String to Integer Java

In Java, we often receive numeric values in the form of text called strings. But to do some calculations or store them as numbers, we have to change them to an integer. For example, if the user inputs “25”, it comes as a string, and we have to convert it into a number to use it. Java provides two easy ways to do this: parseInt() and valueOf(). Both can change a string into a number. One returns a primitive int, while the other returns an Integer object

In this guide, you will learn how and when to use different methods to convert a string to int in Java with examples, and also know how to fix errors if the string is not valid.

Table of Contents:

• Why Do We Convert Strings to Integers?
• How to Convert a String to an Integer in Java
  • Method 1: Use Integer.parseInt() to Convert a String to an Integer
  • Method 2: Use Integer.valueOf() to Convert a String to an Integer
• Cases of parseInt() and Integer.valueOf() Methods
• Difference Between parseInt() and valueOf()
• Primitive int vs Integer Class
• When to Use parseInt() vs valueOf()?

Why Do We Convert Strings to Integers?

We convert strings to integers because sometimes we need to do some calculations with the numbers that are hidden inside the text.

There are many reasons to convert a string to an integer. Some of them are as follows:

1. To Perform Math Operations: You cannot perform math calculations on strings.

For example,

String age = "25"; // This is string

You cannot perform calculations directly on this text-based variable related to age; hence, there is a need to first convert the string to an integer to perform maths on this.

2. User Input is usually a String: When users enter data using a form or keyboard, it is captured as a string, like age or marks. The data entered in the form cannot perform calculations, hence it has to be converted to an Integer.

For example,

Scanner sc = new Scanner(System.in);
String age = sc.nextLine();

In the above example, if the user enters the number 30, it will be stored as “30”.

To calculate or validate it, we must convert it:

int realAge = Integer.parseInt(age);

3. Data from Files or APIs Comes as Strings: When reading from files, databases, or APIs, numeric data is often stored as strings. You need to convert it if you want to do calculations, make decisions, or sort or filter by number.

4. Comparisons Work Better with Integers: Comparing strings lexicographically can yield unexpected results, such as returning a negative value when the string ‘100’ is compared to ’20’.

System.out.println("100".compareTo("20"));

will return -1 as an output

While comparing the int as,

System.out.println(100 > 20);

will return the boolean value true

5. Memory Efficiency and Performance: Numbers take up less memory and are faster to process than strings during operations.

How to Convert a String to an Integer in Java

When you receive a number in the form of a string, like “123”, you can convert it to an integer by using the following 2 methods.

Method 1: Use Integer.parseInt() to Convert a String to an Integer

Integer.parseInt() is a method in Java that converts a String into an int, and takes a string that contains digits and returns it as a number. If the string has non-numeric characters, it throws a NumberFormatException.

How to use Integer.parseInt()

To use an Integer.parseInt() method, you have to follow the following syntax.

int var = Integer.parseInt(str);

The var is the int variable, and str is the String variable that has to be converted to an int.

Example:

Number Format Exceptions of the parseInt() Method

A NumberFormatException occurs when the string is passed to Integer.parseInt() is not a valid integer, which means the string contains some characters that can not be converted into a number.

NumberFormatException can occur due to the following reasons:

• If a string contains any letter or symbol.
• If the string has any whitespace.
• If the string is empty.
• If the string is null
• If the number is too large for an int.

A try-catch block is used to handle NumberFormatException in a Java program.

• The try block contains code that may throw an exception.
• The catch block handles the exception if it occurs.

Example:

Output:

Method 2: Use Integer.valueOf() to Convert a String to an Integer

Integer.valueOf() is a method in Java that is used to convert a String into an Integer. It works just like Integer.parseInt(), but instead of returning a primitive int, it returns an Integer object.

If the string has non-numeric characters, it throws a NumberFormatException.

How to use Integer.valueOf()

To use an Integer.valueOf() method, you have to follow the following syntax.

Integer var= Integer.valueOf( str);

The var is the Integer variable, and str is the String variable that has to be converted to an int.

Number Format Exceptions of Integer.valueOf() Method

Output:

In the above Java program, the string str is ” 123″, which has a space at its beginning, due to which the Integer.valueOf() method is unable to convert it into an Integer, and a NumberFormatException is thrown

Cases of parseInt() and Integer.valueOf() Methods

1. Valid positive number: This successfully converts the string into an integer because it contains only valid numeric characters.

For example,

Integer.parseInt("100"); // 100

2. Valid Negative number: This method also works for negative numbers.

For example,

Integer.parseInt("-50"); // -50

3. String with whitespace (invalid): If the string contains whitespace, it is considered invalid and throws a NumberFormatException because Java does not automatically remove the spaces when parsing the integers.

For example,

Integer.parseInt(" 42 "); // NumberFormatException

4. Alphabets in a string: If the string contains any non-numeric characters, it throws a NumberFormatException.

For example,

Integer.parseInt("1a2"); // NumberFormatException

Note: Always validate or clean your string before converting it with Integer.parseInt() to avoid exceptions.

5. Decimal Numbers: If the String has a decimal value in it, then NumberFormatException is thrown.

For example,

Integer num = Integer.valueOf("99.9");

6. Empty String: If the string is empty the NumberFormatException will be thrown.

For example,

Integer num = Integer.valueOf(" ");

Difference Between parseInt() and valueOf()

Both parseInt() and valueOf() are used to convert a string to an int, but they have key differences in return type and use cases.

Key difference between parseInt() and valueOf():

Note: Autoboxing is the automatic conversion of primitive types into their corresponding wrapper class objects.

Primitive int vs Integer Class

When to Use parseInt() vs valueOf()?

The Integer.parseInt() method in Java is used to store numbers as a primitive int. It is good for situations where you only need to perform the calculations or just store the number in a variable. It cannot be used in Java collections like ArrayList, and it cannot be assigned null.

The Integer.valueOf() method in Java is used when you need to store the number in an object form, such as when using collections like ArrayList<Integer>, or when dealing with APIs, databases, or frameworks that expect objects.

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Conclusion

So far in this article, we have learned how we can convert a string to an int in Java using the parseInt() and valueOf() methods. Use parseInt() when you just need a number for math, and use valueOf() when you need the number as an object, like for storing in an ArrayList. Both methods throw a NumberFormatException if the string is not a valid numeric representation

String to Integer Java – FAQs

if (str.matches("\d+")) {
    int num = Integer.parseInt(str);
}