How to Convert a String Class to an Integer Class in Java?

String in Java is used to store the sequence of characters in Java. A string can contain a character, a word, a sentence, or a paragraph. Integer is a Wrapper class that is used to store Integer values in Java. The range of the Integer class is from -2,147,483,648 to 2,147,483,647 (-2^31 to 2^31 - 1).

In this article, we will learn to Convert a String to an Integer in Java.

Examples of Converting a String to an Integer

Input: "213456"
Output:  213456

Input: "2134567"
Output: 2134567

To implement this we have two methods as given below.

1. User Implementation

In this method, we will write a user-defined function to convert the given string into an Integer. The idea is to iterate the string from start to end. First, we will declare an Integer variable result to store the Integer value. Then we will simply loop through the string and at each iteration we will multiply the current result by 10 and then add the current character to the result by converting the character to int. We will perform the following tasks to implement this algorithm.

1. Declare an Integer variable called result to store the ans.
2. Loop from start to end on the given string.
3. At each iteration multiply the previous result by 10 so that it will shift by one position on the left.
4. Convert the current character of the string to int and add it to the result.
5. Return the result.

Below is the Program to Convert a String to an Integer with User Implementation:

public class StringToInteger {

    public static Integer convertStringToInteger(String str) {
        Integer res = 0;
        
        for (int i = 0; i < str.length(); i++) {
            // Multiply the given result by 10 so that it will shift it by one position on left
            res = res * 10;
            
            // Convert the current character to Integer and then add it to the previous result
            res += (s.charAt(i) - '0');
        }
        
        return res;
    }
    
    public static void main(String[] args) {
        String inp1 = "213456";
        System.out.println( convertStringToInteger(inp1));
        
    }
}

Output:

21456

Complexity of the above Program:

Time complexity: O(N)
Space complexity: O(1)

2. Using InBuilt Java function

In this method instead of writing custom logic we will use inbuilt method given by Integer class in java. The method is "Integer.parseInt()". We will give the string to this method and it will give us the resulting int as the ans. Following are the steps to implement the same.

1. Declare a Integer variable result.
2. Pass the given string to the "Integer.parseInt()" function.
3. Store it int the result variable.
4. Return the result.

Below is the Program to Convert a String to an Integer with User InBuilt Function:

public class Helper {
    public static Integer convertStringToInteger(String s) {
        // Parse the string to an integer
        Integer result = Integer.parseInt(s);
        return result;
    }

    public static void main(String[] args) {
        // Example input string
        String inp1 = "213456";

        // Convert the input string to an integer
          // Using the convertStringToInteger method
        Integer output = convertStringToInteger(inp1);

        // Print the result
        System.out.println(output);
    }
}

Output:

21456

Complexity of the above Program:

Time complexity: O(N)
Space complexity: O(1)