Question

I created a class called Foo. Foo has three fields, which are all ints: x, y, and z. I want to make a PriorityQueue<Foo> which prioritizes the Foo objects differently in different situations. For instance, I might want to prioritize by x value, or maybe by y value, or maybe by z. But I won’t know which value I want to prioritize by until runtime. I’ve heard you can use comparators to somehow impose an ordering on the fly, which I think would be perfect here.

I’m confused as to exactly how I would do this though. Could someone please show me an example if say I wanted to prioritize on x using a comparator (without having to override the compareTo function in my Foo class)?

Thank you very much.

Answer 1

A comparator is a parameterized interface that allows you to define how two instances of the parameterized type can be compared. let’s assume you have the following class

class Foo {
   int x;
   int y;
   int z;
}

Then to define a comparator that orders elements based on their x value then y then z we’d do the following

class XyzComparator implements Comparator<Foo> {
    @Override
    public int compare(Foo foo1, Foo foo2) {
          if(foo1.x != foo2.x) {
               return Integer.compare(foo1.x, foo2.x);
          }
          if(foo1.y != foo2.y) {
               return Integer.compare(foo1.y, foo2.y);
          }
          return Integer.compare(foo1.z, foo2.z);
    }
}

Similarly you can define comprators that compare elements based first on their y value then x the z…etc
Finally at runtime you can instantiate a PriorityQueue with that comparator

PriorityQueue<Foo> queue;
if(x_then_y_then_z) {
    queue = new PriorityQueue<Foo>(10, new XyzComparator());
} else if (y_then_x_then_z) {
    queue = new PriorityQueue<Foo>(10, new ZxyComparator());
}

For more information take a look at the priority queue javadoc as well as the comparator javadoc

Edit: Please see @buritos comment regarding integer overflow.