Noob Java Math Question...

[tomtrom77]

I am working through the Java course over at wibit.net (GREAT programming tutorials!) Given that their forums seem to be locked for the moment, I have a question about one of their String examples here:

import java.util.Random;

class StringExample3
{
	public static void main(String[] args)
	{
		Random rand = new Random();
		String outputStr = "";
		Integer n1 = new Integer(rand.nextInt());
		Integer n2 = new Integer(rand.nextInt());
		Integer n3 = new Integer(n1.intValue() + n2.intValue());
		outputStr = n1.toString() + " + " + n2.toString() + " = " + n3.toString();
		System.out.println(outputStr);
	}
}

So when I run the program, most of the time n1 + n2 really does = n3. However, about one in 10 times, it does not. Basically, the output will sometimes look like this: 1603065481 + 1105661559 = -1586240256. This is clearly not right. Like I said, it's a one in 10 occurrence, but this has me wondering. Can someone explain what is going on here? I doubt that there is a math issue within the language itself, or is there? What am I missing?

Thanks,

Tom

[tomtrom77]

[foulkelore]

You might try using long or double instead in int. I got the following info from the JAVA Tutorials

minimum value of -2,147,483,648 and a maximum value of 2,147,483,647

[Tolls]

Yep.
Your ints are wrapping, so use longs or change to using BigIntegers if you can't guarantee a long will be big enough.

[tomtrom77]

Ah! That make sense!

This is one of those things that I have learned but hadn't ever seen in practice, so I didn't recognize it when it popped up!

I appreciate your help!

-Tom

[tomtrom77]

Here is the revised and now-properly-functioning code:

import java.util.Random;

class StringExample3
{
	public static void main(String[] args)
	{
		Random rand = new Random();
		String outputStr = "";
		Integer n1 = new Integer(rand.nextInt());
		Integer n2 = new Integer(rand.nextInt());
		Long n3 = new Long((long)n1.intValue() + (long)n2.intValue());
		outputStr = n1.toString() + " + " + n2.toString() + " = " + n3.toString();
		System.out.println(outputStr);
	}
}