I’m glad you’re back. The last few lessons were heavy, so in this one we’re going to slow down and integrate. We’ll discuss object equality and how to make object copies. While these are new, they are largely straightforward applications of things that we already know.
What does it mean for two things to be the same? On one hand this is a deep epistemological question. But in our computer programs, it has practical importance.
For example, I might have a list of items and want to remove all of the duplicates, or find something. I might require a key of some kind to access some information, and need to be able to tell whether the key you present is the same as the one that is required. These are examples of places in computer code where equality matters.
Java’s notion of equality is left up to us, the class
designer, to define.
Let’s look at an example of how.
Object.equals
Object.equals
equals
is one of the methods defined by Object
.
That means, if we don’t override equals
, our class
es inherit the equals
method defined by Object
.
This doesn’t do nothing, but it’s not particularly useful.
Let’s see how it works:
Create a public class named Course
.
Course
should define a single public constructor that accepts three fields: a String
(the department),
a String
(the course number, and yes, this must be a String
) and an int
(the enrollment).
You are welcome to name these fields however you like!
Your constructor should reject null
departments and numbers and negative enrollments.
You do not need to provide any getters and setters!
Instead, define a single method named equals
.
Your equals
method accepts an Object
as a single parameter.
It should return true
if the passed object is a Course
and has the same values for the department and number,
and false
otherwise.
It may surprise you to learn that Java has no built-in way to copy an object.
Object
does provide a clone
method, but even the Java architects themselves recommend against it.
The reasons for this are somewhat out of scope for us at this point. Put simply, objects may have complex internal state that is difficult or impossible to copy. Put another way, not every object may represent something that can be copied.
However, when our objects can be copied we can enable this using a pattern called a copy constructor. Let’s look at how to do that together:
Create a public class named Restaurant
.
Restaurant
should define a single public constructor that accepts three fields: a String
(the name),
a String
(the type of cuisine) and an int
(the capacity).
You are welcome to name these fields however you like!
Your constructor should reject null
names and cuisines and negative or zero capacities.
You do not need to provide any getters and setters!
Instead, define a single method named equals
.
Your equals
method accepts an Object
as a single parameter.
It should return true
if the passed object is a Restaurant
and has the same values for the name and cuisine,
and false
otherwise.
Create a public class named Location
.
Location
should define a single public constructor that accepts three fields: a String
(the description),
and a latitude and longitude position, as double
values, in that order.
You are welcome to name these fields however you like!
Your constructor should reject null
descriptions and invalid latitude and longitude values.
Valid longitude values are between -180.0
and 180.0
, inclusive, while valid latitude values are between
-90.0
and 90.0
, inclusive.
You do not need to provide any getters and setters!
Instead, define a single method named equals
.
Your equals
method accepts an Object
as a single parameter.
It should return true
if the passed object is a Location
and has the same position as the passed object, and
false
otherwise.
James Mickens is a Harvard professor who does work on distributed systems and large-scale web services. He’s had papers published in the most prestigious venues in systems and networking, and received tenure at Harvard in 2019(1).
James is also funny. Very funny! You can find his technology-driven humor in written form, but at least I think he’s at his best when giving a talk. Here’s one brief example, that also touches on an important issue:
(And if you have more time, this longer talk is a personal favorite.)
Need more practice? Head over to the practice page.