How to Validate Integer Input and Handle Invalid Input in Java using Scanner?

How to Validate Integer Input Using Scanner and Handle Invalid Input

In Java, when working with user input, ensuring the validity of numerical data is crucial for program robustness. This article demonstrates how to use the Scanner class to enforce valid integer input and gracefully handle invalid input, such as non-numerical characters.

Problem:

Consider the following code:

Scanner kb = new Scanner(System.in);
int num1;
int num2 = 0;

System.out.print("Enter number 1: ");
num1 = kb.nextInt();

while(num2 < num1) {
    System.out.print("Enter number 2: ");
    num2 = kb.nextInt();
}

This code aims to prompt users for two integers, ensuring num2 is greater than num1. However, it faces two challenges:

1. It doesn't validate whether user input is actually an integer. This can lead to runtime exceptions when the user enters a non-numerical character.
2. It doesn't handle the case where the user enters num2 as a value less than num1.

Solution:

To address these challenges, we can utilize Scanner's hasNextInt() method. This method returns a boolean indicating whether the next token in the input stream can be interpreted as an integer. If it returns false, we know the input is invalid and can prompt the user to try again.

Here's an improved version of the code:

Scanner sc = new Scanner(System.in);
System.out.print("Enter number 1: ");
while (!sc.hasNextInt()) sc.next();
int num1 = sc.nextInt();
int num2;
System.out.print("Enter number 2: ");
do {
    while (!sc.hasNextInt()) sc.next();
    num2 = sc.nextInt();
} while (num2 < num1);
System.out.println(num1 + " " + num2);

In this code, we check for valid integers using hasNextInt() before attempting to read them. If the input is invalid, we consume the input using sc.next() and prompt the user again.

Advantages:

• Ensures that only valid integers are accepted as input.
• Handles invalid input gracefully, preventing runtime exceptions.
• Forces num2 to be greater than num1.
• Eliminates the need for explicit parseInt() and error handling for NumberFormatException.

Related Questions:

• How do I ignore invalid input and read the next valid value from input stream with Java Scanner?

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