Function Interface in Java with Examples

The Function Interface is a part of the java.util.function package which has been introduced since Java 8, to implement functional programming in Java. It represents a function which takes in one argument and produces a result. Hence this functional interface takes in 2 generics namely as follows:

• T: denotes the type of the input argument
• R: denotes the return type of the function

The lambda expression assigned to an object of Function type is used to define its apply() which eventually applies the given function on the argument.

Methods in Function Interface

The Function interface consists of the following 4 methods as listed which are later discussed as follows:

1. apply()
2. andThen()
3. compose()
4. identity()

Method 1: apply()

Syntax:

R apply(T t)

Parameters: This method takes in only one parameter t which is the function argument

Return Type: This method returns the function result which is of type R.

Example

Java

// Java Program to Illustrate Functional Interface
// Via apply() method

// Importing interface
import java.util.function.Function;

// Main class
public class GFG {

    // Main driver method
    public static void main(String args[])
    {
        // Function which takes in a number
        // and returns half of it
        Function<Integer, Double> half = a -> a / 2.0;

        // Applying the function to get the result
        System.out.println(half.apply(10));
    }
}

Output

5.0

Method 2: andThen()

It returns a composed function wherein the parameterized function will be executed after the first one. If evaluation of either function throws an error, it is relayed to the caller of the composed function.

Syntax:

default <V> Function<T, V> 
andThen(Function<? super R, ? extends V> after)

where V is the type of output of the after function, and of the composed function

Parameters: This method accepts a parameter after which is the function to be applied after the current one.

Return Value: This method returns a composed function that applies the current function first and then the after function

Exception: This method throws NullPointerException if the after function is null.

Example 1:

Java

// Java Program to illustrate addThen() method

// Importing interface
import java.util.function.Function;

// Main class
public class GFG {

    // main driver method
    public static void main(String args[])
    {
        // Function which takes in a number and
        // returns half of it
        Function<Integer, Double> half = a -> a / 2.0;

        // Now treble the output of half function
        half = half.andThen(a -> 3 * a);

        // Applying the function to get the result
        // and printing on console
        System.out.println(half.apply(10));
    }
}

Output

15.0

Example 2: To demonstrate when NullPointerException is returned.

Java

// Java Program to illustrate addThen() method
// When NullPointerException occurs

// Importing interface
import java.util.function.Function;

// Main class
public class GFG {

    // Main driver method
    public static void main(String args[])
    {

        // Function which takes in a number and
        // returns half of it
        Function<Integer, Double> half = a -> a / 2.0;

        // Try block to check for exceptions
        try {

            // Trying to pass null as parameter
            half = half.andThen(null);
        }

        // Catch block to handle exceptions
        catch (Exception e) {

            // Print statement
            System.out.println("Exception thrown "
                               + "while passing null: "
                               + e);
        }
    }
}

Output

Exception thrown while passing null: java.lang.NullPointerException

Method 3: compose()

It returns a composed function wherein the parameterized function will be executed first and then the first one. If evaluation of either function throws an error, it is relayed to the caller of the composed function.

Syntax:

default <V> Function<V, R> 
compose(Function<? super V, ? extends T> before)

Where V is the type of input of the before function, and of the composed function

Parameters: This method accepts a parameter before which is the function to be applied first and then the current one

Return Value: This method returns a composed function that applies the current function after the parameterized function

Exception: This method throws NullPointerException if the before function is null.

Example 1:

Java

// Java Program to illustrate compose() method

// Importing Function interface
import java.util.function.Function;

// Main class
public class GFG {

    // Main driver method
    public static void main(String args[])
    {
        // Function which takes in a number and
        // returns half of it
        Function<Integer, Double> half = a -> a / 2.0;

        // However treble the value given to half function
        half = half.compose(a -> 3 * a);

        // Applying the function to get the result
        System.out.println(half.apply(5));
    }
}

Output

7.5

Example 2: When NullPointerException is returned.

Java

// Java Program to illustrate compose() method

// Importing Function interface
import java.util.function.Function;

// Main class
public class GFG {

    // Main driver method
    public static void main(String args[])
    {
        // Function which takes in a number and
        // returns half of it
        Function<Integer, Double> half = a -> a / 2.0;

        // Try block to check for exceptions
        try {

            // Trying to pass null as parameter
            half = half.compose(null);
        }

        // Catch block to handle exceptions
        catch (Exception e) {

            // Print statement
            System.out.println("Exception thrown "
                               + "while passing null: "
                               + e);
        }
    }
}

Output

Exception thrown while passing null: java.lang.NullPointerException

Method 4: identity()

This method returns a function that returns its only argument.

Syntax:

static <T> Function<T, T> identity()

where T denotes the type of the argument and the value to be returned

Returns: This method returns a function that returns its own argument

Example

Java

// Java Program to Illustrate identity() method

// Importing Function interface
import java.util.function.Function;

// Main class
public class GFG {

    // Main driver method
    public static void main(String args[])
    {
        // Function which takes in a number and
        // returns it
        Function<Integer, Integer> i = Function.identity();

        // Print statement
        System.out.println(i.apply(10));
    }
}

Output

10