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There are collection types that implement the map interface.

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For example, a hash map directly implements the map interface.

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Linked to hash map as a child class of hash map, which implements the map interface.

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Tree map implements these sort map interface, which in turn implements the map interface.

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So one way or another, all three classes implement the map interface, whether it's directly or indirectly.

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In this lesson, you're in a see polymorphism from the lens of map.

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Open the folder for this lesson by following this path in your course resources.

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In our map is an interface defines methods that a class must implement.

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The hash map class implements this interface, it also implement a bunch of other interfaces, but namely

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map.

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Here are some of the methods that map the phones.

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Once again, the familiar sighs method hash map implements the interface, so we know the hash map class

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overrides this method.

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You can go back here and call size from the hash map.

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Here is another one is empty, and it follows that we can call is empty.

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Contains key, and so we know hash map already overrides contains key, which we can use to check for

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a particular key.

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Anyways, the hash map class implements this interface, so it's basically signing a contract that it's

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going to override all of the methods inside the interface.

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And it does.

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For example, we know our hash map is going to be forced to override or remove books that remove.

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And yeah.

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So a separate hash map from link to hash map and tree map.

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The most striking difference is that hash map has no order length.

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Hash map and tree map do follow in order.

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If you print the hash map, you'll notice that they appear in a random order.

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Tree map entries are sorted according to the compare to method.

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Based on the book classes compared to Method, we can expect tree map to sort its entries from lowest

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the highest price.

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So declare this as a true map instead.

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And check it out, all of the entries inside the tree map are sorted from lowest to highest price.

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Now for a link to hash map, the entries appear according to their insertion order.

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If I declare this as a linked hash map.

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And run the app.

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The entries appear according to the order that you inserted them in.

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So tree map and link to hash map, follow and order hash map doesn't have an order.

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Here I can convert this to a key set.

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And to an array.

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And grabbed the object index to.

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And I know it's going to be Game of Thrones.

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If I make this into a tree map.

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I know in next two is going to return Harry Potter, because that's the way it's going to be sorted.

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But if we use a hash map.

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I have no idea what in next two is going to give me because everything is going to be out of order.

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Random.

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So when do you use a map, when there is parity between data?

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But specifically, when do we use a hash map?

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If you don't care about order, use a hash map because it offers the best performance.

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Use a tree map if you need the entries to be sorted, according to what you specify.

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Use a linked hash map if you need the entries to be sorted according to their insertion order.

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Now, it's worth noting that I always use hash mark because it offers the best performance out of the

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three, and it's quite rare that indeed the entries to be sorted.

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And in this course, we're probably going to keep using hash map.

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Now, it's interesting, as you remember, that interfaces allow for polymorphism, why is polymorphism

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important, as always, flexibility.

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In this case, print map expects a map.

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And our hash map can take the form of a map so we can directly pass it in.

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We can even go a step further and declare this as type map.

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And even here, I can define this as a trauma.

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And what I have to change anything because I know Trump can take the form of map once again.

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I can define this as a linked hash map and everything should still work.

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So once again, we see that polymorphism helps make our code reusable because all three of our collections

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share one common form.

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Use a map collection when there is parity between data.

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We can drill further and say, use a hash map if you don't care about order because it offers the best

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performance.

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Use a tree map if you need the entries to be sorted, according to what you specify.

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Use a linked hash map if you need the entries to be sorted according to their insertion order.

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In this course, we're likely going to keep using hash map because it offers the best performance out

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of the three, and it's not common that we're going to need the entries to be sorted.