30. Relational Operators

Similar to the Equality Operators, the expressions involving Relational Operators return a boolean result (true or false).

All of the following operations resolve to true:

assert 5 > 2
assert 4 >= 3
assert 4 >= 4
assert 8 < 9
assert 6 <= 7
assert 7 <= 7

Ordinarily, the operands used in a relational comparison can be compared in a meaningful manner. If they are different data types then the operands need to be able to find a common type for comparison (such as both being numbers) - the following code will cause and exception because Groovy can’t be expected compare a string with a number in this way:

if ('easy' < 123) println "It's easier than 123"

Spaceship

The spaceship operator comes from the Perl programming language. The Spaceship operator is most often seen where sorting is done.

In the example below the sort function uses the closure to define the sort algorithm and this is where the spaceship lands:

def nums = [42, -99, 6.3, 1, 612, 1, -128, 28, 0]

//Descending
println nums.sort{n1, n2 -> n2<=>n1 }

//Ascending
println nums.sort{n1, n2 -> n1<=>n2 }

The following table indicates the result for spaceship expressions (LHS = left-hand side, RHS = right-hand side):

The following assertions all resolve as true:

assert 2 <=> 2 == 0
assert 1 <=> 2 == -1
assert 2 <=> 1 == 1

Overloading the relational operators

The compareTo method is used by Groovy to assess the result of relational operations:

	assert 1.compareTo(2) == -1

Java’s Comparable interface is implemented by classes that allow instances to be compared. Custom classes can determine their own appropriate algorithm for the Comparable’s compareTo method and this will be available when you use the relational operators.

class Num implements Comparable {
    def val

    @Override
    int compareTo(obj) {
         if (val < obj.val) {
             return -1
         } else if (val > obj.val) {
             return 1
         } else {
             return 0
         }
    }
}

def a = new Num(val: 2)
def b = new Num(val: 5)
def c = new Num(val: 2)

assert a < b
assert b > a
assert a != b
assert a == c

You’ll notice that I’ve tested a != b and a == c - these equality operators actually call the compareTo method. There’s been a bit of discussion about how Groovy handles == and the underlying equals and compareTo methods so if you’re looking to overload these operators it’d be worth your time checking up on what the Groovy developers are planning.