Java Lesson — Scanner, stdin, stdout, and stderr

What are stdin, stdout, and stderr?

Many programming languages use three standard streams for communication. Java also uses them.

In simple classroom programs, stdout and stderr may both appear on the same console, but they are conceptually different streams.

Using Scanner with System.in

To read input from the keyboard in Java, we usually use the Scanner class. The scanner reads from System.in, which represents standard input.

Import Scanner

import java.util.Scanner;

Basic example

import java.util.Scanner;

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

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

        System.out.println("Hello, " + name + "!");

        sc.close();
    }
}

In this example, the program asks for a name, reads the full line using nextLine(), and then prints a greeting.

Common Scanner methods

Scanner provides different methods for different kinds of input.

Reading numbers

import java.util.Scanner;

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

        System.out.print("Enter first number: ");
        int a = sc.nextInt();

        System.out.print("Enter second number: ");
        int b = sc.nextInt();

        int sum = a + b;

        System.out.println("Sum = " + sum);

        sc.close();
    }
}

stdout with System.out

System.out is used for normal output. It sends output to the standard output stream. Two very common methods are print() and println().

System.out.print("Hello ");
System.out.println("World");

print() keeps the cursor on the same line. println() prints and then moves to the next line.

Formatted output

String name = "Aman";
int marks = 92;

System.out.printf("Student: %s, Marks: %d%n", name, marks);

printf() is useful when you want cleaner formatted output.

stderr with System.err

System.err is used for error messages. It sends output to the standard error stream. This is helpful when you want to separate normal program output from warnings or errors.

System.err.println("Invalid input. Please enter a positive number.");

In some IDEs and terminals, error output may appear in a different color or may be handled separately.

Example using stdout and stderr together

import java.util.Scanner;

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

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

        if (age < 0) {
            System.err.println("Error: age cannot be negative.");
        } else {
            System.out.println("Your age is " + age);
        }

        sc.close();
    }
}

Important note about nextInt() and nextLine()

One common beginner problem happens when we use nextInt() and then immediately use nextLine(). After reading the integer, the newline character is still left in the input buffer.

import java.util.Scanner;

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

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

        sc.nextLine(); // consume leftover newline

        System.out.print("Enter full name: ");
        String name = sc.nextLine();

        System.out.println("Name: " + name);
        System.out.println("Age: " + age);

        sc.close();
    }
}

That extra sc.nextLine() is used to consume the leftover newline before reading the actual text line.

Complete example

import java.util.Scanner;

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

        System.out.println("=== Student Information Form ===");

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

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

        System.out.print("Enter your marks: ");
        double marks = sc.nextDouble();

        if (age < 0) {
            System.err.println("Error: age cannot be negative.");
        } else {
            System.out.println();
            System.out.println("=== Output ===");
            System.out.println("Name  : " + name);
            System.out.println("Age   : " + age);
            System.out.println("Marks : " + marks);
        }

        sc.close();
    }
}

Assignments

Interactive MCQ Quiz

Choose one answer for each question and click Check Answers.

Quick revision

• System.in is standard input.
• Scanner is commonly used to read input from System.in.
• System.out is used for normal output.
• System.err is used for error messages.
• nextLine() reads a full line.
• nextInt() reads an integer.
• After nextInt(), you may need an extra nextLine() before reading text.