4- The Iterable Interface

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The iterable interface represents an object that is iterable.

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That means we can iterate or loop over it without knowing anything

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about its implementation detail.

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So we are using an array here.

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If we decide to replace this with an array list or a different data structure

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in the future,

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the clients of this class are not going to get affected because they are going to code

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against the iterable interface.

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Back to our remote control metaphor,

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think of the buttons for changing the volume.

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If the internal implementation of this remote control changes,

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you're not going to get affected.

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You're still going to use the same buttons.

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These buttons are the interfaces that you work with.

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So we want to implement the iterable interface in this class and then

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have other classes that use our generic list program against the iterable interface.

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It's like saying that they use the volume control buttons on a remote control.

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Okay, now

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Here's the documentation for the iterable interface.

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As you can see,

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this interface declares three methods.

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The first and last methods have a default implementation so we don't

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have to implement them.

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The only method that we have to implement

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is this method over here,

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iterator.

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It returns an iterator object.

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We're going to talk about iterators in the next video,

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but very briefly an iterator is an object that we use to iterate

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over an iterable.

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Now I know the name is confusing,

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but don't worry,

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I'm going to make it super simple for you.

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So,

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back to our generic list class.

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Here we type implements iterable of

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t.

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So this is also a generic interface.

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Now we press alt and enter and have IntelliJ implement

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the methods.

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So here you can see the same three methods that I showed

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you. The iterator method is highlighted because it's a required method.

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So let's press ok and here you can see the iterator

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method.

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For now don't worry about the code we're going to write in this method.

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We're going to come back and complete this in the next video.

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All I want you to pay attention to is the return type of this method.

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It returns an iterator object.

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So, back to our main method.

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Now that our list implements the iterable interface,

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we know that it has the iterator method.

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So we call it to get an iterator,

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and with this we can iterate or loop over our list

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without knowing anything about its internal implementation.

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So we can write code like this.

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While iterator has next,

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we're going to call iterator

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dot next.

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This will return the current object.

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So we store it over here and then print it on

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the console.

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What's going on here?

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Well, imagine our list looks like this.

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So we have a,

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b, and c.

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This iterator is like a pointer that is pointing at one object.

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So initially it's pointing at the first object.

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At the beginning of our loop,

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we ask, do you have more items?

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It says yes.

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So here we call the next method.

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This will return the current item,

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which is the A,

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and then it will advance the pointer to point to B.

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In the next iteration,

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we ask the iterator,

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do you have more items?

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We still do.

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So this method will return true.

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Then we read the current item and advance the pointer.

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One more time,

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we read the C, and then the iterator ends up here.

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So in the next iteration,

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when we call the hasNext method,

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we'll get false.

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So this iterator is simply an object that we

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use to iterate over a list,

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or more accurately,

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an object that is iterable.

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Now let me show you something really interesting.

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Let's compile the code,

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then from the project window,

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let's select the main class and look at its bytecode representation.

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Take a look.

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So over here we have a reference to iterator that has next

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and iterator dot next.

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Now I'm going to delete all the code for

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using this iterator object and instead use a for each loop.

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So for each item and list we're going to print

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each item.

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One more time we're going to compile the code and

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look at the bytecode representation of the main class.

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Take a look.

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We still have a reference to the iterator object.

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So the for each loop is actually a syntactical sugar over

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the iterator object.

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When we use the for each loop,

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the Java compiler will convert our code to use an iterator object.

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This is the reason why we can iterate over this list using

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a for each loop.

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Now,

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if I remove this bit of code over here,

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implements iterable,

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and go back to the main class

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you can see we have a compilation error saying for each not applicable to type

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generic list.

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So this is the benefit of implementing the iterable interface.

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It allows us to iterate over an object without knowing anything about

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its internal implementation.

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So I'm going to bring this back.

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Now if tomorrow we decide to replace this array with let's say an array

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list, our for each loop is not going to get affected.

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Now our implementation is not complete.

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We still have to implement the iterator method and that's

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what we're going to do next.

4- The Iterable Interface

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