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The ANTLR 4 grammar compiler.

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/*
 * Copyright (c) 2012 The ANTLR Project. All rights reserved.
 * Use of this file is governed by the BSD-3-Clause license that
 * can be found in the LICENSE.txt file in the project root.
 */

package org.antlr.v4.runtime.misc;

import java.lang.annotation.Documented;
import java.lang.annotation.ElementType;
import java.lang.annotation.Retention;
import java.lang.annotation.RetentionPolicy;
import java.lang.annotation.Target;

/**
 * This annotation marks a field, parameter, local variable, or method (return
 * value) as potentially having the value {@code null}. The specific semantics
 * implied by this annotation depend on the kind of element the annotation is
 * applied to.
 *
 * 
 * Field or Local Variable: Code reading the field or local
 * variable may not assume that the value is never {@code null}.
 * Parameter: Code calling the method might pass
 * {@code null} for this parameter. The documentation for the method should
 * describe the behavior of the method in the event this parameter is
 * {@code null}.
 * 
 * Method (Return Value): Code calling the method may not
 * assume that the result of the method is never {@code null}. The documentation
 * for the method should describe the meaning of a {@code null} reference being
 * returned. Overriding methods may optionally use the {@link NotNull}
 * annotation instead of this annotation for the method, indicating that the
 * overriding method (and any method which overrides it) will never return a
 * {@code null} reference.
 * 
 *
 * 
 * The {@link NullUsageProcessor} annotation processor validates certain usage
 * scenarios for this annotation, with compile-time errors or warnings reported
 * for misuse. For detailed information about the supported analysis, see the
 * documentation for {@link NullUsageProcessor}.
 */
@Documented
@Retention(RetentionPolicy.CLASS)
@Target({ElementType.FIELD, ElementType.METHOD, ElementType.PARAMETER, ElementType.LOCAL_VARIABLE})
public @interface Nullable {
}