Java - program not reading 'else' properly? (logic error)

Question

I have been working on this program for determining if a right triangle can be formed by three user-inputted side lengths. My program uses the Pythagorean Theorem , being a2 + b2 = c2 . I can get the program to recongnise when a triangle CANNOT be built, but I can't seem to get it to recognise when it can be built.

Any suggestions? I am unsure if this is an error with my if statements, or a simple logic error. (I am a total beginner at coding, so I apologise for the simplicity of this question).

String myInputA = JOptionPane.showInputDialog(null, "Hello, and welcome to the 'Right' Triangle Tester.\nThis program will determine if three side lengths form a right triangle. \nPlease input the first side length below.","Right Triangle Tester",JOptionPane.INFORMATION_MESSAGE);
    String myInputB = JOptionPane.showInputDialog(null, "Great. Please enter the second side below.","Right Triangle Tester", JOptionPane.INFORMATION_MESSAGE);
    String myInputC = JOptionPane.showInputDialog(null, "Please enter the last side below.","Right Triangle Tester", JOptionPane.INFORMATION_MESSAGE);
    double sideA = Double.parseDouble(myInputA);
    double sideB = Double.parseDouble(myInputB);
    double sideC = Double.parseDouble(myInputC);
    if ((sideA * sideA) != ((sideB * sideB) + (sideC * sideC)))
    {
JOptionPane.showMessageDialog(null,"I am sorry. Those side lengths do not form a right triangle.","Right Triangle Tester", JOptionPane.ERROR_MESSAGE);
    }
    else if ((sideB * sideB) != (sideC * sideC) + (sideA * sideA))
    {
JOptionPane.showMessageDialog(null,"I am sorry. Those side lengths do not form a right triangle.","Right Triangle Tester", JOptionPane.ERROR_MESSAGE);
    }
    else if ((sideC * sideC) != (sideA * sideA) + (sideB * sideB))
    {
        JOptionPane.showMessageDialog(null,"I am sorry. Those side lengths do not form a right triangle.","Right Triangle Tester", JOptionPane.ERROR_MESSAGE);
    }
        else 
    {
    JOptionPane.showMessageDialog(null, "Congratulations, those side lengths form a right triangle.","Right Triangle Tester", JOptionPane. INFORMATION_MESSAGE);
}

Answer 1

Your logic is wrong. If at least one value^2 is the value of the other two, it's a triangle.

In your logic, you'll get to the else part only if all three values are sum of the other two (which can only happen if all are 0)

Answer 2

Change your condition checking to be like below. Check for right angle condition in if block; else show the error message.

bool IssideA = (sideA * sideA) == ((sideB * sideB) + (sideC * sideC));
bool IssideB = (sideB * sideB) == (sideC * sideC) + (sideA * sideA);
bool IssideC = (sideC * sideC) == (sideA * sideA) + (sideB * sideB);

    if (IssideA || IssideB || IssideC)
    {
JOptionPane.showMessageDialog(null, "Congratulations, those side lengths form a right triangle.","Right Triangle Tester", JOptionPane. INFORMATION_MESSAGE);
    }
    else 
    {
        JOptionPane.showMessageDialog(null,"I am sorry. Those side lengths do not form a right triangle.","Right Triangle Tester", JOptionPane.ERROR_MESSAGE);    
    }

Answer 3

Mathematically, if a ² + b ² = c ² , then c must be larger than a or b. If you determine which length is the hypotenuse, your logic will simplify, because if a ² + b ² = c ² , then a ² + b ² + c ² = 2 * c ² ; you don't actually have to know which is which.

Given:

double a,b,c;

To determine if the triangle is a right angle, find the hypotenuse and compare the sum of squares with twice the hypotenuse squared:

double h = a > b ? (a > c ? a : c) : (b > c ? b : c); 
if (h * h * 2 == a * a + b * b + c * c)
    // yes!

Note that you may fall foul of the imprecision of the double type. I would recommend converting this code to us BigDecimal to avoid this problem.