Function.java

/*
 * 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 transformation from one object to another. For example, a {@code
 * StringToIntegerFunction} may implement
 * <code>Function<String,Integer></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<Float,Float></code>.
 * <p>
 * Implementations which may cause side effects upon evaluation are strongly
 * encouraged to state this fact clearly in their API documentation.
 * 
 * @param <F>
 *        the type of the function input
 * @param <T>
 *        the type of the function output
 * @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(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 <em>not</em> to override
     * {@link Object#equals}.
     */
    boolean equals(Object obj);
}