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The Checker Framework enhances Java's type system to make it more powerful and useful. This lets software developers detect and prevent errors in their Java programs. The Checker Framework includes compiler plug-ins ("checkers") that find bugs or verify their absence. It also permits you to write your own compiler plug-ins.

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package org.checkerframework.checker.nullness.qual;

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;
import org.checkerframework.framework.qual.DefaultFor;
import org.checkerframework.framework.qual.LiteralKind;
import org.checkerframework.framework.qual.QualifierForLiterals;
import org.checkerframework.framework.qual.SubtypeOf;

/**
 * {@link Nullable} is a type annotation that makes no commitments about whether the value is {@code
 * null}. Equivalently, the type includes the {@code null} value.
 *
 * The Nullness Checker issues an error if {@code null} is assigned an expression of {@link
 * NonNull} type.
 *
 * Programmers typically write {@code @Nullable} to indicate that the value is not known to be
 * {@link NonNull}. However, since {@code @Nullable} is a supertype of {@code @NonNull}, an
 * expression that never evaluates to {@code null} can have a declared type of {@code @Nullable}.
 *
 * @see NonNull
 * @see MonotonicNonNull
 * @see org.checkerframework.checker.nullness.NullnessChecker
 * @checker_framework.manual #nullness-checker Nullness Checker
 */
@Documented
@Retention(RetentionPolicy.RUNTIME)
@Target({ElementType.TYPE_USE, ElementType.TYPE_PARAMETER})
@SubtypeOf({})
@QualifierForLiterals(LiteralKind.NULL)
@DefaultFor(types = Void.class)
public @interface Nullable {}