Why Console Input and Output Matter
Console programs interact with users through text: you print prompts (output) and read what the user types (input). In Java, a common beginner-friendly way to read console input is the Scanner class. You will typically: import the class, create a Scanner connected to System.in, read values (text and numbers), then print results—often with formatting.
Importing the Scanner Library
Scanner lives in the java.util package, so you must import it at the top of your file.
import java.util.Scanner;If you forget this import, you will usually see an error like “Scanner cannot be resolved to a type”.
Creating a Scanner
Create one Scanner object for reading from the keyboard (standard input). A common pattern is to create it inside main.
import java.util.Scanner; public class Main { public static void main(String[] args) { Scanner scanner = new Scanner(System.in); // read input here scanner.close(); } }Note: In small learning programs, calling scanner.close() is fine. In larger applications, closing System.in can affect other input later, but for single-run console exercises it is acceptable.
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Printing Output: Prompts and Formatted Results
Basic printing
Use System.out.print to keep the cursor on the same line, and System.out.println to move to the next line.
System.out.print("Enter your name: "); System.out.println("Thanks!");Formatted printing with printf
System.out.printf lets you insert values into a format string. Common placeholders include %s for strings, %d for integers, and %.2f for decimals with 2 digits after the decimal point.
String name = "Sam"; int count = 3; double price = 12.5; System.out.printf("Hello %s, you bought %d items. Total: $%.2f%n", name, count, price);%n prints a platform-independent newline.
Reading Strings
The two most used string-reading methods are:
next(): reads one “token” (stops at whitespace). Good for single words.nextLine(): reads the entire line (including spaces) until Enter is pressed. Good for full names or sentences.
System.out.print("Enter a one-word username: "); String username = scanner.next(); System.out.print("Enter your full name: "); scanner.nextLine(); // consume leftover newline if needed (explained later) String fullName = scanner.nextLine();In many programs you will prefer nextLine() for user-friendly input because it allows spaces.
Reading Numbers (int and double)
Scanner can parse numeric input directly.
nextInt()reads an integer (e.g., 10, -3).nextDouble()reads a decimal number (e.g., 3.14).
System.out.print("Enter an integer: "); int a = scanner.nextInt(); System.out.print("Enter a decimal number: "); double b = scanner.nextDouble();If the user types something that cannot be parsed (like “ten”), Scanner throws an InputMismatchException. In beginner projects, you can assume correct input; later you can add validation loops.
Guided Project: Name + Two Numbers, Friendly Output + Calculation
Goal: ask the user for their name and two numbers, then print a friendly message and a calculated result. This project also demonstrates a common pitfall: mixing numeric reads with nextLine().
Step 1: Set up imports and Scanner
import java.util.Scanner; public class Main { public static void main(String[] args) { Scanner scanner = new Scanner(System.in); // steps go here scanner.close(); } }Step 2: Ask for the name (use nextLine)
Using nextLine() allows the user to type a full name with spaces.
System.out.print("What is your name? "); String name = scanner.nextLine();Step 3: Ask for two numbers
Read two numbers. Here we will use double so the user can enter either whole numbers or decimals.
System.out.print("Enter the first number: "); double num1 = scanner.nextDouble(); System.out.print("Enter the second number: "); double num2 = scanner.nextDouble();Step 4: Compute and print formatted results
Compute a result (for example, the sum) and print a friendly message using printf.
double sum = num1 + num2; System.out.printf("Hi %s! The sum of %.2f and %.2f is %.2f.%n", name, num1, num2, sum);Full project code (ready to run)
import java.util.Scanner; public class Main { public static void main(String[] args) { Scanner scanner = new Scanner(System.in); System.out.print("What is your name? "); String name = scanner.nextLine(); System.out.print("Enter the first number: "); double num1 = scanner.nextDouble(); System.out.print("Enter the second number: "); double num2 = scanner.nextDouble(); double sum = num1 + num2; System.out.printf("Hi %s! The sum of %.2f and %.2f is %.2f.%n", name, num1, num2, sum); scanner.close(); } }Common Pitfall: Mixing nextLine() with Numeric Reads
A frequent issue happens when you read a number with nextInt() or nextDouble() and then try to read a line of text with nextLine(). The numeric methods read the number but leave the newline (the Enter key) in the input buffer. Then nextLine() immediately reads that leftover newline and returns an empty string.
Example of the problem
System.out.print("Enter your age: "); int age = scanner.nextInt(); System.out.print("Enter your full name: "); String fullName = scanner.nextLine(); // often becomes "" (empty) System.out.println("Name: " + fullName);Fix: Consume the leftover newline
After reading a number, call nextLine() once to consume the remaining newline, then call it again to read the real text.
System.out.print("Enter your age: "); int age = scanner.nextInt(); scanner.nextLine(); // consume leftover newline System.out.print("Enter your full name: "); String fullName = scanner.nextLine(); System.out.printf("Age: %d, Name: %s%n", age, fullName);Alternative approach: Read everything with nextLine and parse
Another reliable pattern is to read user input as strings using nextLine() and then convert to numbers. This avoids the leftover newline issue entirely.
System.out.print("Enter the first number: "); double num1 = Double.parseDouble(scanner.nextLine()); System.out.print("Enter the second number: "); double num2 = Double.parseDouble(scanner.nextLine());If the user types invalid input, parsing methods like Double.parseDouble throw a NumberFormatException. You can later add checks and re-prompting to make the program more robust.
