Java: general function X->Y interface

Question

I need an interface like:

interface Function<X,Y> {
    Y eval(X obj);
}

Is there something like this in Java already or do I need to define my own?

Answer 1

Check out Guava, it has a Function interface:

public interface Function<F, T> {
  /**
   * Applies the function to an object of type {@code F}, resulting in an object of     type {@code T}.
   * Note that types {@code F} and {@code T} may or may not be the same.
   *
   * @param from the source object
   * @return the resulting object
   */
  T apply(@Nullable F from);

  /**
   * Indicates whether some other object is equal to this {@code Function}. This     method can return
   * {@code true} <i>only</i> if the specified object is also a {@code Function} and, for every
   * input object {@code o}, it returns exactly the same value. Thus, {@code
   * function1.equals(function2)} implies that either {@code function1.apply(o)} and {@code
   * function2.apply(o)} are both null, or {@code function1.apply(o).equals(function2.apply(o))}.
   *
   * <p>Note that it is always safe <i>not</i> to override {@link Object#equals}.
   */
  boolean equals(@Nullable Object obj);
 }

Answer 2

Unfortunately, there's no such thing in the core Java libraries. As a consequence, many libraries define their own function-like interface. If you happen to use such a library already, you can re-use the function it uses.

Answer 3

There is a built-in interface like this, although it's only compatible with Java 8 onwards. You can find it here.

From the Javadocs:

public interface Function<T,R>

Represents a function that accepts one argument and produces a result.

This is a functional interface whose functional method is apply(Object).

Type Parameters:
T - the type of the input to the function
R - the type of the result of the function

Answer 4

You can use a library such as Apache Commons Functor which has useful functions such as:

UnaryFunction

T evaluate(A obj);

BinaryFunction

T evaluate(L left, R right);