9- The Comparator Interface

Jun 21, 2026 10:07 · 3:51 · English · Whisper Turbo · 2 المتكلمون

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So you have learned how to sort objects by implementing the comparable interface.

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Now,

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this implementation is not very flexible because it's always comparing

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customers by their name.

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What if tomorrow we introduce new fields like email,

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address,

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and so on?

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We're not going to be able to sort our customers by their email or by their address.

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That's where the comparator interface comes to the rescue.

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This interface is very similar to the comparable interface,

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so it's a generic interface and here we specify the type of objects

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that should be compared with each other.

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So we can create a new class called email comparator that compares

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two customers by their email.

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Let me show you.

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First I'm going to introduce a new field here,

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private string email

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Now let's initialize this through the constructor.

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So we press alt and enter and add a constructor parameter.

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Let's click on refactor.

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There you go.

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Now we can initialize the email via the constructor.

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Of course,

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our main method has broken because IntelliJ didn't know what to pass

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for these emails.

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Let's quickly supply a few values like E3,

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E2,

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and E1.

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Now to compare customers by their email,

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we're going to create a class that implements the comparator interface.

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So,

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in this package,

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I'm going to add a new class.

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Let's call this email comparator.

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Here we should type implements comparator

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of customer.

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Let's import the type and then

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implement this interface.

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Now as you can see,

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this interface has quite a few methods,

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but all of these except the compare method have a default implementation,

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so we don't have to worry about them.

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Now let's implement this method.

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So this method takes two customer objects and returns

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an integer.

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The logic we're going to write here is exactly like the logic we wrote when

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implementing the comparable interface.

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So if the first object is smaller than the second object,

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we're going to return a negative value.

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If it's greater,

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we're going to return a positive value.

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And if they're equal,

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we're going to return zero.

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Now,

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here I'm going to get the email of the first

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customer.

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Now,

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because this is a private field,

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we cannot access it here.

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So we have to go back to our customer class and create a getter for

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that field.

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So,

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over here,

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I'll put the caret on the field name and press alt

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and enter.

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Let's create a getter and a setter for the email field.

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That's pretty easy.

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Now we have this getter,

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getEmail.

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So,

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back to our email comparator.

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We get the email of the first customer.

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This returns a string.

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Now,

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you know that strings are comparable,

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so here we can call the compareTo method and pass the email

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of the second customer.

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So,

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O2, let getEmail.

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And then we return the result.

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As simple as that.

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Now,

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Back to our main method.

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When calling the sort method,

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we can pass a comparator object.

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So the sort method has two overloads,

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one that takes a list and the other that takes a list along with a

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comparator.

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So I'm going to use the second overload and here we pass our

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email comparator.

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Now, when we run this program,

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we're going to see the customer with the name C first,

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because its email is going to come first.

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Then we're going to save the customer with the name A,

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and finally the customer with the name B.

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Take a look.

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So we got C,

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A, and B.

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So this is how we can use the comparator interface.

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