How to Loop a Java Program With A Prompt

A Beginner’s Guide to User-Controlled Loops

Before You Start: How to Run Java Code

If you’ve never coded in Java before, follow these steps:

Install Java:
- Download from Oracle’s official site
- Verify installation with java -version in Command Prompt/Terminal
Install a Code Editor:
- Beginners: Use VS Code with the “Extension Pack for Java”
- Alternative: IntelliJ Community Edition
Create Your First File:
- Make a new file named RetryLoop.java
- Copy-paste the code examples below
- Press ▶️ in your editor or run javac RetryLoop.java && java RetryLoop in Terminal

Why This Matters

User interaction loops help create programs that:
Repeat actions until users are done
Handle errors gracefully
Build interactive experiences like games and tools

The Core Solution: `while` + `Scanner`

What this does: Creates a simple “Try Again?” prompt
How to use: Replace System.out.println("Running program...") with your actual code

java<code>import java.util.Scanner;  <em>// Required for user input</em>

public class RetryLoop {
    public static void main(String&#91;] args) {
        <em>// Step 1: Create input scanner</em>
        Scanner scanner = new Scanner(System.in);
        boolean retry;
        
        <em>// Step 2: Create the retry loop</em>
        do {
            <em>// YOUR CODE GOES HERE - REPLACE THIS LINE!</em>
            System.out.println("Running program...");
            
            <em>// Step 3: Ask to continue</em>
            System.out.print("Try again? (yes/no): ");
            String input = scanner.nextLine().trim().toLowerCase();
            retry = input.equals("yes");
            
        } while (retry);  <em>// Step 4: Repeat if user says "yes"</em>
        
        <em>// Step 5: Clean up</em>
        scanner.close();
    }
}
</code>

java<code>import java.util.Scanner; // Required for user input

public class RetryLoop {

public static void main(String[] args) {

// Step 1: Create input scanner

Scanner scanner = new Scanner(System.in);

boolean retry;

// Step 2: Create the retry loop

do {

// YOUR CODE GOES HERE - REPLACE THIS LINE!

System.out.println("Running program...");

// Step 3: Ask to continue

System.out.print("Try again? (yes/no): ");

String input = scanner.nextLine().trim().toLowerCase();

retry = input.equals("yes");

} while (retry); // Step 4: Repeat if user says "yes"

// Step 5: Clean up

scanner.close();

}

</code>

Key Features:
▸ Works for basic programs
▸ Easy to understand
▸ Requires no extra tools

Advanced Implementation: Error Handling

What this adds:

Number validation
Max 3 attempts
Clear error messages

java<code>public class RetryWithLimits {
    public static void main(String&#91;] args) {
        Scanner scanner = new Scanner(System.in);
        int maxAttempts = 3;  <em>// Change to allow more/fewer tries</em>
        int attempts = 0;
        boolean success = false;
        
        do {
            try {
                System.out.print("Enter a number (1-100): ");
                int num = Integer.parseInt(scanner.nextLine());
                
                if(num &lt; 1 || num &gt; 100) {
                    throw new IllegalArgumentException();
                }
                
                System.out.println("Valid number: " + num);
                success = true;
                
            } catch (Exception e) {
                System.out.println("Invalid input! Attempts left: " + (maxAttempts - attempts));
                attempts++;
            }
            
        } while (!success &amp;&amp; attempts &lt; maxAttempts);
        
        scanner.close();
    }
}
</code>

java<code>public class RetryWithLimits {

public static void main(String[] args) {

Scanner scanner = new Scanner(System.in);

int maxAttempts = 3; // Change to allow more/fewer tries

int attempts = 0;

boolean success = false;

do {

try {

System.out.print("Enter a number (1-100): ");

int num = Integer.parseInt(scanner.nextLine());

if(num < 1 || num > 100) {

throw new IllegalArgumentException();

}

System.out.println("Valid number: " + num);

success = true;

} catch (Exception e) {

System.out.println("Invalid input! Attempts left: " + (maxAttempts - attempts));

attempts++;

}

} while (!success && attempts < maxAttempts);

scanner.close();

}

</code>

How to Modify:

Change maxAttempts to allow more tries
Adjust number range (1-100) in the if-statement

Pro Tips for Beginners

Test Your Code:

Try entering “YES”, “no”, and “maybe”

Test with numbers like 0, 50, and 101

Common Errors:

java<code><em>// WRONG: Forgets to close scanner</em> <em>// scanner.close();</em> <em>// WRONG: Uses wrong comparison</em> <em>// if(input == "yes") // Use .equals() instead!</em></code>

1	java<code><em>// WRONG: Forgets to close scanner</em> <em>// scanner.close();</em> <em>// WRONG: Uses wrong comparison</em> <em>// if(input == "yes") // Use .equals() instead!</em></code>

Next Steps:

Add a score counter for games

Create multiple retry points

Add a delay between tries with Thread.sleep(1000)

Real-World Examples

Quiz Game:

java

<code>do { askQuestion(); checkAnswer(); System.out.print("Next question? (yes/no): "); } while (scanner.nextLine().equalsIgnoreCase("yes"));</code>

1	<code>do { askQuestion(); checkAnswer(); System.out.print("Next question? (yes/no): "); } while (scanner.nextLine().equalsIgnoreCase("yes"));</code>

Data Entry Tool:

java

<code>while (true) { try { enterData(); break; <em>// Exit on success</em> } catch (Exception e) { System.out.println("Error! Try again."); } }</code>

1	<code>while (true) { try { enterData(); break; <em>// Exit on success</em> } catch (Exception e) { System.out.println("Error! Try again."); } }</code>

Troubleshooting:
“Cannot find Scanner” error?
→ Ensure import java.util.Scanner; is at the top
Program closes immediately?
→ Use scanner.nextLine() instead of scanner.next()

Practice Exercise:
Create a password checker that:

Gives 3 attempts
Shows “Access granted” on correct input
Exits on too many failures

Need Help?
Java Installation Guide
VS Code Java Setup Tutorial
Official Java Tutorials