When it comes to converting a string of numbers, like: "1234" into a number the first question that jumps to your mind is:
"They look like a number! why do I need to convert a number to a number?!"

Well, the computer doesn't see things the way we see them, a string consists of numbers is not a number! or at least according to the computer.

A string of numbers is a bunch of characters and they do not present a numeric value.

So in Java we have two ways to convert a string of 'numbers' to a real number, and here is how this could be done:

In this post I'm going to talk about converting string to int

Using Integer.parseInt()

This method will return the primitive numeric value of a string that contains only numbers, otherwise it will throw an error (NumberFormatException)

For example:

String testStr = "150";
        try{
            System.out.println(Integer.parseInt(testStr));
        } catch (NumberFormatException e) {
            System.out.print("Error: String doesn't contain a valid integer. " + e.getMessage());
        }

Using Integer.valueOf()

This method will return an integer object of the passed parameter,
if the passed parameter isn't valid it will throw an error.
For example:

String testStr = "200";
        try{
            System.out.println(Integer.valueOf(testStr));
        } catch (NumberFormatException e) {
            System.out.print("Error: String doesn't contain a valid integer. " + e.getMessage());
        }