Java User Input
Introduction to User Input
Java provides multiple ways to capture input from users, with the Scanner class being the most common for console-based applications.
Other options include BufferedReader for efficient text input, the Console class for secure password entry, and JOptionPane for graphical dialog input. Handling user input is essential for building interactive programs.
Scanner Class Basics
The Scanner class (from the java.util package) offers convenient methods to read different types of input such as strings, integers, and doubles from the console.
import java.util.Scanner;
public class ScannerBasicExample {
public static void main(String[] args) {
Scanner scanner = new Scanner(System.in);
System.out.print("Enter your name: ");
String name = scanner.nextLine();
System.out.print("Enter your age: ");
int age = scanner.nextInt();
System.out.print("Enter your height (in meters): ");
double height = scanner.nextDouble();
System.out.println("\nHello " + name + "!");
System.out.println("You are " + age + " years old");
System.out.println("Your height is " + height + " meters");
scanner.close();
}
}Enter your name: John Doe Enter your age: 25 Enter your height (in meters): 1.75 Hello John Doe! You are 25 years old Your height is 1.75 meters
Scanner Methods
| Method | Description | Example Input |
|---|---|---|
| next() | Reads a single word (ignores spaces) | "Hello World" → "Hello" |
| nextLine() | Reads an entire line including spaces | "Hello World" → "Hello World" |
| nextInt() | Reads an integer | "25" → 25 |
| nextDouble() | Reads a double | "3.14" → 3.14 |
| nextBoolean() | Reads a boolean | "true" → true |
| hasNext() | Checks if more input is available | Returns true/false |
Handling Input Validation
Validating input ensures that the program handles unexpected values gracefully and avoids runtime errors like InputMismatchException.
import java.util.Scanner;
public class InputValidationExample {
public static void main(String[] args) {
Scanner scanner = new Scanner(System.in);
int age = 0;
boolean validInput = false;
while (!validInput) {
System.out.print("Enter your age: ");
if (scanner.hasNextInt()) {
age = scanner.nextInt();
if (age > 0 && age < 120) {
validInput = true;
} else {
System.out.println("Please enter a valid age (1-119).");
}
} else {
System.out.println("Invalid input! Please enter a number.");
scanner.next(); // Clear invalid token
}
}
System.out.println("Your age is: " + age);
scanner.close();
}
}Enter your age: abc Invalid input! Please enter a number. Enter your age: 150 Please enter a valid age (1-119). Enter your age: 25 Your age is: 25
BufferedReader Alternative
BufferedReader provides efficient reading of text input, especially when reading large inputs line by line. Unlike Scanner, it always reads data as strings, so parsing is required.
import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;
public class BufferedReaderExample {
public static void main(String[] args) {
BufferedReader reader = new BufferedReader(new InputStreamReader(System.in));
try {
System.out.print("Enter your name: ");
String name = reader.readLine();
System.out.print("Enter your age: ");
int age = Integer.parseInt(reader.readLine());
System.out.println("Hello " + name + ", you are " + age + " years old");
} catch (IOException e) {
System.out.println("Error reading input: " + e.getMessage());
} catch (NumberFormatException e) {
System.out.println("Invalid number format.");
} finally {
try {
reader.close();
} catch (IOException e) {
System.out.println("Error closing reader: " + e.getMessage());
}
}
}
}Enter your name: Jane Smith Enter your age: 30 Hello Jane Smith, you are 30 years old
Console Class
The Console class allows secure input of sensitive information such as passwords, as characters are not displayed while typing.
import java.io.Console;
public class ConsoleExample {
public static void main(String[] args) {
Console console = System.console();
if (console == null) {
System.out.println("Console not available");
return;
}
String username = console.readLine("Enter username: ");
char[] password = console.readPassword("Enter password: ");
System.out.println("Username: " + username);
System.out.println("Password length: " + password.length);
java.util.Arrays.fill(password, ' '); // Clear password
}
}Enter username: john_doe Enter password: Username: john_doe Password length: 8
Best Practices
- Always validate user input to prevent runtime errors and improve reliability
- Use try-with-resources for automatic closing of Scanner or BufferedReader
- Prefer Console for sensitive input such as passwords
- Release input resources after use to prevent memory leaks
- Give clear prompts and helpful error messages
- Handle number format exceptions when converting string input to numbers