strToInt.java

package test;

public class StringToInt {
	public static int strToInt( String str ){
		  int i = 0;
		  int num = 0;
		  
		  
		  boolean isNeg = false;

		  //check for negative sign; if it's there, set the isNeg flag
		  if( str.charAt(0) == '-') {
		    isNeg = true;
		    i = 1;
		  }

		  //process each char of the string; 
		  while( i < str.length()) {
		    num *= 10;
		    num += str.charAt(i) - '0'; //minus the ASCII code of '0' to get the value of the charAt(i++)
		    i++;
		    //System.out.println(str.charAt(i++) - '0');
		  }
		  

		  if (isNeg)
		    num = -num;
		  return num;
		}
	
	   public static void main(String[]args){
		   String n = "123";
		   System.out.println(strToInt(n));
	   }

}

We should validate our string char may be they not 0-9 so whenever u access a char check it

str[i] >= '0' && str[i] <= 9 then only process it else raise an exception......

so here we done string to int ==> "1234" -->1234

will u try int to string logic

1234 --->"1234" just try it little tricky this your string to int logic.........