Object Equality

What does it mean for two things to be equal? For primitive types (int, double, etc.) this is a simple concept. The two values are either equivalent or not. For objects, this is not always as straightforward. Consider the following code:

Person p1 = new Person("John", "Smith");
Person p2 = new Person("John", "Smith");

Are the two Person objects created above equal? This is not a simple question to answer. In one sense we can see the equality: the name “John Smith” is being used for each person that’s being created. On the other hand, we also know that it is possible for two people to have the same name, so it is possible that the above code is intended to represent two similarly named individuals.

In the last module we learned how to define our own objects using classes. Just like we are able to define the instance variables and methods within our classes, we can also define what it means for our objects to be equal. To see why this matters, consider this following extension of the previous example:

Person p1 = new Person("John", "Smith");
Person p2 = new Person("John", "Smith");

Set<Person> pSet = new HashSet<>();
pSet.add(p1);
pSet.add(p2);

How many Person objects will the HashSet contain? If equality was determined by name, then we might expect the set to only contain one Person since the other person would be ignored as a duplicate. Is this what we want? Do we know for certain what would happen?

If we wish to use a different definition of equality, then we can do this by implementing our own equals() method.

When adding objects into a Set it uses the equals() method to determine equality of two objects and thus determine if a duplicate already exists in the set. The definition of equals() is critical. It is important to base equals() on the instance variables that determine true, meaningful equality for the object. Sometimes that may mean using all or most of the instance variables to determine equality, sometimes maybe just one or two. Fortunately IDEs (like Eclipse) make generating these methods a lot easier, as you will see in the following exercise:

coding exercise Coding Exercise

In this exercise, you use Eclipse to auto-generate useful parts of a class.

  1. Find and open the Point class.

  2. Based on the comment you see there concerning has-a, declare the instance variables for this class, calling them x and y.

  3. Auto-generate the constructor by choosing Source, then Generate Constructor using Fields, then select Constructor from the pop-up menu.

  4. Choose the fields x and y, and OK to generate the constructor.

  5. These fields will be immutable, so edit the declarations for x and y and add the final attribute. This ensures that the values of x and y can be set only by the constructor.

  6. Now use Eclipse to generate the getters for x and y.

  7. In the main() method, construct and print a Point or two and verify that things look good.

They should not look so good. When you print a Point, you getan ugly and uninformative String. To get something better, we must override the toString() method.

  1. Use Eclipse similarly to generate a simple toString() method.

This, and the subsequent work below, is accomplished also via the Source menu.

  1. OK we are ready to make a Set of some points. Put the following code in the main method:

Set<Point> set = new HashSet<Point>();
set.add(new Point(131, 132));
set.add(new Point(131,132));
System.out.println("Set has " + set);

  1. Recalling that sets should have no duplicates, we can see that there’s a problem here since this set will show the same point (131, 132) twice. This is because equality for a Point has not yet been defined.

  2. Now let’s use Eclipse to generate hashCode() and equals(Object other) methods.

You must pick the attributes (instance variable names) upon which you wish equality to be based.

  1. Rerun your code and make sure that two Point objects with the same coordinates cannot both be contained in the same set.

Further exploration

  1. What happens if hashCode() returns a random integer? Try:

public int hashCode() {
   return (int)(Math.random()*100000);
}

The above code makes hashCode() inconsistent, breaking a portion of the contract concerning object equality.

  1. What happens if hashCode() always returns the same result?

public int hashCode() {
   return 0;
}

Now try lists

Go back and change the Set and HashSet of your code to use List and LinkedList, respectively.

  1. You should see duplicates. But that’s OK for lists.

  2. Intentionally break .equals(Object other) by returning false always.

What aspect of the contract does this break for `.equals(Object other)?

  1. What behavior do you see?

Adding to a list does not consult .equals(Other object), so you should see no difference.

  1. Now ask whether the list .contains(new Point(131,132)).

Nothing should be found in the list with the broken .equals(Object other).

  1. Revert the code back to its proper hashCode and .equals(Object other).

equals() vs. ==

We have now seen two ways of determining equality: == and equals(). Why are there two ways for determining equality? And how do you know which one to use?

For primitive types (ints, doubles, etc.), the answer is simple: always use ==. This will work every time for these simple values.

For objects, you need to think a bit more about what it is that you are trying to determine. equals() will examine the contents (instance variables) of the objects being compared and use that to make its determination. This means that two separate objects can be considered equal if some or all of their instance variables are equal, as defined by the equals() method.

When comparing objects, == will result in true only if the objects on the left and right side are the same object.

coding exercise Coding Exercise

Open the Person program, and work through the provided examples. Make sure that you understand the difference between equals() and == before moving on!

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