1 /* 2 * Copyright (C) 2021 The Guava Authors 3 * 4 * Licensed under the Apache License, Version 2.0 (the "License"); 5 * you may not use this file except in compliance with the License. 6 * You may obtain a copy of the License at 7 * 8 * http://www.apache.org/licenses/LICENSE-2.0 9 * 10 * Unless required by applicable law or agreed to in writing, software 11 * distributed under the License is distributed on an "AS IS" BASIS, 12 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. 13 * See the License for the specific language governing permissions and 14 * limitations under the License. 15 */ 16 17 package com.google.common.base; 18 19 import static java.lang.annotation.ElementType.FIELD; 20 import static java.lang.annotation.ElementType.METHOD; 21 import static java.lang.annotation.ElementType.PARAMETER; 22 import static java.lang.annotation.RetentionPolicy.RUNTIME; 23 24 import com.google.common.annotations.GwtCompatible; 25 import java.lang.annotation.Retention; 26 import java.lang.annotation.Target; 27 28 /** 29 * Annotates a "top-level" type-variable usage that takes its nullness from the type argument 30 * supplied by the user of the class. For example, {@code Multiset.Entry.getElement()} returns 31 * {@code @ParametricNullness E}, which means: 32 * 33 * <ul> 34 * <li>{@code getElement} on a {@code Multiset.Entry<@NonNull String>} returns {@code @NonNull 35 * String}. 36 * <li>{@code getElement} on a {@code Multiset.Entry<@Nullable String>} returns {@code @Nullable 37 * String}. 38 * </ul> 39 * 40 * This is the same behavior as type-variable usages have to Kotlin and to the Checker Framework. 41 * Contrast the method above to: 42 * 43 * <ul> 44 * <li>methods whose return type is a type variable but which can never return {@code null}, 45 * typically because the type forbids nullable type arguments: For example, {@code 46 * ImmutableList.get} returns {@code E}, but that value is never {@code null}. (Accordingly, 47 * {@code ImmutableList} is declared to forbid {@code ImmutableList<@Nullable String>}.) 48 * <li>methods whose return type is a type variable but which can return {@code null} regardless 49 * of the type argument supplied by the user of the class: For example, {@code 50 * ImmutableMap.get} returns {@code @Nullable E} because the method can return {@code null} 51 * even on an {@code ImmutableMap<K, @NonNull String>}. 52 * </ul> 53 * 54 * <p>Consumers of this annotation include: 55 * 56 * <ul> 57 * <li>Kotlin, for which it makes the type-variable usage (a) a Kotlin platform type when the type 58 * argument is non-nullable and (b) nullable when the type argument is nullable. We use this 59 * to "undo" {@link ElementTypesAreNonnullByDefault}. It is the best we can do for Kotlin 60 * under our current constraints. 61 * <li>NullAway, which will <a 62 * href="https://github.com/google/guava/issues/6126#issuecomment-1204399671">treat it 63 * identically to {@code Nullable} as of version 0.9.9</a>. To treat it that way before then, 64 * you can set {@code 65 * -XepOpt:NullAway:CustomNullableAnnotations=com.google.common.base.ParametricNullness,...,com.google.common.util.concurrent.ParametricNullness}, 66 * where the {@code ...} contains the names of all the other {@code ParametricNullness} 67 * annotations in Guava. Or you might prefer to omit Guava from your {@code AnnotatedPackages} 68 * list. 69 * <li><a href="https://developers.google.com/j2objc">J2ObjC</a> 70 * <li>{@code NullPointerTester}, at least in the Android backport (where the type-use annotations 71 * {@code NullPointerTester} would need are not available) and in case of <a 72 * href="https://bugs.openjdk.java.net/browse/JDK-8202469">JDK-8202469</a> 73 * </ul> 74 * 75 * <p>This annotation is a temporary hack. We will remove it after we're able to adopt the <a 76 * href="https://jspecify.dev/">JSpecify</a> nullness annotations and <a 77 * href="https://github.com/google/guava/issues/6126#issuecomment-1203145963">tools no longer need 78 * it</a>. 79 */ 80 @GwtCompatible 81 @Retention(RUNTIME) 82 @Target({FIELD, METHOD, PARAMETER}) 83 @javax.annotation.meta.TypeQualifierNickname 84 @javax.annotation.Nonnull(when = javax.annotation.meta.When.UNKNOWN) 85 @interface ParametricNullness {} 86