How to Convert String to int in Java

Last Updated on October 22, 2024 by jt

String to int conversions are a common task in Java, especially when dealing with user input or API calls. Java provides several ways to perform this conversion. This blog post will cover the most common methods for converting a String to an Integer and cover best practices to avoid potential errors.

String to int Using Integer.parseInt()

Using Integer.parseInt() is the most common way to convert a String into an int. This method accepts a String value and returns its integer value.

Example:

public class StringToIntExample {
    public static void main(String[] args) {
        String str = "123";
        int i = Integer.parseInt(str);
        System.out.println("Converted integer: " + i);
    }
}

-----------------output----------------

Converted integer: 123

This is a simple example. This will throw a NumberFormatException if the input string cannot be parsed as a valid integer.

Following is an example catching the exception:

try {
    int i = Integer.parseInt(str);
} catch (NumberFormatException e) {
    System.out.println("Invalid input: " + e.getMessage());
}

String to int Number Base Conversion

The Integer.parseInt() method also supports converting from other number bases (e.g., binary, hexadecimal). You can specify the number base as a second argument.

Here is an String to int example of converting from binary and hexadecimal.

int binary = Integer.parseInt("101", 2);  // Binary to decimal
int hex = Integer.parseInt("7F", 16);     // Hexadecimal to decimal

Using Integer.valueOf()

Another way to convert a string to an integer is using Integer.valueOf(). It works similarly to parseInt(), but instead of returning a primitive int, it returns an Integer object (a wrapper class).

Here is an example of using Integer.ValueOf():

Integer number = Integer.valueOf("456");
System.out.println("Integer value: " + number);

Using Integer.decode() for Different Number Formats

We can also use Integer.docode() to convert Strings in different formats to Integers.

For example:

int decimal = Integer.decode("123");
int hex = Integer.decode("0x7F");     // Hexadecimal
int octal = Integer.decode("077");    // Octal

Handling Conversion Safely with Apache Commons

For more robust error handling, you can use libraries like Apache Commons, which provide utility methods that avoid throwing exceptions. These help reduce blocks of verification and exception handling in your code.

Example using Apache Commons:

import org.apache.commons.lang3.math.NumberUtils;

int number = NumberUtils.toInt("789", -1);  // Returns -1 if conversion fails

String to Integer Best Practices

Check for Null Values – Passing a null string to parseInt() or valueOf() will throw a NullPointerException. Always check for null before attempting conversion.
Check For Non-Numeric Characters: Strings containing non-numeric characters will cause a NumberFormatException. You should validate input to ensure it’s numeric before attempting conversion.
Empty Strings: Attempting to parse an empty string ("") will also throw a NumberFormatException. Ensure the string is not empty before converting.
Consider Using Third Party Libraries – Libraries like Apache Commons or Google Guava can simplify validation and error handling