User Input in Java using Scanner

Reading input in Java

To make a program interactive, it must read input from the user. In Java, the easiest way is the Scanner class. You create a Scanner tied to the keyboard (System.in) and use its methods to read text and numbers.

Import and create Scanner

import java.util.Scanner;   // at the top

public class Main {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        // ready to read input
    }
}

First import java.util.Scanner;, then create Scanner sc = new Scanner(System.in);. System.in means the keyboard. Now sc can read what the user types.

Reading text and numbers

String name = sc.nextLine();   // whole line of text
String word = sc.next();       // one word
int    age  = sc.nextInt();    // whole number
double pay  = sc.nextDouble(); // decimal number

Each method reads a specific type. Use nextLine() for text with spaces, next() for a single word, and nextInt()/nextDouble() for numbers.

A complete 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.print("Enter your age: ");
        int age = sc.nextInt();

        System.out.println("Hi " + name + ", age " + age);
        sc.close();
    }
}

We prompt, read a name and an age, then print them. sc.close() at the end frees the Scanner.

The nextLine after nextInt bug

int age = sc.nextInt();
sc.nextLine();              // FIX: consume leftover newline
String name = sc.nextLine();

A classic trap: after nextInt(), the Enter key is left in the buffer, so the next nextLine() reads an empty line. The fix is one extra sc.nextLine() right after the number, to swallow that leftover newline.

Practice programs with Scanner

These three programs are common in school and college practical files. Each was compiled with javac and run with the input shown (typed values appear after each prompt).

Program 1: Sum and average of three numbers

import java.util.Scanner;

public class SumAverage {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        System.out.print("Enter three numbers: ");
        int a = sc.nextInt();
        int b = sc.nextInt();
        int c = sc.nextInt();
        int sum = a + b + c;
        double avg = sum / 3.0;
        System.out.println("Sum = " + sum);
        System.out.printf("Average = %.2f%n", avg);
        sc.close();
    }
}

nextInt() can read several numbers typed on one line, separated by spaces. Dividing by 3.0 (not 3) gives a decimal average.

Program 2: Marksheet – name, 5 subjects, percentage and grade

import java.util.Scanner;

public class Marksheet {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        System.out.print("Student name: ");
        String name = sc.nextLine();
        int total = 0;
        for (int i = 1; i <= 5; i++) {
            System.out.print("Marks in subject " + i + ": ");
            total += sc.nextInt();
        }
        double percent = total / 5.0;
        String grade = percent >= 90 ? "A1" : percent >= 75 ? "A2" : percent >= 60 ? "B1" : percent >= 33 ? "C" : "Needs improvement";
        System.out.println(name + ": total " + total + "/500, " + percent + "%, grade " + grade);
        sc.close();
    }
}

The name is read first with nextLine(), before any nextInt(), so the newline trap does not occur. The ternary chain picks the grade.

Program 3: Simple interest calculator

import java.util.Scanner;

public class SimpleInterest {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        System.out.print("Principal (Rs): ");
        double p = sc.nextDouble();
        System.out.print("Rate (% per year): ");
        double r = sc.nextDouble();
        System.out.print("Time (years): ");
        double t = sc.nextDouble();
        double si = p * r * t / 100;
        System.out.printf("Simple interest = Rs %.2f%n", si);
        System.out.printf("Total amount = Rs %.2f%n", p + si);
        sc.close();
    }
}

nextDouble() reads decimal values such as 7.5; printf("%.2f") prints money with two decimal places.

Try it yourself: change Program 2 to read the number of subjects from the user, and Program 3 to also print the interest for each year.

Common mistakes

• Forgetting the import. You must import java.util.Scanner; at the top.
• The nextLine-after-nextInt bug. Add an extra sc.nextLine() to fix it.
• Using next() for full names. next() stops at a space — use nextLine().
• Wrong method for the type. Reading text with nextInt() throws an error.

Summary

• Use the Scanner class to read user input.
• Import it, then create new Scanner(System.in).
• nextInt(), nextDouble(), next(), nextLine() read different types.
• Fix the newline bug with an extra nextLine() after nextInt().
• Close the Scanner with sc.close() when done.

Next, use the input in decisions with if-else, or process it in Java loops.