# Functional Interface and Lambda Expressions in Java 8

### Having trouble with abstract methods in Java 8? Check out this tutorial on using the functional interface and lambda expressions in Java.

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Join For Free**Functional Interface**

A functional interface is an interface with only one abstract method. This means that the interface implementation will only represent one behavior. Examples of a functional interface in Java are:

java.lang.Runnable

java.util.Comparator

java.util.concurrent.Callable

java.io.FileFilter

An important point to remember is that the functional interface can have a number of default methods but only one abstract method.

**Lambda Expressions**

Lambda expressions are introduced in Java 8, and they can represent the instance of a functional interface.

To explain the above statement, we are defining a functional interface named "SquareRoot." It has only one abstract method, "findSquareRoot." `@FunctionalInterface`

annotations are used to declare an interface as a functional interface.

```
@FunctionalInterface
public interface SquareRoot {
abstract double findSquareRoot(int n);
}
```

Before Java 8, we could do the following to use the SquareRoot interface:

Create an implementation class called

`SquareRootImpl`

and implement the`findSquareRoot`

method and use the instance of`SquareRootImpl`

to find the square root of input values.

```
class SquareRootImpl implements SquareRoot {
@Override
public double findSquareRoot(int n) {
return Math.sqrt(n);
}
}
```

Usage of the above implementation can be done below:

```
SquareRootImpl squareRootImpl = new SquareRootImpl();
squareRootImpl.findSquareRoot(in);
```

2. Create an anonymous implementation of the SquareRoot interface and use it to find the square root value.

```
SquareRoot squareRoot = new SquareRoot() {
@Override
public double findSquareRoot(int n) {
return Math.sqrt(n);
}
};
```

Here is the usage:

`squareRoot.findSquareRoot(in); `

The above approaches are very verbose and have a boilerplate code. The same can be done using the lambda expression in fewer lines of code

```
SquareRoot squareRoot = (n) -> (Math.sqrt(n));
squareRoot.findSquareRoot(in);
```

`squareRoot`

is instance of the functional interface "SquareRoot." The interface has only one abstract method and the `(n) -> (Math.sqrt(n)) `

expression provides the implementation of the abstract method `findSquareRoot`

.

**Explained: **` (n) -> (Math.sqrt(n))`

**Explained:**

` (n) -> (Math.sqrt(n))`

In this example method, `findSquareRoot`

expects one input of type double.

The left side of the expression is (n), "n" represents the value expected by the method ` findSquareRoot`

.

The right side of the expression followed by the `->`

sign is `(Math.sqrt(n))`

.

This is the actual method code, using the `Math.sqrt`

function to find the `squareRoot`

.

In the example of the abstract function, `findSquareRoot`

accepts one input of the type `int`

and returns value of type double.

**Abstract method**: abstract double `findSquareRoot(int n)`

;

**Lambda expression**: `SquareRoot squareRoot = (n) -> (Math.sqrt(n))`

"n" is the input and `Math.srqrt(n)`

is the output. The input and output type should match the abstract method in the interface.

**Block Lambda Expression**

Lambda expressions can have multiple lines; the following is the syntax:

```
SquareRoot squareRoot = (n) -> {
double result = Math.sqrt(n);
return result;
};
```

Here is an example of the usage:

` squareRoot.findSquareRoot(in);`

**Lambda Expressions as an Argument to Function**

You can pass the `squareRoot`

as input to any function. This is done the way we could pass an implementation instance of an interface in earlier versions of Java.

```
private static double squareRootJava8LambdaAsArgument(int i) {
SquareRoot squareRoot = (n) -> (Math.sqrt(n));
return displayData(squareRoot, i);
}
private static double displayData(SquareRoot squareRoot, int i) {
return squareRoot.findSquareRoot(9);
}
```

In the above example, the square root is passed as an input to the method `displayData`

.

**Lambda Expression With the Type of Input Syntax**

We can use the type on input in syntax, e.g. `int n, `

in this case.

`SquareRoot squareRoot = (int n) -> (Math.sqrt(n));`

**Lambda Expression With No Input **

A function can have no input, which can be seen below:

```
Abstarct method in functionalInterface
abstract double sayHello();
Lambda expression for abstract method will look like
() -> (System.out.println(“Hellow”))
```

The () represents an empty input, and the response is `String Hellow`

.

**Lambda Expression With More Than One Input **

The abstract method sum takes two inputs of type `int`

.

```
abstract method sum in functional interface
abstract int sum(int a, int b);
Lambda expression for abstract method will look like
(a,b) -> (a+b) OR
(int a,int b) -> (a+b)
```

The detailed code can be found here.

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