Question

I am trying to create a program in which – I am putting this as simply as possible – I find the sides of a triangle.

The problem with doing so is that there might be another triangle that has already been found with the exact same sides. For example:

If the program find a triangle with sides 1, 2, 1 and another with 1, 1, 2, then, because of the rules of what I am doing, I will not need to use this triangle again since it has the same sides, just in a different order.

I would like to create a class that I can enter the triangle values in, like = new triangle(x,y,z) where x, y, and z are all integers of the sides.

This is not the problem, as I already have the class. The question is how I can check to see if the triangle has already been made with the same sides as the one I am entering?

Thanks.

Answer 1

Override equals and hashCode of Triangle to detect “same” triangles, and keep a HashSet<Triangle> with all Triangle created so far.

For instance, if you want to print the Triangles without duplicates, you could do:

Set<Triangle> triangles = new HashSet<Triangle>();
for (String line : file) {
    Triangle t = makeTriangle(line);
    triangles.add(t); // does nothing if an equal Triangle is already in the set
}
System.out.println("The distinct triangles are:");
for (Triangle t : triangles) {
    System.out.println(t);
}

where

class Triangle {
    private int[] sides;

    public Triangle(int x, int y, int z) {
        sides = new int[] {x,y,z};
        Arrays.sort(sides);
    }

    @Override public boolean equals(Object o) {
        return o instanceof Triangle && Arrays.equals(sides, ((Triangle) o).sides);
    }

    @Override public int hashCode() {
        return Arrays.hashCode(sides);
    }

    @Override public String toString() {
        return Arrays.toString(sides);
    }
}

Note: This approach requires that you create a new Triangle to check whether it is a duplicate. One could extends the approach to work around object allocation, but this would complicate the code for little performance gain.