2- Functional Interfaces

Jun 26, 2026 22:45 · 3:51 · English · Whisper Turbo · 2 સ્પીકર

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S… Speaker 2 (2- Functional Interfaces)

Let's start this section by defining functional interfaces.

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S… Speaker 2 (2- Functional Interfaces)

A functional interface is an interface with only a single

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S… Speaker 2 (2- Functional Interfaces)

abstract method.

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S… Speaker 2 (2- Functional Interfaces)

So earlier we talked about the comparable interface.

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S… Speaker 2 (2- Functional Interfaces)

This interface has a single abstract method called compareTo.

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S… Speaker 2 (2- Functional Interfaces)

So this is a functional interface.

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S… Speaker 2 (2- Functional Interfaces)

We also talked about the comparator interface.

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S… Speaker 2 (2- Functional Interfaces)

That is a functional interface as well.

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S… Speaker 2 (2- Functional Interfaces)

Now in this video,

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I'm going to create a functional interface from scratch.

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S… Speaker 2 (2- Functional Interfaces)

We're going to use this functional interface throughout this section.

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S… Speaker 2 (2- Functional Interfaces)

So in this package lambdas,

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S… Speaker 1 (2- Functional Interfaces)

I'm going to add a new class Let's call this printer and

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S… Speaker 1 (2- Functional Interfaces)

change the kind to interface So this

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S… Speaker 2 (2- Functional Interfaces)

is the interface of a printer in one implementation We may want to print on the

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S… Speaker 2 (2- Functional Interfaces)

terminal in another implementation We may want to talk to an actual

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S… Speaker 2 (2- Functional Interfaces)

printer so we could have different implementations of this interface Now

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S… Speaker 2 (2- Functional Interfaces)

what operations do we need here?

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S… Speaker 2 (2- Functional Interfaces)

We need a single method for printing a message.

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S… Speaker 1 (2- Functional Interfaces)

So void print message.

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S… Speaker 2 (2- Functional Interfaces)

Now we have an interface with a single abstract

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S… Speaker 1 (2- Functional Interfaces)

method.

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S… Speaker 2 (2- Functional Interfaces)

So this is a functional interface.

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S… Speaker 2 (2- Functional Interfaces)

Now in the second part of this series,

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S… Speaker 2 (2- Functional Interfaces)

I told you that in the recent years,

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S… Speaker 2 (2- Functional Interfaces)

Java introduced the concept of default methods in interfaces.

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S… Speaker 2 (2- Functional Interfaces)

These default methods can have implementation.

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S… Speaker 2 (2- Functional Interfaces)

So we can have a default method like this default void

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S… Speaker 2 (2- Functional Interfaces)

print twice,

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S… Speaker 1 (2- Functional Interfaces)

which takes a message and prints it twice.

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S… Speaker 1 (2- Functional Interfaces)

So we can have code,

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S… Speaker 2 (2- Functional Interfaces)

we can have implementation in an interface.

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S… Speaker 2 (2- Functional Interfaces)

I told you that in my opinion,

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S… Speaker 2 (2- Functional Interfaces)

this is a bad approach that the Java team has taken because interfaces

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S… Speaker 2 (2- Functional Interfaces)

should not have implementation.

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S… Speaker 2 (2- Functional Interfaces)

They should only be used as contracts.

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S… Speaker 1 (2- Functional Interfaces)

But that aside,

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S… Speaker 2 (2- Functional Interfaces)

what I want to point out here is that this interface has two methods.

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S… Speaker 2 (2- Functional Interfaces)

One of them is a default method with implementation.

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S… Speaker 2 (2- Functional Interfaces)

The other is an abstract method.

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S… Speaker 2 (2- Functional Interfaces)

This interface is still a functional interface because it has a single

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S… Speaker 2 (2- Functional Interfaces)

abstract method.

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S… Speaker 2 (2- Functional Interfaces)

So it doesn't matter if you have many default or even static methods in

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S… Speaker 2 (2- Functional Interfaces)

our interfaces.

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S… Speaker 2 (2- Functional Interfaces)

As long as we have a single abstract method,

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S… Speaker 2 (2- Functional Interfaces)

we refer to that interface as a functional interface.

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S… Speaker 1 (2- Functional Interfaces)

Now,

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S… Speaker 2 (2- Functional Interfaces)

let me delete this ugly default method.

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S… Speaker 1 (2- Functional Interfaces)

Alright,

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S… Speaker 2 (2- Functional Interfaces)

so let's go ahead and implement this interface in a class.

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S… Speaker 1 (2- Functional Interfaces)

I'm going to add a new class called

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S… Speaker 2 (2- Functional Interfaces)

console printer.

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In this implementation,

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S… Speaker 2 (2- Functional Interfaces)

we're going to print on the console or terminal.

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S… Speaker 2 (2- Functional Interfaces)

So let's have this class implement the printer interface.

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S… Speaker 1 (2- Functional Interfaces)

Here we're going to add a print statement

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S… Speaker 1 (2- Functional Interfaces)

and print this message.

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S… Speaker 1 (2- Functional Interfaces)

Pretty straightforward,

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S… Speaker 1 (2- Functional Interfaces)

right?

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S… Speaker 1 (2- Functional Interfaces)

Now,

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S… Speaker 2 (2- Functional Interfaces)

back to our demo class.

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S… Speaker 2 (2- Functional Interfaces)

Here I'm going to create a static method,

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S… Speaker 2 (2- Functional Interfaces)

public static.

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S… Speaker 2 (2- Functional Interfaces)

I'm using static so I can easily call it from this other static method.

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S… Speaker 2 (2- Functional Interfaces)

It doesn't have to be static.

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S… Speaker 1 (2- Functional Interfaces)

So public static void greet.

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S… Speaker 1 (2- Functional Interfaces)

this method needs a printer.

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S… Speaker 2 (2- Functional Interfaces)

So here we're going to talk to the printer interface,

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S… Speaker 2 (2- Functional Interfaces)

we don't care about the implementation,

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S… Speaker 2 (2- Functional Interfaces)

we are programming to an interface,

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S… Speaker 1 (2- Functional Interfaces)

okay?

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S… Speaker 2 (2- Functional Interfaces)

So printer,

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then we call printer dot print,

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S… Speaker 2 (2- Functional Interfaces)

and here we can say hello world.

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S… Speaker 1 (2- Functional Interfaces)

Now in our show method I'm going to call the greet

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S… Speaker 2 (2- Functional Interfaces)

method, here we need to pass an object that implements the printer interface,

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S… Speaker 2 (2- Functional Interfaces)

so we can use our console printer object.

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S… Speaker 1 (2- Functional Interfaces)

Now when we run this program,

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S… Speaker 2 (2- Functional Interfaces)

we see the hello world message.

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S… Speaker 1 (2- Functional Interfaces)

So pretty straightforward.

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S… Speaker 1 (2- Functional Interfaces)

Now,

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S… Speaker 2 (2- Functional Interfaces)

sometimes we don't want to explicitly create a class to implement an interface because

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S… Speaker 1 (2- Functional Interfaces)

this requires writing a bit of code.

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S… Speaker 2 (2- Functional Interfaces)

Sometimes we may not want to reuse this class.

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S… Speaker 1 (2- Functional Interfaces)

In the next video,

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S… Speaker 2 (2- Functional Interfaces)

I'm going to show you how to use anonymous inner classes for this purpose.

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