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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.common.subtyping.qual;

import java.lang.annotation.Documented;
import java.lang.annotation.ElementType;
import java.lang.annotation.Target;
import org.checkerframework.framework.qual.SubtypeOf;
import org.checkerframework.framework.qual.TargetLocations;
import org.checkerframework.framework.qual.TypeUseLocation;

/**
 * A special annotation intended solely for representing the bottom type in the qualifier hierarchy.
 * It should not be used! Instead, each type system should define its own dedicated bottom type.
 *
 * This qualifier is used automatically if the existing qualifiers do not have a bottom type.
 * This only works the user never runs two type systems together. Furthermore, because it has no
 * {@code @RetentionPolicy} meta-annotation, this qualifier will not be stored in bytecode. So, only
 * use this qualifier during prototyping of very simple type systems. For realistic systems,
 * introduce a top and bottom qualifier that gets stored in bytecode.
 *
 * To use this qualifier, the type system designer needs to use methods like {@code
 * org.checkerframework.framework.type.treeannotator.ImplicitsTreeAnnotator#addTreeKind(com.sun.source.tree.Tree.Kind,
 * javax.lang.model.element.AnnotationMirror)} to add default annotations and needs to manually add
 * the bottom qualifier to the qualifier hierarchy.
 *
 * @see org.checkerframework.framework.type.QualifierHierarchy#getBottomAnnotations()
 * @checker_framework.manual #subtyping-checker Subtyping Checker
 */
@Documented
@SubtypeOf({})
@Target({ElementType.TYPE_USE, ElementType.TYPE_PARAMETER})
@TargetLocations({TypeUseLocation.EXPLICIT_LOWER_BOUND, TypeUseLocation.EXPLICIT_UPPER_BOUND})
public @interface Bottom {}