How to Convert String to Integer in Java: Tips and Examples

Working with numbers and text in Java is a common task in programming. Sometimes, we need to convert text (strings) into numbers (integers) so we can perform mathematical operations. In this guide, you’ll learn how to turn a string into an integer step-by-step, why this is important, and some key things to watch out for.

Why Do We Convert Strings to Integers?

In programming, strings are used to represent text, while integers are used to work with numbers. For example:

• A user might enter their age into a text box, and it will be stored as a string ("25").
• To check if they are old enough to register for an event, you need to convert the string "25" into the integer 25 and then compare it with the required age limit.

This process ensures the program can correctly process and calculate numerical data.

Methods to Convert Strings to Integers in Java

Java provides simple and efficient methods to handle this task. Let’s explore these methods with examples.

Using Integer.parseInt()

The Integer.parseInt() method is one of the easiest ways to convert a string to int in Java.

public class Main {
    public static void main(String[] args) {
        String numberString = "123";
        int number = Integer.parseInt(numberString);
        System.out.println("The integer value is: " + number);
    }
}

What happens here?

• The string "123" is passed to Integer.parseInt(), which turns it into the integer 123.
• The resulting integer can now be used in mathematical operations.

Common Use Case: If you’re reading user input or fetching data from a database or file, it’s often stored as a string. Using Integer.parseInt() ensures you can work with it as a number.

Using Integer.valueOf()

Another method to convert a string to integer in Java is Integer.valueOf(). This method is very similar to Integer.parseInt(), but it returns an Integer object instead of a primitive int.

public class Main {
    public static void main(String[] args) {
        String numberString = "456";
        Integer number = Integer.valueOf(numberString);
        System.out.println("The Integer object value is: " + number);
    }
}

Key Difference:

• Use Integer.parseInt() if you need a primitive int.
• Use Integer.valueOf() if you need an Integer object, for example, to work with collections like ArrayList<Integer>.

Handling Invalid Strings

What happens if the string cannot be converted into a number? For example:

public class Main {
    public static void main(String[] args) {
        String invalidString = "abc";
        try {
            int number = Integer.parseInt(invalidString);
            System.out.println("The number is: " + number);
        } catch (NumberFormatException e) {
            System.out.println("Invalid string: Cannot convert to an integer.");
        }
    }
}

Why is this important?

• If the string contains non-numeric characters, Java will throw a NumberFormatException.
• Using a try-catch block helps your program handle errors gracefully instead of crashing.

What if the String Has Extra Spaces?

Sometimes, a string may have spaces at the beginning or end. These need to be removed before conversion. You can use the trim() method to clean the string:

public class Main {
    public static void main(String[] args) {
        String numberString = " 789 ";
        int number = Integer.parseInt(numberString.trim());
        System.out.println("The integer value is: " + number);
    }
}

A Practical Example: Adding Two Numbers

Let’s combine everything we’ve learned into a simple program that takes two numbers as strings, converts them to integers, and adds them together:

import java.util.Scanner;

public class Main {
    public static void main(String[] args) {
        Scanner scanner = new Scanner(System.in);

        System.out.println("Enter the first number:");
        String firstNumber = scanner.nextLine();

        System.out.println("Enter the second number:");
        String secondNumber = scanner.nextLine();

        try {
            int num1 = Integer.parseInt(firstNumber);
            int num2 = Integer.parseInt(secondNumber);

            int sum = num1 + num2;
            System.out.println("The sum is: " + sum);
        } catch (NumberFormatException e) {
            System.out.println("Please enter valid numbers.");
        }
    }
}

Explanation:

1. The program reads two strings from the user.
2. It converts them into integers using Integer.parseInt().
3. If the input is invalid, an error message is displayed.