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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 com.google.errorprone.annotations.CanIgnoreReturnValue;
import org.checkerframework.checker.nullness.compatqual.NullableDecl;

/**
 * Determines an output value based on an input value; a pre-Java-8 version of {@link
 * java.util.function.Function java.util.function.Function}.
 *
 * The {@link Functions} class provides common functions and related utilities.
 *
 * 
See the Guava User Guide article on the use of {@code Function}.
 *
 * 
For Java 8+ users
 *
 * This interface is now a legacy type. Use {@code java.util.function.Function} (or the
 * appropriate primitive specialization such as {@code ToIntFunction}) instead whenever possible.
 * Otherwise, at least reduce explicit dependencies on this type by using lambda expressions
 * or method references instead of classes, leaving your code easier to migrate in the future.
 *
 * 
To use an existing function (say, named {@code function}) in a context where the other
 * type of function is expected, use the method reference {@code function::apply}. A future
 * version of {@code com.google.common.base.Function} will be made to extend {@code
 * java.util.function.Function}, making conversion code necessary only in one direction. At that
 * time, this interface will be officially discouraged.
 *
 * @author Kevin Bourrillion
 * @since 2.0
 */
@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
   */
  @CanIgnoreReturnValue // TODO(kevinb): remove this
  @NullableDecl
  T apply(@NullableDecl F input);

  /**
   * May return {@code true} if {@code object} is a {@code Function} that behaves identically
   * to this function.
   *
   * Warning: do not depend on the behavior of this method.
   *
   * Historically, {@code Function} instances in this library have implemented this method to
   * recognize certain cases where distinct {@code Function} instances would in fact behave
   * identically. However, as code migrates to {@code java.util.function}, that behavior will
   * disappear. It is best not to depend on it.
   */
  @Override
  boolean equals(@NullableDecl Object object);
}