6- The Collection Interface

Jun 21, 2026 09:22 · 9:35 · English · Whisper Turbo · 2 স্পিকার

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The next interface we're going to talk about is the collection interface,

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which represents an object that acts as a collection or

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a container for other objects.

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What are the operations that we need to work with a collection?

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We should be able to add an object to this collection,

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we should be able to remove an existing object,

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we should be able to check for the existence of an object and so on.

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These are the operations that the collection interface declares.

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Here on the documentation you can see that the collection interface is also a generic

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interface because here we have a generic type parameter,

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but instead of t we have e which is short for element that

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is a common convention for declaring interfaces or classes that have a collection

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semantic because a collection can have multiple elements,

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okay?

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Now, over here you can see that this interface extends the iterable

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interface, so every collection is also iterable.

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And that means we can use a forage loop to iterate over all collections

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in Java.

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Now over here you can see various classes that implement this interface.

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As you can see,

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there are over 20 implementations of this interface.

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Most of the time,

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like 90 % of the time,

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you only need to use the ArrayList class.

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These other implementations are for special purposes and we're not going

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to cover them in this course.

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You can learn about them on your own.

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I just want to give you a strong foundation.

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So back to our project,

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I'm going to add a new package,

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a new package called collections.

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We're going to write all the code in this section.

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in this package.

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I'm going to add a new class called collections demo.

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In this class we're going to have a public static method called

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show.

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This is where I'm going to show you various operations in the collection interface.

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If you download the source code that I've included at the beginning of this course,

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you can see all examples in this class.

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But while I'm recording this video,

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I'm going to delete some of these examples to keep the screen clean and easy to read.

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Now back in the main class,

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we're going to call collections demo dot

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show.

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Now here I'm going to declare a variable of type collection

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of string.

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Let's import this type.

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So that is an interface.

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We call this variable collection.

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Now here we should set this to a new instance of ArrayList

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because this is one of the classes that implements

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this interface.

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Now let's add a few items to this collection.

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So collection dot add.

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can add a string like A,

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then B,

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and then C.

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Because this collection is iterable,

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we can use a for each loop here.

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So for item in collection,

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we can print each item.

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So we get ABC.

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We can also print the collection as a whole.

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So print collection.

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Now,

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what if you want to add multiple items in one go,

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instead of calling the add method three times?

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Well,

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Here we have another class called collections.

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That is a utility class declared in the java .util package.

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This class has a method called addAll.

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The first parameter of this method is the collection that we want to modify.

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So we pass our collection over here.

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Now look at the second argument.

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Here we can see the list of parameters by pressing command and P

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on Mac or control and P on Windows.

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If this shortcut doesn't work for you,

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you can find it under view

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parameter info.

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So look at the second parameter,

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string dot dot dot.

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When you see dot dot dot,

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that means you can pass a variable number of arguments.

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You can pass zero or more strings.

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So here we can pass A and then B

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and then C.

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So instead of calling the add method three times,

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we're passing multiple arguments to this method.

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Now,

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if we run our program,

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we get the same result as before.

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Another useful method is the size method that returns the size of a collection.

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So here we have three items.

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We also have a method for removing an object

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from a collection.

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So we can call collection .removeA and

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then we can print the collection.

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So A is gone.

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We can also remove all items by calling

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the clear method.

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Now the collection is empty.

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We can verify that by calling the is empty

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method.

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it returns true.

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If you want to check for the existence of an item,

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you can call the contains method.

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So we want to see if this collection contains an

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So contains A,

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and then we can print contains A.

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Get true.

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There are times you want to convert a collection to a regular

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array.

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So here we call collection .toArray

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As you can see,

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this method has three forms or three overloads.

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If you don't pass any arguments here,

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this will return an object array.

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So we can store it in an object array.

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So every item in this array is going to be an instance

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of the object class.

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That means if we get the first item and

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use the dot operator,

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we don't see any of the string methods.

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If that's what you need,

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then you need to convert this collection to a string array instead of an object array.

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That's very easy.

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We call collection .toArray.

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We're going to use this form,

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this overload that takes a tArray.

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So if we pass a new string array

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here, how many items do we have in this collection?

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2, 3.

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So we pass a string array over here,

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and this will return a string array.

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Also, we don't really have to specify the size here.

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We could simply pass 0 and that is a common convention because

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you don't want to worry about counting the number of items in your collection.

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You can simply pass an empty array and this method would automatically

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create an array with enough capacity to hold all these items.

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Now,

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if we get the first item,

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we can access all the string methods.

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For example,

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we can convert the string to the lowercase or uppercase and so on.

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And by the way,

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we can use var here,

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just as usual.

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Now some people don't like var,

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they argue that we cannot see the type of this object,

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but that is not right.

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So if you put the caret over here,

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and then go to view,

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quick documentation,

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the shortcut is F1,

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you can easily see that this is an object array.

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Or if you put the caret over here,

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and one more time,

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press F1,

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you can see that this is a string array.

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So I personally prefer var because it makes our code cleaner.

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So this is how we can use the two array methods.

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Now,

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there are times you want to compare two collections for equality.

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So I'm going to create another collection of type

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string.

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Let's call it other and set it to a new ArrayList.

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Now what would happen if we use var

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over here?

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Then the type of this variable is going to be the ArrayList class,

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not the collection interface.

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So it's going to include the additional methods that are declared in the ArrayList

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class.

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Now, as I explained in the second part of this series,

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we should program against interfaces.

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This will make our applications more flexible and loosely coupled.

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So,

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I'm going to revert this back to collection of string.

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Now,

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I want to add every object that we have in this collection in

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our second collection.

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So, we type other that add all.

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Now look at the type of this parameter.

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It's collection of unknown extends string.

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So we have a wild card again.

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That means we can pass a collection of string or any type

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that extends the collection of string.

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So,

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I'm going to pass our first collection.

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Now,

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you want to check to see if these two collections are equal.

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So I'm going to do a print statement here.

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What do you think is the result of this expression?

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Collection equals other.

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Are we going to get true or false?

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We're going to get false because these are two different objects in memory.

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Look, here we have two new operations.

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So these are going to be different objects in memory and this equality

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operator compares these objects by their references or their addresses

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in memory.

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Now if you want to check to see if two collections have the exact same content,

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you should use the equals method.

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So print collection equals other.

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Take a look.

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So we get false as a result of evaluating this expression because these

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are two different objects in memory,

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but we get true because these two collections have the exact same

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content.

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So this was all about the collection interface.

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Once again,

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don't worry about memorizing any of these.

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All you need to take away from this video is the kind of operations that are provided

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by the collection interface.

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We can add an object,

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we can remove an object,

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we can check to see if an object exists and so on.

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Don't worry about memorizing,

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you can always look at these operations on the documentation.

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Next we're going to look at the list interface.

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