/*
 * Copyright (C) 2007 The Guava Authors
 *
 * 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;

import javax.annotation.Nullable;

import com.google.common.annotations.GwtCompatible;

/**
 * Determines an output value based on an input value.
 *
 * <p>
 * The {@link Functions} class provides common functions and related utilites.
 *
 * <p>
 * See the Guava User Guide article on <a href=
 * "http://code.google.com/p/guava-libraries/wiki/FunctionalExplained">the use
 * of {@code
 * Function}</a>.
 *
 * @author Kevin Bourrillion
 * @since 2.0 (imported from Google Collections Library)
 */
@GwtCompatible
public interface Function<F, T> {
	/**
	 * Returns the result of applying this function to {@code input}. This method is
	 * <i>generally expected</i>, but not absolutely required, to have the following
	 * properties:
	 *
	 * <ul>
	 * <li>Its execution does not cause any observable side effects.
	 * <li>The computation is <i>consistent with equals</i>; that is,
	 * {@link Objects#equal Objects.equal}{@code (a, b)} implies that
	 * {@code Objects.equal(function.apply(a),
	 *     function.apply(b))}.
	 * </ul>
	 *
	 * @throws NullPointerException if {@code input} is null and this function does
	 *                              not accept null arguments
	 */
	@Nullable
	T apply(@Nullable F input);

	/**
	 * Indicates whether another object is equal to this function.
	 *
	 * <p>
	 * Most implementations will have no reason to override the behavior of
	 * {@link Object#equals}. However, an implementation may also choose to return
	 * {@code true} whenever {@code object} is a {@link Function} that it considers
	 * <i>interchangeable</i> with this one. "Interchangeable" <i>typically</i>
	 * means that {@code Objects.equal(this.apply(f), that.apply(f))} is true for
	 * all {@code f} of type {@code F}. Note that a {@code false} result from this
	 * method does not imply that the functions are known <i>not</i> to be
	 * interchangeable.
	 */
	@Override
	boolean equals(@Nullable Object object);
}