10- The Queue Interface

Jun 21, 2026 10:23 · 4:47 · English · Whisper Turbo · 2 സ്പീക്കറുകള്‍
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കാണിക്കല്‍ മാത്രം.
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The next interface we're going to talk about is the queue interface.
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Similar to the list interface,
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it extends the collection interface,
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so every queue is a collection as well.
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We use queues in situations where we want to process jobs based
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on the order we receive them.
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For example,
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think of the printer at your organization.
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Your printer receives various jobs,
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puts them in a queue,
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and processes them one by one in the order it receives them.
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Here on the documentation you can see that the queue interface has these sub -interfaces
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like blocking deck,
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blocking queue,
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deck,
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and transfer queue.
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Again,
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these are for special purposes and chances are you're not going to use them for the most of
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your career.
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And here you can see various implementations of the queue interface.
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The two implementations you use most of the time are array deck and
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priority queue.
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Deck is short for double ended queue.
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It's a special type of queue that has two ends.
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So items can enter the queue from either end.
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This is different from the queues that we have in real life.
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For example,
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when we go in a queue for a concert or a movie,
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we always enter the queue from the back.
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With double ended queues or decks,
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items can enter from either end.
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Priority queue is another common implementation and again it's a
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special type of queue.
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It's a queue where each item gets a priority and this priority
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determines the position of this item in the queue.
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So items with a higher priority move to the front of the queue.
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A common application of this is the queue that your operating system uses to
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manage the processes.
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Some processes have a higher priority so they get more CPU time.
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So let's declare a variable of type queue of string
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We call it queue and set it to a new array deck.
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Now,
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by the way, I didn't specify this generic type argument over here because
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this is redundant.
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The Java compiler can look at what we have on the left side and
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figure out that this is an array deck of strings.
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This is the reason why a string is grayed out,
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so we can remove it.
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Now in this queue let's add a few items.
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Let's imagine that C entered the queue first.
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followed by A,
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and then B.
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So our queue looks like this.
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B,
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A,
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and C.
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So C is at the front of the queue.
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Now we have another method for adding an item to a queue,
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and that is offer.
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So queue .offer,
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we can now offer D,
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so D will be at the end or the rear
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of the queue.
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Now what is the difference between add and offer?
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The difference depends on the implementation of the queue that we are using.
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In case of an array deck,
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there is no difference,
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but in some implementations,
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the queue might have a limited size.
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In those cases,
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if the queue gets full,
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the add method throws an exception,
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whereas the offer method returns false.
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So it's kind of less aggressive.
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Now let me delete this line.
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So our queue looks like this.
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Now we can get the item at the front of the queue by calling the
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peak method.
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So this will return the item at the front.
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We can print this.
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So at the front we have C.
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So now we should see the C.
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Beautiful.
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We have another similar method that is element.
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The difference between peak and element is that peak returns null
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if the queue is empty,
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whereas element throws an exception.
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Let me show you.
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So I'm going to comment out these few lines.
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Now our queue is empty.
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So the element method
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throws an exception.
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In contrast,
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if we call the peak method,
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we'll get null.
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There you go.
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Now,
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let me bring this back.
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Alright.
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Now,
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to remove the item at the front of the queue,
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we call the remove method.
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This will remove the item at the front and return it.
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So now,
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when we run this program,
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we get the C,
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and if we print the entire queue,
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you can see,
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that the C is gone and our queue only has A and B.
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We have a similar method called pole.
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The difference is that pole returns null if the queue is empty,
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whereas remove throws an exception.
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So once again,
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let me show you,
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I'm going to comment out these few lines.
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So pole is going to return null.
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There you go.
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Whereas if we call the remove method,
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we get an exception.
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There you go.
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So this is how we can use cues. In the next video we're going to talk about sets.

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