Convert String To Int In Java Without Using Integer ParseInt()

First Approch

To convert string to an integer.

First, we need to know what place value each digit must be multiplied by.

Scan the digits from right to left because the rightmost position is always the one's place, the next to rightmost is always the ten's, and next too.

This allows us to start at the right end of the string with a place value of 1 and work backward through the string, multiplying the place value by 10 each time we move to a new place.

Converting String to Integer: Algorithm 


       1.   Start number at 0

       2.   If the first character is '-'
                       Set the negative flag
                       Start scanning with the next character
              For each character in the string 
                       Multiply number by 10
                       Add( digit number - '0' ) to number
                If  negative flag set
                        Negate number
                        Return number

StringtoInt1.java 


package com.javacodestuffs.core.strings;
 public class StringtoInt1 {
    
    
    public static void main (String args[])
    {
        String  convertingString = "753211137";
        System.out.println("Before converting String to Integer is  :  "+ convertingString);
        int convetedInt = convertStringToInt( convertingString );
     
        System.out.println("String converted to Integer "+ convetedInt);
        
    }
    
    
    public static int convertStringToInt( String str ){
        int i = 0, number = 0;
        boolean isNegative = false;
        int len = str.length();
        if( str.charAt(0) == '-' ){
            isNegative = true;
            i = 1;
        }
        while( i < len ){
            number *= 10;
            number += ( str.charAt(i++) - '0' );
        }
        if( isNegative )
        number = -number;
        return number;
    }   
 
}
output:
Before converting String to Integer is  :  753211137
String converted to Integer 753211137

Second Way

From the given string, create an array of characters for each element, keep the index saved, and multiply its ASCII value by the power of the actual reverse index. Sum the partial factors and you get it.

We need to cast to use Math.pow (since it returns a double), but you can avoid it by creating your own power function.

StringtoInt2.java 


package com.javacodestuffs.core.strings;
public class StringtoInt2 {

	public static void main(String args[]) {
		String convertingString = "753211137";
		System.out.println("Before converting String to Integer is  :  " + convertingString);
		int convetedInt = convertStringToInt(convertingString);

		System.out.println("String converted to Integer " + convetedInt);

	}

	public static int convertStringToInt(String str) {
		int res = 0;
		char[] chars = str.toCharArray();
		System.out.println(str.length());
		for (int i = str.length() - 1, j = 0; i >= 0; i--, j++) {
			int temp = chars[j] - 48;
			int power = (int) Math.pow(10, i);
			res += temp * power;
			System.out.println(res);
		}
		return res;
	}

}
Before converting String to Integer is  :  753211137
9
700000000
750000000
753000000
753200000
753210000
753211000
753211100
753211130
753211137
String converted to Integer 753211137

Third Approch - Using Java 8

1). Convert srtNum to char.

2). For each char (represented as 'b') -> 'b' -'0' will give the relative number.

3). Sum all in a (initial value is 0) (each time we perform an opertaion on a char do -> a=a*10 .

StringtoInt3.java 


package com.javacodestuffs.core.strings;
public class StringtoInt3 {

	public static void main(String args[]) {
		String convertingString = "753211137";
		System.out.println("Before converting String to Integer is  :  " + convertingString);
		int convetedInt = convertStringToInt(convertingString);

		System.out.println("String converted to Integer " + convetedInt);

	}

	public static int convertStringToInt(String strNum) {
		return strNum.chars().reduce(0, (a, b) - >10 * a + b - '0');
	}

}
output:

Before converting String to Integer is  :  753211137
String converted to Integer 753211137

In this article, we have seen how to  Convert String To Int In Java Without Using Integer ParseInt() with examples. All source code in the article can be found in the GitHub repository.