/*
 * Copyright (C) 2007 Google Inc.
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 * http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

package com.google.common.base;

/**
 * A Function provides a transformation on an object and returns the resulting
 * object.  For example, a {@code StringToIntegerFunction} may implement
 * <code>Function&lt;String,Integer&gt;</code> and transform integers in
 * {@code String} format to {@code Integer} format.
 *
 * <p>The transformation on the source object does not necessarily result in
 * an object of a different type.  For example, a
 * {@code FarenheitToCelsiusFunction} may implement
 * <code>Function&lt;Float,Float&gt;</code>.
 *
 * <p>Implementations which may cause side effects upon evaluation are strongly
 * encouraged to state this fact clearly in their API documentation.
 *
 * @author Kevin Bourrillion
 * @author Scott Bonneau
 */
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} <em>only</em> 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 function1.apply(o)} are both {@code null}, or
   * {@code function1.apply(o).equals(function2.apply(o))}.
   *
   * <p>Note that it is <em>always</em> safe <em>not</em> to override
   * {@code Object.equals(Object)}.
   */
  boolean equals(Object obj);
}