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/*
 * Copyright (C) 2021 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.graph;

import static java.lang.annotation.ElementType.FIELD;
import static java.lang.annotation.ElementType.METHOD;
import static java.lang.annotation.ElementType.PARAMETER;
import static java.lang.annotation.RetentionPolicy.RUNTIME;

import com.google.common.annotations.GwtCompatible;
import java.lang.annotation.Retention;
import java.lang.annotation.Target;

/**
 * Annotates a "top-level" type-variable usage that takes its nullness from the type argument
 * supplied by the user of the class. For example, {@code Multiset.Entry.getElement()} returns
 * {@code @ParametricNullness E}, which means:
 *
 * 
    *
  • {@code getElement} on a {@code Multiset.Entry<@NonNull String>} returns {@code @NonNull * String}. *
  • {@code getElement} on a {@code Multiset.Entry<@Nullable String>} returns {@code @Nullable * String}. *
* * This is the same behavior as type-variable usages have to Kotlin and to the Checker Framework. * Contrast the method above to: * *
    *
  • methods whose return type is a type variable but which can never return {@code null}, * typically because the type forbids nullable type arguments: For example, {@code * ImmutableList.get} returns {@code E}, but that value is never {@code null}. (Accordingly, * {@code ImmutableList} is declared to forbid {@code ImmutableList<@Nullable String>}.) *
  • methods whose return type is a type variable but which can return {@code null} regardless * of the type argument supplied by the user of the class: For example, {@code * ImmutableMap.get} returns {@code @Nullable E} because the method can return {@code null} * even on an {@code ImmutableMap}. *
* *

Consumers of this annotation include: * *

    *
  • Kotlin, for which it makes the type-variable usage (a) a Kotlin platform type when the type * argument is non-nullable and (b) nullable when the type argument is nullable. We use this * to "undo" {@link ElementTypesAreNonnullByDefault}. It is the best we can do for Kotlin * under our current constraints. *
  • NullAway, which will treat it * identically to {@code Nullable} as of version 0.9.9. To treat it that way before then, * you can set {@code * -XepOpt:NullAway:CustomNullableAnnotations=com.google.common.base.ParametricNullness,...,com.google.common.util.concurrent.ParametricNullness}, * where the {@code ...} contains the names of all the other {@code ParametricNullness} * annotations in Guava. Or you might prefer to omit Guava from your {@code AnnotatedPackages} * list. *
  • J2ObjC *
  • {@code NullPointerTester}, at least in the Android backport (where the type-use annotations * {@code NullPointerTester} would need are not available) and in case of JDK-8202469 *
* *

This annotation is a temporary hack. We will remove it after we're able to adopt the JSpecify nullness annotations and tools no longer need * it. */ @GwtCompatible @Retention(RUNTIME) @Target({FIELD, METHOD, PARAMETER}) @javax.annotation.meta.TypeQualifierNickname @javax.annotation.Nonnull(when = javax.annotation.meta.When.UNKNOWN) @interface ParametricNullness {}





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