Java How to Convert String to int with Examples

In Java, you can convert a string to an int using Integer.parseInt(yourString), which returns the integer value represented by the string.

Examples

Input"123"

Output123

Input"0"

Output0

Input"-456"

Output-456

How to Think About It

To convert a string to an integer, think of the string as a number written in text form. You want to change it into a number type that Java can use for math. The method Integer.parseInt() reads the string and turns it into an int if the string is a valid number.

Algorithm

1

Take the input string that represents a number.

2

Use a built-in method to read the string and convert it to an integer.

3

Return or use the integer value for further operations.

Code

java

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

Output

123

Dry Run

Let's trace converting the string "123" to an int using Integer.parseInt.

1

Input string

numberString = "123"

2

Convert string to int

number = Integer.parseInt("123") → 123

3

Print result

Output: 123

Step	Operation	Value
1	Input string	"123"
2	Convert to int	123
3	Print output	123

Why This Works

Step 1: Reading the string

The method Integer.parseInt() reads the characters in the string as digits.

Step 2: Converting to int

It converts those digits into an integer number type that Java can use for math.

Step 3: Using the int

The integer can then be stored in a variable or used in calculations.

Alternative Approaches

Using Integer.valueOf()

java

public class Main {
    public static void main(String[] args) {
        String numberString = "123";
        int number = Integer.valueOf(numberString);
        System.out.println(number);
    }
}

Returns an Integer object which is unboxed to int automatically; useful if you want an Integer object.

Using Scanner to read int from string

java

import java.util.Scanner;
public class Main {
    public static void main(String[] args) {
        String numberString = "123";
        Scanner scanner = new Scanner(numberString);
        int number = scanner.nextInt();
        System.out.println(number);
        scanner.close();
    }
}

Useful when parsing input streams or more complex input, but more code than parseInt.

Complexity: O(n) time, O(1) space

Time Complexity

The method reads each character of the string once, so it takes time proportional to the string length, O(n).

Space Complexity

It uses a fixed amount of extra space, so space complexity is O(1).

Which Approach is Fastest?

Integer.parseInt() is the fastest and simplest for converting strings to int compared to alternatives like Integer.valueOf() or using Scanner.

Approach	Time	Space	Best For
Integer.parseInt()	O(n)	O(1)	Simple string to int conversion
Integer.valueOf()	O(n)	O(1)	When Integer object needed
Scanner.nextInt()	O(n)	O(1)	Parsing input streams or complex input