yea the string contains only integer values....what is regex???and how to use it??

Here is the tutorial for Regular Expressions (regex is short for them) you will get to know a lot about them and how to use them on the link.

To just provide a brief about what a regex is and how they would be useful in such operations, take the current example where you just want to check wheter a string is an integer, in other words you want to check whether it contains just digits or other characters too.
So you could define a regex with a pattern such as [\\d*] Here '\\d' means digits (0-9), and * means any number of occurrences of them. After such a pattern has been defined, you just check whether the string satisfies this pattern or not, any string satisfying this pattern would certainly contain only digits and hence would classify to be an integer.

I'm still not sure what you are asking, are you trying to determine if each position in a given string is a digit (numeric)? If so then what was stated above will do just fine. If not what is the purpose of comparing a gigantor number such as xyz...512 characters to an integer.

In your example loop you would simply loop up to the size of the the string and not necessarily the position.

IE: if you need to compare each position in a string to determine if it is in fact a digit you would want something that loops up to the string.length and then use the [] accessors to determine if a specific point is a digit IE: is string[i] a digit.

If you need a mathematical comparison, and the numbers are too large to store in a long, then no, there is no other (uncomplicated) way than with BigInteger. Now, if you think you must do it some other way, than convert the int that you are to compare it to, to a String, than check the lengths of the String. The longer one is the larger. If they are same, start taking one character at time from the front of each String and comparing them (Strings compareTo is good enough for this). As soon as you find one that is larger than the other, you've found the larger number. Now, is it wise to do it this way? No. It is wise to use BigInteger, as already said.

But, as asked once already, what is it that you are actually trying to acheive? If you are attempting to loop some 1000000000000 times, than be prepared to wait, no matter what method you use. And, if you are trying to loop that many times, than the real question is why? What are you doing that you feel you need to loop that long?

convert i to String and use compareTo() function

here is how it works

Try it with

You might be surprised by the outcome.

Anyways it gives the same outcome which is "-1"

Which is saying that s1 is less than s2. Well, is it? ;-)

Yes you are right.

@lipun4 : Read what the docs for compareTo() have to say to understand the point masijade makes here cleverly.

Like I said, though, "-1" says that s1 (which is the lesser number the first time, but the greater number the second time) is less than s2, whereby s1 is significantly greater than s2 the second time, not less than s2 as "-1" indicates.

What I am saying, is that you cannot String compare the entire String at once. That comparison is done alphabetically, not numerically. For single characters, alphabetically and numerically are the same, but not for multicharacter Strings. (E.G. alphabetically 10 is less than 2).