
String sentence = sc.nextLine();
System.out.println("You entered: " + sentence);Code language: JavaScript (javascript)

5. Handling Input Errors

When users enter the wrong data type, a java.util.InputMismatchException may occur.

Example: Handling Errors

try {
    System.out.println("Enter an integer:");
    int number = sc.nextInt();
} catch (InputMismatchException e) {
    System.out.println("Invalid input! Please enter an integer.");
}Code language: JavaScript (javascript)

6. Using BufferedReader for Efficient Input

The BufferedReader class can also be used for input, especially when fast reading is needed.

Example: Using BufferedReader

import java.io.*;

public class BufferedReaderExample {
    public static void main(String[] args) throws IOException {
        BufferedReader br = new BufferedReader(new InputStreamReader(System.in));

        System.out.println("Enter a line:");
        String line = br.readLine();

        System.out.println("You entered: " + line);
    }
}
/*
Enter a line:
Hello, Java!
You entered: Hello, Java!
*/

7. Printing Errors (System.err)

The System.err stream is used to print error messages. It works similarly to System.out but is intended for error output.

Example: Printing Errors

public class ErrorPrintingExample {
    public static void main(String[] args) {
        System.err.println("Error: Invalid operation.");
    }
}
/*
Error: Invalid operation.
*/

8. Difference between System.out, System.err, and System.in

9. Closing Resources

It is essential to close resources like Scanner and BufferedReader to avoid memory leaks.

Example:

Scanner sc = new Scanner(System.in);
try {
    System.out.println("Enter your age:");
    int age = sc.nextInt();
    System.out.println("Your age is: " + age);
} finally {
    sc.close();
}Code language: JavaScript (javascript)

Java’s System.out, System.err, and System.in are fundamental for standard input and output operations. While System.out and System.err handle output (normal and error, respectively), System.in is used for input. The Scanner class is typically used to read input efficiently, while BufferedReader is preferable when performance is crucial.