8- The Consumer Interface

Jun 29, 2026 19:16 · 4:06 · English · Whisper Turbo · 4 Cynhadledd

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The consumer interface represents an operation that takes a single argument

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and doesn't return a value.

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Now,

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why does this matter?

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Why do you have to learn about the consumer interface?

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Well, a lot of methods in Java expect an object that implements

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the consumer interface.

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I will show you an example of this later in this video.

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But first,

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let's look at the methods of this interface.

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So as you can see

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Here we have a single abstract method called except that

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takes an argument of type T and here T is our generic type parameter and

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this method doesn't return a value.

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We also have a default method as I told you before these default

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methods have an implementation.

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I'm going to talk about this method in the next video.

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So the consumer interface represents an operation that takes a single

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argument and doesn't return a value.

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Now we have a few variations of this interface.

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One of them is by consumer that takes two arguments.

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So if you look at the except method,

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you can see here we have two arguments t and u.

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We also have int consumer.

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This is one of the primitive specializations of this interface.

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the accept method takes a primitive integer.

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So if you're dealing with a lot of integer values,

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it is more efficient to use this interface as opposed to the regular consumer

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interface because with this interface,

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you don't pay the cost of auto boxing.

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You also have long consumer and double consumer.

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Now let me show you how to use the consumer interface.

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I'm going to create a list of integers.

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And here I'm going to use list of to initialize this list with

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a few items.

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So one,

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two, and three.

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Now there are two ways to iterate over this list.

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We can use a for each loop.

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So for each item in list print each

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item. Now there is another way to iterate over

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this list.

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We can call list .foreach and take

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a look.

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this method expects a consumer object.

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In fact, a consumer of integer.

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As you saw,

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the consumer interface is a functional interface because it has a single

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abstract method.

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So we can represent it using a lambda expression.

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In this case,

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we want to pass a lambda expression that takes an integer and doesn't

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return a value.

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So let's pass a lambda expression here.

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First we add the parameter integer item.

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then the lambda operator that goes to a code block.

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Now in this case,

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we don't really need to specify the type of this parameter because the

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Java compiler knows that every item in this list is an instance of

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the integer class.

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So this type is redundant.

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Let's remove it.

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Now we have a single parameter so we can get rid of the parentheses.

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That is better.

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Now in the body of this lambda,

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I just want to print this item.

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Because we have a single statement.

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We don't even need the code block.

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So print item So these are two different ways

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to iterate over a list what we have on the top is an example

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of imperative programming

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That basically means implementing logic using instructions.

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For example,

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whenever we have for loops or if and else statements or switch

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on case, we are writing code in imperative style.

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So imperative programming is a style of programming or a programming paradigm.

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Now we have so many different programming paradigms or styles of programming like

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functional, object oriented,

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event driven and so on.

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What we have on the bottom is an example of declarative programming.

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So instead of using instructions to specify how something should be done,

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we specify what needs to be done.

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So here we don't have a for statement.

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We're simply saying for each item in this list,

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print it.

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Now we're going to talk more about this concept,

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so if this is new to you,

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don't worry.

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Next we're going to talk about chaining consumers.

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