8- Comparable Interface

Jun 20, 2026 06:18 · 5:38 · English · Whisper Turbo · 2 speakers

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Lecture Index

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Earlier I briefly mentioned the comparable interface.

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We use this interface to compare two objects like two user objects.

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Now,

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what is the benefit of this?

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Well,

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a lot of sorting algorithms are based on comparing objects.

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So with these comparisons,

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we can determine what objects should come first.

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I've talked about this in detail in my ultimate data structures and algorithms

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course, which is the course that I recommend you to watch after finishing the advanced Java

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course.

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Now let's see how we can use the comparable interface for comparing objects.

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So here on Java documentation,

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you can see that comparable is a generic interface,

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because here we have a type parameter.

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And if you scroll down,

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you can see that this interface has a single method called compareTo,

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which takes an object.

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Now the type of this object is t,

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which is the type parameter for this generic interface.

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So to be able to compare two user objects,

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we should implement this interface in our user class.

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So here in the user class we type implements comparable

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of user.

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Now we have a compilation error because we haven't implemented this interface yet.

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So we press alt and enter and ask IntelliJ to

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implement the methods of this interface.

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IntelliJ is telling us that the comparable interface has a single method,

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which is highlighted,

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which means we're going to implement this.

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So,

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okay,

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and here's our comparative method.

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Now look at the type of parameter of this method.

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It's user,

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because we passed the user class over here.

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Now let me show you what happens if you forget to pass this generic

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type argument.

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So,

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I'm going to undo the changes.

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and simply type implements comparable without

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the generic type argument.

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Okay, now let's implement the methods.

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Look at the type of this parameter.

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It's object.

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So instead of comparing a user object with another user object,

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now we are comparing this with an instance of the object class.

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Earlier I told you the problems with the object class.

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We have to do an explicit cast here and it is possible to get a casting exception

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at runtime.

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So this approach is unsafe and that's why we should always

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specify the generic type argument.

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So here we pass the user class as the generic type argument.

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Now,

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here's the logic we should implement here.

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If the current user object is less than this

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other user object,

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that is this parameter here,

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then we should return a negative value.

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It could be negative one,

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negative two,

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whatever, it doesn't really matter.

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If they're equal,

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then we should return 0.

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And if the current user object is greater than the other user object,

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then we should return a positive value.

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Again, it could be 1,

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2, it doesn't matter.

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So on what basis are we going to compare these two user objects?

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In other words,

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how can we say if a user object is greater than another user object

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or not?

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Well,

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let's assume that we're going to compare users by their points.

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Later,

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we'll be able to sort users by their points.

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We'll look at that in the next section.

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So let's declare a private field of type

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integer called points.

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Now we want to initialize this from the constructor.

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So let's quickly add a constructor public user integer

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points, and here we set this .points equals

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points.

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Now back to our compareTo method.

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Here's one way to implement this logic.

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If points is less than o

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.points, then we return negative 1.

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If they're equal,

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then we return 0 and otherwise

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we return negative 1.

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But there is a better way.

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We can simply return points minus

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O dot points.

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So if this user has more points than the other user,

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the result of this expression is going to be a positive number.

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If they're equal,

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it's going to be 0.

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And if this user has fewer points than the other user,

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the result of this expression is going to be a negative number.

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So we don't really need these few lines over here.

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Okay?

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Now also for clarity,

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I would prefer to rename this argument to other.

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That's better.

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So this is how we can implement the

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comparable interface.

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Now back in the main class,

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let's compare two user objects.

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So here's one user,

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new user with 10 points.

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And here's another user with 20 points.

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Now in Java we cannot write code like this if user1 is

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less than user2.

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We can only use this operator to compare numbers and characters.

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So to compare two user objects,

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we'll have to use our compareTo method.

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We pass user2 here,

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then we check to see if the result is less than 0.

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We can print a message like user1 is

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smaller than user2.

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else if user 1 .compared to user 2 equals

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0, then we can print user 1 equals

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user 2.

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Otherwise,

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we're going to print user 1 is greater than user

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2. Let's run the program.

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So user 1 is less than user 2

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because it has fewer points.

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