Produce a Java program which, given three sides of a triangle,
determines whether the triangle is:

Equilateral - all sides the same length, 
Isosceles - two sides the same length or 
Scalene - no sides the same length 

Assume that any individual side must be a positive number not greater
than 100.0.

Hi there strongbow.

I've written the program for you. I've included comments, so it should
be fairly self-explanatory, but if anything is unclear, then feel free
to request clarification before rating this answer.

You can download it from: http://xult.org/Triangle.java

Or you can copy and paste from below.

Good luck

--seizer-ga

To compile this code, simply type at the command line: 

javac Triangle.java

To run it, type:

java Triangle

-------- CODE IS BELOW THIS LINE --------

// We need to import this for user input
import java.io.*;

public class Triangle {

	public static void main(String args[]) {
		// declare our variables
		int side1=0, side2=0, side3=0;
		// Read in the lengths of the sides
		try {
			side1 = Integer.parseInt(getInput("Please enter the length of the
first side: "));
			side2 = Integer.parseInt(getInput("Please enter the length of the
second side: "));
			side3 = Integer.parseInt(getInput("Please enter the length of the
third side: "));
		} catch (NumberFormatException e) {
			// We might get this error if the user types in a string, rather
than a valid integer
			System.out.println("Error: " + e.getMessage());
			System.exit(1);
		} catch (IOException e) {
			// We might get this error if there was a problem reading from the
keyboard
			System.out.println("Error: " + e.getMessage());
			System.exit(1);
		}
		
		// Here we do a sanity check. If the user does not type in three
valid sides, we warn them and exit
		if (side1 + side2 + side3 != 180) {
			System.out.println("Warning: sides should add up to 180 to be a
valid triangle");
			System.exit(1);
		}
		
		/* 
		 * Now we determine which triangle it is.
		 *
		 * If side1 is equal to side2, AND  side2 is equal to side3, we know
that
		 * side1 must be equal to side3 because of the rule of inference.
This
		 * means that the triangle is equilateral.
		 *
		 * If side1 is equal to side2, OR side1 is equal to side3, OR side2
is equal to
		 * side3, then the triangle fits the criteria for being an
isosceles.
		 *
		 * If neither of the above are true, then we know that the triangle
must be
		 * scalene.
		 *
		 */
		if (side1 == side2 && side2 == side3) {
			System.out.println("Equilateral");
			System.exit(0);
		} else if (side1 == side2 || side1 == side3 || side2 == side3) {
			System.out.println("Isosceles");
			System.exit(0);
		} else {
			System.out.println("Scalene");
			System.exit(0);
		}
        }
	
	// We use this method to obtain console input from the user.
	public static String getInput(String question) throws IOException {
		// Display the question to the user
		System.out.print(question);
		// obtain their input
		BufferedReader in = new BufferedReader(new
InputStreamReader(System.in));
		// return that input
		return in.readLine();
	}
}

-------- CODE IS ABOVE THIS LINE --------

---

Clarification of Answer by seizer-ga on 13 Nov 2002 16:32 PST

As the commenters below have pointed out, the check

if (side1 + side2 + side3 != 180)

is wrong. I was of course thinking of angles - please excuse my absent
mindedness!

larryjay7's suggestion is an excellent one, and I would recommend the
substitution of his conditional for my erroneous one.

--seizer-ga

Thanks good answer and quick. Had to change bit about sides adding up to 180 though.

What is this line for?

if (side1 + side2 + side3 != 180) { 
   System.out.println("Warning: sides should add up to 180 to be a valid triangle");

Nice program - Is there a way we can get extra credit on this assignment? =)

I think there is a confusion of sides with angles here....

if (side1 + side2 + side3 != 180) {  
   System.out.println("Warning: sides should add up to 180 to be a
valid triangle");

Instead, what you should check for is that no side is greater than the
sum of the other two sides)

if ( (side1 > (side2 + side3)) ||  (side2 > (side1 + side3)) || (side3
> (side2 + side1)) ) {
 System.out.println("Warning: sides do not form a valid triangle");