8- The Comparable Interface

Jun 21, 2026 09:49 · 4:28 · English · Whisper Turbo · 2 Højttalere

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Forelæsningsindeks

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In this video I'm going to show you how to sort data using the comparable interface.

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So first we're going to add a new class to our collections package.

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We're going to call that customer.

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In this class we're going to add a private field,

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private string,

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name,

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and we want to initialize this from the constructor.

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So we press command and N on Mac or control and N

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on Windows to bring up the generate palette and select constructor and

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then select the name field.

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So, we're initializing this field through our constructor.

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Super easy.

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Now,

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back to our main class,

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I'm going to create a list of customers.

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Let's import these types,

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and then set this to a new array

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list.

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Now let's add three customers to

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this list.

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So customers .add,

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here's one customer.

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I want to set the name to B,

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another customer with the name A,

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and finally the last customer with the name C.

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S… Speaker 1 (8- The Comparable Interface)

Now, to sort this list,

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we can use the collections class.

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You learned about this earlier.

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So earlier we talked about the addAll method.

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Here we have another method for sorting lists.

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So,

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we can pass the customer's object over here,

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but we get a compilation error.

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No instances of type variables t exists so that customer confirms

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to comparable.

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I know this is very confusing.

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Basically all this is telling us is that the customer class does not implement

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the comparable interface.

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Now why does this matter?

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Well, a lot of sorting algorithms work by comparing objects.

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So if we can compare two objects,

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we can determine which object should come first,

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which one should come next,

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and so on.

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So to fix this problem,

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we should implement the comparable interface in our customer class and that

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is super easy.

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You learned this in the last section.

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So over here we type implements comparable

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of customer because we

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want to compare a customer with another customer.

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Now we press alt and enter and implement this

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interface.

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Now,

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what should we do in this method?

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Let me quickly refresh your memory.

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We want to compare two objects.

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If this object is smaller or less than the other object,

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we should return a negative value,

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like minus 1.

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It could be anything.

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If these two objects are equal,

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we should return 0.

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And if this object is greater than the other object,

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we should return a positive value.

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It could be 1 or any positive values.

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Now,

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on what basis are we going to compare these customers?

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Currently,

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we only have the name field.

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So,

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we should compare the name of this customer with the other customer.

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However,

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we cannot write code like this,

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if name is less than o .name.

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Because this comparison operator can only be applied between numbers

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and characters,

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nothing else.

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So,

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to solve this problem,

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we're going to get the name of this customer,

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this is a string object,

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it's comparable,

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so it has the compareTo method.

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We can compare this with the name of the other customer,

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so o .name.

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dot name.

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Okay?

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Now whatever value this method returns,

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we simply return that.

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Okay?

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Also,

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let's rename this parameter to other.

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That's better.

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So we're done with the implementation of the comparable interface

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in our customer class.

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Now back to the main method.

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The error is gone.

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So our list should be sorted.

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Let's verify it by printing the customer's list on the

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console. We get this weird strain because

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This is the default representation of a customer object as a string.

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We talked about this many times before.

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So to solve this problem,

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we should overwrite the toString method in this class.

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That is super easy.

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We bring up the generate palette and select toString.

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Now I'm going to simplify this

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expression.

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We only want to return the name of this customer when

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we try to convert it to a string.

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Let's run our program one more time.

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As you can see,

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our list is sorted.

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Beautiful.

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In the next video,

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we're going to talk about another interface that is used for sorting data.

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