Java Scanner — How to Take User Input

So far, all values in our programs have been hardcoded. Real programs need to accept input from users. Java’s Scanner class makes this easy.

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Importing Scanner

Scanner lives in the java.util package:

import java.util.Scanner;

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Creating a Scanner

Scanner scanner = new Scanner(System.in);

System.in represents the keyboard (standard input).

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Reading Different Types

Reading a String (one word)

System.out.print("Enter your name: ");
String name = scanner.next();  // reads one word
System.out.println("Hello, " + name);

Enter your name: Alice
Hello, Alice

Reading a Full Line

System.out.print("Enter your full name: ");
String fullName = scanner.nextLine();  // reads entire line
System.out.println("Hello, " + fullName);

Enter your full name: Alice Johnson
Hello, Alice Johnson

Reading Numbers

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

System.out.print("Enter your GPA: ");
double gpa = scanner.nextDouble();

System.out.println("Age: " + age + ", GPA: " + gpa);

All Scanner Methods

Method	Reads
next()	One word (until whitespace)
nextLine()	Entire line (until Enter)
nextInt()	An int
nextLong()	A long
nextDouble()	A double
nextFloat()	A float
nextBoolean()	true or false
nextByte()	A byte
nextShort()	A short

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The nextLine() Trap

This is the most common pitfall with Scanner:

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

System.out.print("Enter name: ");
String name = scanner.nextLine();  // ⚠️ SKIPPED!

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

Enter age: 25
Enter name: Age: 25, Name:

Why? nextInt() reads 25 but leaves the newline character (\n) in the buffer. nextLine() then reads that leftover newline and returns an empty string.

The Fix

Add a dummy nextLine() after nextInt() (or any nextXxx()):

System.out.print("Enter age: ");
int age = scanner.nextInt();
scanner.nextLine();  // consume the leftover newline

System.out.print("Enter name: ");
String name = scanner.nextLine();  // now works correctly

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

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Checking Before Reading — hasNextXxx()

You can check if valid input is available:

System.out.print("Enter a number: ");
if (scanner.hasNextInt()) {
    int num = scanner.nextInt();
    System.out.println("You entered: " + num);
} else {
    System.out.println("That's not a valid number!");
    scanner.next();  // discard invalid input
}

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Reading Input in a Loop

Scanner scanner = new Scanner(System.in);
System.out.println("Enter numbers (type 'quit' to stop):");

while (scanner.hasNextInt()) {
    int num = scanner.nextInt();
    System.out.println("Got: " + num);
}

System.out.println("Done!");

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Complete Example: Simple Calculator

import java.util.Scanner;

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

        System.out.print("Enter first number: ");
        double a = scanner.nextDouble();

        System.out.print("Enter operator (+, -, *, /): ");
        String op = scanner.next();

        System.out.print("Enter second number: ");
        double b = scanner.nextDouble();

        double result = switch (op) {
            case "+" -> a + b;
            case "-" -> a - b;
            case "*" -> a * b;
            case "/" -> b != 0 ? a / b : Double.NaN;
            default -> {
                System.out.println("Unknown operator");
                yield 0;
            }
        };

        System.out.printf("%.2f %s %.2f = %.2f%n", a, op, b, result);
        scanner.close();
    }
}

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Closing the Scanner

Always close the scanner when done:

scanner.close();

Or use try-with-resources (covered later):

try (Scanner scanner = new Scanner(System.in)) {
    // use scanner
}  // automatically closed

⚠️ After closing a Scanner on System.in, you cannot reopen it. Only close it when you’re completely done with all input.

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Quick Exercises

1. Write a program that asks for the user’s name and age, then prints: “Hello [name], you will be [age+1] next year!”
2. Read 5 numbers in a loop and print their sum.
3. Handle the nextLine() trap: read an int, then a full sentence.

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Summary

Task	Method
Read one word	scanner.next()
Read full line	scanner.nextLine()
Read integer	scanner.nextInt()
Read decimal	scanner.nextDouble()
Check before read	scanner.hasNextInt()
Consume leftover newline	scanner.nextLine() (dummy call)

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What’s Next?

Next, let’s learn about comments — how to annotate your code so humans (including future you) can understand it: Comments in Java.