Java Program: User input validation for duplicate integers

Java Exception Handling: Exercise-6 with Solution

Write a Java program that reads a list of integers from the user and throws an exception if any numbers are duplicates.

Sample Solution:

Java Code:

import java.util.*;

public class Duplicate_Number_Check {
  public static void main(String[] args) {
    try {
      List < Integer > numbers = readNumbersFromUser();
      checkDuplicates(numbers);
      System.out.println("No duplicate numbers!");
    } catch (Duplicate_Number_Exception e) {
      System.out.println("Error: " + e.getMessage());
    }
  }

  public static List < Integer > readNumbersFromUser() {
    List < Integer > numbers = new ArrayList < > ();
    Scanner scanner = new Scanner(System.in);

    System.out.print("How many numbers do you want to input? ");
    int count = scanner.nextInt();

    System.out.println("Input the integers:");
    for (int i = 0; i < count; i++) {
      int num = scanner.nextInt();
      numbers.add(num);
    }

    scanner.close();
    return numbers;
  }

  public static void checkDuplicates(List < Integer > numbers) throws Duplicate_Number_Exception {
    Set < Integer > uniqueNumbers = new HashSet < > ();

    for (int num: numbers) {
      if (uniqueNumbers.contains(num)) {
        throw new Duplicate_Number_Exception("Duplicate number found: " + num);
      }
      uniqueNumbers.add(num);
    }
  }
}
class Duplicate_Number_Exception extends Exception {
  public Duplicate_Number_Exception(String message) {
    super(message);
  }
}

Sample Output:

How many numbers do you want to input? 5
Input the integers:
1
2
3
4
5
No duplicate numbers!

How many numbers do you want to input? 6
Input the integers:
1
2
3
3
4
5
Error: Duplicate number found: 3

Explanation:

In the above exercise,

• The program includes three classes: Duplicate_Number_Check, Duplicate_Number_Exception, and the main class.
• The main method is defined in the Duplicate_Number_Check class and serves as the entry point for the program.
• In the main method, we call the readNumbersFromUser method to read a list of integers from the user. We store them in a List<Integer> called numbers.

• Next we call the checkDuplicates method, passing the numbers list as an argument. If there are duplicate numbers in the list, a Duplicate_Number_Exception is thrown.
• If no exception is thrown, we print the message "No duplicate numbers!".
• The readNumbersFromUser method prompts the user to enter the number of integers they want to input and reads the integers from the user. The method returns a list of entered numbers.
• The checkDuplicates method takes a list of integers as a parameter and checks for duplicates using a Set<Integer> called uniqueNumbers. Upon iterating over each number in the list, a Duplicate_Number_Exception is thrown if that number already exists in the uniqueNumbers set.
• The Duplicate_Number_Exception class is a custom exception class that extends the base Exception class. Using the super keyword, it passes a message parameter to the constructor of the superclass.
• IDuplicate_Number_Exceptions are caught in the main method, an appropriate error message is printed.

Flowchart:

Java Code Editor:

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