What is the purpose of the BiFunction<T, U, R>…

BiFunction<T, U, R> is used to represent a function that takes two arguments of types T and U, and returns a result of type R. It is a special functional interface from the java.util.function package.

Its abstract method has the following signature:

// Represents a function that takes two arguments and produces a result.
// @since 1.8
public interface BiFunction<T, U, R> {

    // Applies this function to the given arguments.
    // @param t the first function argument
    // @param u the second function argument
    // @return the function result
    R apply(T t, U u);

    // Returns a composed function that first applies this function to its input,
    // and then applies the after function to the result.
    // If evaluation of any of the expressions throws an exception, it is propagated.
    // @param after the function to apply after this function
    // @throws NullPointerException if after is null
    // @return a composed function that first applies this function and then applies the after function
    default <V> BiFunction<T, U, V> andThen(Function<? super R, ? extends V> after) {
        Objects.requireNonNull(after);
        return (T t, U u) -> after.apply(apply(t, u));
    }
}

Main usage scenarios:

Aggregation and data combination: Merging or calculating a value based on two input data.
Applying logic to two elements: Performing an operation that requires two arguments.
Use in streams: In operations like reduce or collect, where elements need to be combined pairwise.

Example usage:

// Example BiFunction for adding two integers
BiFunction<Integer, Integer, Integer> sum = (a, b) -> a + b;

// Using BiFunction
int result = sum.apply(5, 3); // result will be 8

System.out.println(result);

Another example using andThen:

// Example BiFunction for adding two numbers
BiFunction<Integer, Integer, Integer> adder = (a, b) -> a + b;

// Function example for converting the sum result to a string
Function<Integer, String> toStringConverter = i -> "Result: " + i;

// Using andThen to combine BiFunction and Function
BiFunction<Integer, Integer, String> composedFunction = adder.andThen(toStringConverter);

// Applying the composite function
String finalResult = composedFunction.apply(10, 20); // finalResult will be "Result: 30"

System.out.println(finalResult);

BiFunction provides a concise and readable way to pass behavior (as a function) into methods or classes that require working with two input parameters.

What is the purpose of the BiFunction<T, U, R> functional interface in Java?