13- The Map Interface

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So as I told you in the last video,

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in Java we have an interface called map.

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This interface is declared in the java .util package and it's

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part of the collections framework,

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but it's not part of this hierarchy that you saw earlier.

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So it's not a collection,

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it's not an iterable,

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it's something entirely different.

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Now, as you can see over here,

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this interface has two generic type parameters,

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K and V,

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which are short for key and value.

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So with a map,

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you're essentially mapping a key to a value.

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This value could be an object like a customer,

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and this key is something that we use to look up that object,

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like look up that customer.

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Let's say you want to look up customers by their email.

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So we create a map of string to

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customer.

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So our keys are going to be strings and our values are going to be customer objects.

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We call this map and set it to a new hash map.

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Again,

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this interface has many different implementations,

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but the one that you use most of the time is the hash map class.

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Now let me create a couple of customer objects over here.

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So C1,

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we set it to a new customer.

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Here we should pass a name.

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Let's say A.

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And then an email,

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let's say E1.

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I'm going to duplicate this line.

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Let's create the second customer call it B with

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E2 as the email.

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Now we want to store these customers in our hash map.

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So we type map .put,

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this method has two parameters,

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a key which is a string and a value which is a customer.

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So, here I'm going to pass c1 .getEmail.

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I'm going to use the email of this customer as a key to look it up,

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and then I pass the customer object itself.

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Similarly,

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we add the second customer.

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Now,

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to get a customer from this hash map,

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we type map .get,

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we pass the key,

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let's say e1,

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and this will return a customer object.

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So,

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if we print this,

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what do you think we're going to get?

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We're going to see the name of this customer.

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So,

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There you go.

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So instead of using a loop to iterate over a list,

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we call the get method of the hash map to get an object.

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And this is extremely fast.

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It doesn't matter how many objects we have in this hash map,

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we could have 1 trillion objects.

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The cost of finding a customer is constant and it would be very

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very small.

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Now what if we look for a customer that doesn't exist in this list?

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Let's say e10.

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We get null.

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Here we have a similar method called get or default.

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So we can pass the key and a default value if this value

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we're looking for doesn't exist.

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So in this case,

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we can pass,

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let's say,

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an unknown customer.

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So I'm going to create an unknown customer over here,

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new customer,

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the name is unknown and the email is nothing.

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So I'm telling this map,

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look for an object with the key E10 and if you cannot find it,

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return

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this unknown customer.

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Take a look.

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Now we get unknown.

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Another useful method is contains key.

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So here we can call contains key.

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You want to see if you have a customer with this email,

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e10.

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Obviously this returns a boolean,

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so let's rename this to exists.

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Now we run the program.

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We get false.

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We also have a method for replacing an object in a hash table.

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For example,

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We can call replace,

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and we want to replace this customer with the email E1

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with another customer object.

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Let's say we made a mistake,

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and this customer object,

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its name should be A++ with the same email.

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Now,

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this method is going to replace the customer,

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so if we print our entire map,

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You can see that this map has two key value pairs.

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Here's the first key value pair and here's the second key value pair.

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So this is the reason this interface is called map in Java because

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it maps a key to a value.

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in C sharp and python is called a dictionary because it's kind of like a dictionary

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where we have words and their meanings.

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So each piece of data has two components,

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a key and a value.

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Okay?

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Now,

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as I told you before,

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this map interface is not an iterable object,

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so we cannot use it in a for each loop.

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So if you write for each item in map,

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look,

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we get a compilation error because this is not iterable.

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So how can we find the objects we have stored in a map?

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Well, here we have three methods.

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One of them is key set.

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This returns the list of keys in this map.

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This object is iterable so we can iterate over it.

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In each iteration we get a key.

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So we can print it over here.

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There you go.

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These are our keys.

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We have a similar method called entry set.

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Now look at the return type of this method.

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It returns a set of entry of string

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and customer.

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So it returns a set and because sets are iterable,

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we can iterate over them using a forage loop.

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So in each iteration we get an entry,

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this entry if we print it,

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we see something

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like this, key equals value.

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Now we can also get the value by

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calling the get value method or the key.

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So if you need to access both the key and the value in each iteration,

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you should iterate over the entry set.

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And finally we have another method called values.

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It returns a collection of customers,

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so this is also iterable.

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Let's rename this variable to customer.

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So in each iteration we get one customer.

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So A and B.

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And by the way,

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the order of these customers is not guaranteed exactly like our sets.

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So just because we added customer 1 before customer 2,

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doesn't mean when we print these customers,

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they're going to come out in the same order.

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So this is all about the map interface in Java.

13- The Map Interface

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