What are the purposes of the functional interfaces ToDoubleBiFunction<T, U>, ToIntBiFunction<T, U>, and ToLongBiFunction<T, U>?

These interfaces are used to represent functions that take two arguments of different or the same types (T and U) and return a primitive type: ToDoubleBiFunction returns double, ToIntBiFunction returns int, and ToLongBiFunction returns long.

They are used when a binary operation needs to be performed on two input values to produce a primitive numeric result. Typical scenarios include:

• Calculating a numeric value based on two objects.
• Transforming a pair of objects into a primitive number.
• Using in streams for aggregating operations or transformations.

Example of ToDoubleBiFunction usage:

// Calculating distance between two points
ToDoubleBiFunction<Point, Point> distanceCalculator = (p1, p2) ->
    Math.sqrt(Math.pow(p1.getX() - p2.getX(), 2) + Math.pow(p1.getY() - p2.getY(), 2));

// Point class for example
class Point {
    private double x;
    private double y;

    public Point(double x, double y) {
        this.x = x;
        this.y = y;
    }

    public double getX() {
        return x;
    }

    public double getY() {
        return y;
    }
}