/*
 * Copyright 2010-2020 JetBrains s.r.o. and Kotlin Programming Language contributors.
 * Use of this source code is governed by the Apache 2.0 license that can be found in the license/LICENSE.txt file.
 */

package org.jetbrains.annotations;

import java.lang.annotation.*;

/**
 * An element annotated with {@link Nullable} claims {@code null} value is perfectly <em>valid</em>
 * to return (for methods), pass to (parameters) or hold in (local variables and fields).
 * Apart from documentation purposes this annotation is intended to be used by static analysis tools
 * to validate against probable runtime errors or element contract violations.
 * <br>
 * By convention, this annotation applied only when the value should <em>always</em> be checked against {@code null}
 * because the developer could do nothing to prevent null from happening.
 * Otherwise, too eager {@link Nullable} usage could lead to too many false positives from static analysis tools.
 * <br>
 * For example, {@link java.util.Map#get(Object key)} should <em>not</em> be annotated {@link Nullable} because
 * someone may have put not-null value in the map by this key and is expecting to find this value there ever since.
 * <br>
 * On the other hand, the {@link java.lang.ref.Reference#get()} should be annotated {@link Nullable} because
 * it returns {@code null} if object got collected which can happen at any time completely unexpectedly.
 */
@Documented
@Retention(RetentionPolicy.CLASS)
@Target({ElementType.METHOD, ElementType.FIELD, ElementType.PARAMETER, ElementType.LOCAL_VARIABLE, ElementType.TYPE_USE})
public @interface Nullable {
    String value() default "";
}