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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 javax.annotation.CheckForNull;
import org.checkerframework.checker.nullness.qual.Nullable;
/**
* Determines an output value based on an input value; a pre-Java-8 version of {@link
* java.util.function.Function java.util.function.Function}.
*
* <p>The {@link Functions} class provides common functions and related utilities.
*
* <p>See the Guava User Guide article on <a
* href="https://github.com/google/guava/wiki/FunctionalExplained">the use of {@code Function}</a>.
*
* <h3>For Java 8+ users</h3>
*
* <p>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 <i>explicit</i> dependencies on this type by using lambda expressions
* or method references instead of classes, leaving your code easier to migrate in the future.
*
* <p>To use an existing function (say, named {@code function}) in a context where the <i>other
* type</i> 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 <i>extend</i> {@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
@ElementTypesAreNonnullByDefault
public interface Function<F extends @Nullable Object, T extends @Nullable Object> {
/**
* 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
*/
@CanIgnoreReturnValue // TODO(kevinb): remove this
@ParametricNullness
T apply(@ParametricNullness F input);
/**
* <i>May</i> return {@code true} if {@code object} is a {@code Function} that behaves identically
* to this function.
*
* <p><b>Warning: do not depend</b> on the behavior of this method.
*
* <p>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(@CheckForNull Object object);
}
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