Function Functional Interface in Java

In this lesson, we will explore the Function interface from java.util.function package. 

Function interface represents a function that takes in one argument and produces a result.

It has one functional (single abstract) method R apply(T t), which takes one input of type T and returns the function result of type R.

It also contains two default and one static method.

Since it is a Functional Interface, we can implement it with a Lambda expression.

Function<T, R>

@FunctionalInterface
public interface Function<T, R> {
    /**
     * Applies this function to the given argument.
     *
     * @param t the function argument
     * @return the function result
     */
    R apply(T t);

    /**
     * Returns a composed function that first applies the {@code before}
     * function to its input, and then applies this function to the result.
     * If evaluation of either function throws an exception, it is relayed to
     * the caller of the composed function.
     *
     * @param <V> the type of input to the {@code before} function, and to the
     *           composed function
     * @param before the function to apply before this function is applied
     * @return a composed function that first applies the {@code before}
     * function and then applies this function
     * @throws NullPointerException if before is null
     *
     * @see #andThen(Function)
     */
    default <V> Function<V, R> compose(Function<? super V, ? extends T> before) {
        Objects.requireNonNull(before);
        return (V v) -> apply(before.apply(v));
    }

    /**
     * Returns a composed function that first applies this function to
     * its input, and then applies the {@code after} function to the result.
     * If evaluation of either function throws an exception, it is relayed to
     * the caller of the composed function.
     *
     * @param <V> the type of output of the {@code after} function, and of the
     *           composed function
     * @param after the function to apply after this function is applied
     * @return a composed function that first applies this function and then
     * applies the {@code after} function
     * @throws NullPointerException if after is null
     *
     * @see #compose(Function)
     */
    default <V> Function<T, V> andThen(Function<? super R, ? extends V> after) {
        Objects.requireNonNull(after);
        return (T t) -> after.apply(apply(t));
    }

    /**
     * Returns a function that always returns its input argument.
     *
     * @param <T> the type of the input and output objects to the function
     * @return a function that always returns its input argument
     */
    static <T> Function<T, T> identity() {
        return t -> t;
    }
}

T represents the type of the input parameter, and U represents the return type.

Implementing the Function interface in Java

Example 1:

Implementation of the single abstract method R apply(T t)

class Test {

  public static void main(String[] args) {
    Function<String, String> f1 = str -> str.concat("Programming");
    System.out.println(f1.apply("Java"));
  }
}

Note: If we have a single statement inside the Lambda body, then we don’t need to put the curly braces {}. The same for the input parameter, if we are dealing with only one, then we don’t need to put it between the parentheses ().

Also, if we have a single statement that returns some result, we don’t have to put the return keyword, it would be redundant.

Example 2:

Implementation of the <V> Function<T, V> andThen(Function<? super R, ? extends V> after) method that accepts the Function and returns the Function also.

class Test {

  public static void main(String[] args) {
    Function<String, String> concatStrings = str -> str.concat("Programming");
    Function<String, String> toUpperCase = str -> str.toUpperCase();

    System.out.println(concatStrings.andThen(toUpperCase).apply("Java"));
  }
}

Example 3:

Implementing the <V> Function<V, R> compose(Function<? super V, ? extends T> before) method that accept the Function and return the Function also. Just like the andThen() method. 

class Test {

  public static void main(String[] args) {
    Function<String, String> concatStrings = str -> str.concat("Programming");
    Function<String, String> toUpperCase = str -> str.toUpperCase();

    System.out.println(concatStrings.compose(toUpperCase).apply("Java"));
  }
}

Example 4:

Implementing the <T> Function<T, T> identity() method that accepts no input and returns a function which returns its own argument.

class Test {

  public static void main(String[] args) {
    Function<String, String> function = Function.identity();
    System.out.println(function.apply("Hello Java!"));
  }
}