How to Convert String to Int without using Integer.parseInt() method: Code With Example

As integer can be positive and negative, here two cases arise.

Use cases#

Case#1: If string is positive

If user inputs  "12312", then it should give output 12312 as an int number

Case#2: If string is negative

If user inputs "-47939", then it should give output -47939 as an int number.

Case#3: If string contains alphabetic character like "12ab6", it should print an error.

Approach#

We will traverse the character array as we know String is the array of character.

Step#1: int_value = 0;

Step#2: Traverse the Character array from right to left.

Step#3: Find the place_value of the Character because the ASCII value is different of character to string.

Step#4: Multiplying the place value by 10 each time and add to sum.

              int_value = int_value + place_value * 10;

import java.text.ParseException;

public class IntegerParser {

     public static int parseInt(String str) throws ParseException {

           int i = 0, number = 0;

           boolean isNegative = false;

           char[] value = str.toCharArray();

           if(value[0] == '-') {

                isNegative = true;

                i = 1;

           }

           while(i < value.length) {

                char ch = value[i++];

                int place_value = ch - '0';

                if(place_value>=0 && place_value<=9) {

                     number *= 10;

                     number += (ch - '0');

                } else {

                    System.out.println("Wrong input format!!");

                    throw new ParseException("String to int parse",-1);

                }

           }

           if(isNegative) {

                number = -number;

           }

           return number;

     }

     public static void main (String args[]) throws ParseException {

           String  convertingString="1243";

           int integer = parseInt(convertingString);

           System.out.println(integer);

           convertingString="-1243";

           integer = parseInt(convertingString);

           System.out.println(integer);

          

           convertingString="-12a43";

           integer = parseInt(convertingString);

           System.out.println(integer);

     }

}

Output:

1243

-1243

Wrong input format!!

Exception in thread "main" java.text.ParseException: String to int parse

     at IntegerParser.parseInt(IntegerParser.java:39)

     at IntegerParser.main(IntegerParser.java:17)

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