Java User Input

import java.util.Scanner;

public class Main {
    public static void main(String[] args) {
        Scanner input = new Scanner(System.in);
        System.out.print("What's your name? ");
        String name = input.nextLine();
        System.out.println("Hello, " + name + "!");
    }
}

• The import line at the top makes the Scanner class available. It goes above the class.
• new Scanner(System.in) creates a Scanner that reads from the keyboard. Create one and reuse it.
• Using print instead of println for the question keeps the cursor on the same line, which looks natural.

The number methods convert the text for you, so int age = input.nextInt(); gives you a real int that you can do maths with. If the user types something that isn't a number, such as ten, the program stops with an InputMismatchException.

nextInt() reads the number but leaves the Enter key in the input. A nextLine() straight afterwards reads that leftover Enter and returns an empty string, so it looks as if Java skipped your question:

int age = input.nextInt();
input.nextLine();               // throw away the leftover Enter
String city = input.nextLine(); // now this waits properly

The fix is to call nextLine() once to consume the rest of the line after reading a number. Another common approach is to read everything with nextLine() and convert it yourself with Integer.parseInt(...), which avoids the trap completely.

hasNextInt() looks at the next input without reading it and returns true only if it's a valid whole number. Combined with a loop, it lets you keep asking until the input is valid:

while (!input.hasNextInt()) {
    System.out.print("Please enter a number: ");
    input.next();   // discard the bad word
}
int number = input.nextInt();

There are matching methods such as hasNextDouble() and hasNextLine(). Validating input this way keeps your program from crashing when someone makes a typo.

A Scanner splits input into tokens separated by spaces, tabs or new lines. Methods such as nextInt() don't care whether the values were typed on one line or several. If the user types 3 4 and presses Enter, two calls to nextInt() read 3 and then 4:

System.out.print("Width and height: ");
int width = input.nextInt();
int height = input.nextInt();
System.out.println("Area: " + width * height);

When you need the whole line, for example to check how many values were typed, read it with nextLine() and split it yourself: String[] parts = input.nextLine().trim().split("\\s+");. Each piece is a String, so convert it with Integer.parseInt(parts[0]).

nextDouble() follows the computer's locale, its language and region settings. On a computer set to German or French, it expects a decimal comma, 3,5, and treats 3.5 as invalid input. If your program should always accept a decimal point, set the locale once after creating the Scanner:

Scanner input = new Scanner(System.in).useLocale(java.util.Locale.UK);

Reading the text with nextLine() and converting it with Double.parseDouble() also always uses a decimal point, because parseDouble ignores the locale.