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Guava is a suite of core and expanded libraries that include utility classes, google's collections, io classes, and much much more. Guava has only one code dependency - javax.annotation, per the JSR-305 spec.

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/*
 * 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 com.google.common.annotations.GwtCompatible;

import javax.annotation.Nullable;

/**
 * Determines an output value based on an input value.
 *
 * The {@link Functions} class provides common functions and related utilites.
 *
 * 
See the Guava User Guide article on the use of {@code
 * Function}.
 *
 * @author Kevin Bourrillion
 * @since 2.0 (imported from Google Collections Library)
 */
@GwtCompatible
public interface Function {
  /**
   * Returns the result of applying this function to {@code input}. This method is generally
   * expected, but not absolutely required, to have the following properties:
   *
   * 

   * Its execution does not cause any observable side effects.
   * 
The computation is consistent with equals; that is, {@link Objects#equal
   *     Objects.equal}{@code (a, b)} implies that {@code Objects.equal(function.apply(a),
   *     function.apply(b))}.
   * 
   *
   * @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.
   *
   * 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 interchangeable with this one. "Interchangeable"
   * typically 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 not to be interchangeable.
   */
  @Override
  boolean equals(@Nullable Object object);
}