28. Equality Operators

The equality operators return a boolean (true or false) result from a comparison.

All of the following comparisons evaluate as being true:

assert -99 == -99
assert 'koala' == 'koala'
assert 'cat' != 'dog'
assert 6 != 7

def domesticAnimal = 'dog'
def wildAnimal = 'lion'
assert domesticAnimal != wildAnimal

def str1 = 'Hello'
def str2 = 'Hello'
assert str1 == str2

What Is Equality?

Equality can be a little tricky - both for Groovy and humanity. Think about the statement “Hey cool, we have the same car!”. This could mean that we have the same make and model but different instances of a car or it could mean that we share a car.

def obj1 = new Object()
def obj2 = new Object()
def obj3 = obj1

assert obj1.is(obj3)
assert ! obj1.is(obj2)

assert obj1 != obj2
assert obj1 == obj3

assert ! obj1.equals(obj2)
assert obj1.equals(obj3)

Precedence

In the following example, the equality operator (!=) is evaluated before the assignment operator (=), resulting in the value of truth being the boolean value true:

def truth = 'cats' != 'dogs'
assert truth == true

Overloading Equality

It is possible to define a custom implementation of == by overriding the equals(Object obj) method. This can be handy if your object has a simple method for determining equality, such as comparing staff members by their ID:

class StaffMember {
    def id

    @Override
    boolean equals(obj) {
        if (this.id == obj.id) {
            return true
        } else {
            return false
        }
    }
}

def fred = new StaffMember(id: 12)
def jan = new StaffMember(id: 47)
def janet = new StaffMember(id: 47)

assert fred != jan
assert jan == janet

The Groovy package groovy.transform provides a handy annotation that generates an equals implementation which compares the object’s properties. This reduces the previous StaffMember class to even fewer lines of code:

@groovy.transform.EqualsAndHashCode
class StaffMember {
    def id
}

def fred = new StaffMember(id: 12)
def jan = new StaffMember(id: 47)
def janet = new StaffMember(id: 47)

assert fred != jan
assert jan == janet