5- The Iterator Interface

Jun 21, 2026 09:03 · 5:26 · English · Whisper Turbo · 2 Sprekers
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In this video,
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we're going to complete our implementation of the iterator method.
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So as you can see,
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this method returns an iterator object.
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This iterator is actually an interface declared in the java
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.util package.
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So here on the documentation,
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you can see the four methods declared in this interface.
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These two methods over here have a default implementation.
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So we don't have to worry about them.
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The only two methods that we have to implement are has next and
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next, which you saw in the last video.
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So now we should declare a new class that implements the
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iterator interface.
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So back to our generic list,
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we're going to implement this iterator as a private nested class inside
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this generator list class.
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You will see why in a second.
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So after the iterator method we declare a private
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class called list iterator,
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this class should implement the iterator interface which is
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generic,
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so iterator of t.
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Now this t that we have here is the same t that we used
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when declaring our generic list.
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So if the client of this class,
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let's say in our main method,
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we create a new generic list of string,
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this type parameter is going to be a string,
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and we're going to use the same type parameter over here
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because we're going to iterate over a list of strings.
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So that is why I don't want to hard code a type over here
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like string or integer.
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I want to use the same type parameter that we have on the top.
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Okay, so here's our private class.
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Now let's implement the iterator interface.
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So we press alt and enter and select implement methods.
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So the two required methods are highlighted.
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Let's click OK.
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Now in this class,
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we want to iterate over a generic list.
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So we should pass that generic list over here via a constructor.
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So let's add a constructor public list iterator.
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Here we need a parameter of type generic list of t,
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the same type parameter,
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we call it list.
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Now we should store this in a private field in this class.
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So private generic list of t,
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we call it list.
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And here we set this .list to list.
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Now,
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because we have declared this list iterator inside our
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generic list,
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here we have access
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to list .items.
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So we can see the items array.
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And this is perfectly fine because this class is part of the implementation
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of our generic list.
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So if tomorrow we decide to replace this array with,
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let's say an array list,
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this class is the only place where we have to make changes.
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Because this class knows how to iterate over a generic list.
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So if we replace the items array with an array list,
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nowhere else in the code,
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nowhere inside the main method or anywhere else where we have used a generic list,
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we have to make changes,
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okay?
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Now here's a question,
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how can we iterate over an array?
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Well,
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we need an index variable,
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initially we set it to 0,
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as long as the index is less than the number of items in the array,
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we increment it,
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right?
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So,
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let's declare a private field of type integer
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called index
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Now in our hasNext method,
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instead of returning false,
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we're going to return a boolean expression like this.
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If index is less than,
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list .count.
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Then,
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in our next method,
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instead of returning null,
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we're going to return list .items of
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index,
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and then we'll increment the index.
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So initially index is 0,
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let me call the next method,
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this will return list .items of 0.
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Now next time we call this method index is going to be one so we'll return the
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second item in this array and we're going to do this as long as index is less
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than the number of items in our list.
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Okay,
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so pretty simple we're done implementing our list
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iterator now back to our iterator method instead
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of returning null we're going to return a new list iterator
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Now note that here we have another list iterator which is
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an interface declared in the java .util package.
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We don't want this one,
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we want the list iterator that we declared here.
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So,
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let's create a new object,
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and here we should pass our generic list to the constructor of this class.
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What is that?
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That is the current object,
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this.
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So let's quickly recap.
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We made our generic list iterable
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by implementing the iterable interface.
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and here we return a list iterator.
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This is an object that knows how to iterate over a generic list.
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So it has intimate knowledge of the implementation detail of
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our generic list.
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If tomorrow we decide to replace this array with an array
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list or a different data structure,
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this iterator is the only place where we have to modify our code.
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Now,
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back to our main class,
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let's add a couple of items in this list.
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Let's add A and then B.
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Here we can iterate over this list using a for each loop.
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Take a look.
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We get A and B.
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Next we're going to talk about the collection interface.

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