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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.framework.util;

import java.lang.annotation.Annotation;
import java.util.Set;
import org.checkerframework.checker.interning.qual.Interned;
import org.checkerframework.checker.nullness.qual.Nullable;
import org.checkerframework.checker.signature.qual.CanonicalName;
import org.checkerframework.dataflow.qual.Pure;
import org.checkerframework.framework.qual.AnnotatedFor;

/**
 * Represents a kind of qualifier, which is an annotation class. If two qualifiers use the same
 * annotation class, then they have the same qualifier kind. Two qualifiers can have the same "kind"
 * of qualifier but not be the same qualifier; an example is {@code @IndexFor("a")} and
 * {@code @IndexFor("b")}.
 *
 * A {@code QualifierKind} holds information about the relationship between itself and other
 * {@link QualifierKind}s.
 *
 * 
Exactly one qualifier kind is created for each annotation class.
 *
 * The set of all QualifierKinds for a checker is like an enum. One QualifierKind is like an enum
 * constant in that they are immutable after initialization and only a finite number exist per type
 * system.
 */
@AnnotatedFor("nullness")
public @Interned interface QualifierKind extends Comparable {

  /**
   * Returns the canonical name of the annotation class of this.
   *
   * @return the canonical name of the annotation class of this
   */
  @Interned @CanonicalName String getName();

  /**
   * Returns the annotation class for this.
   *
   * @return the annotation class for this
   */
  Class getAnnotationClass();

  /**
   * Returns the top qualifier kind of the hierarchy to which this qualifier kind belongs.
   *
   * @return the top qualifier kind of the hierarchy to which this qualifier kind belongs
   */
  QualifierKind getTop();

  /**
   * Returns true if this is the top qualifier of its hierarchy.
   *
   * @return true if this is the top qualifier of its hierarchy
   */
  boolean isTop();

  /**
   * Returns the bottom qualifier kind of the hierarchy to which this qualifier kind belongs.
   *
   * @return the bottom qualifier kind of the hierarchy to which this qualifier kind belongs
   */
  QualifierKind getBottom();

  /**
   * Returns true if this is the bottom qualifier of its hierarchy.
   *
   * @return true if this is the bottom qualifier of its hierarchy
   */
  boolean isBottom();

  /**
   * Returns the polymorphic qualifier kind of the hierarchy to which this qualifier kind belongs,
   * or null if one does not exist.
   *
   * @return the polymorphic qualifier kind of the hierarchy to which this qualifier kind belongs,
   *     or null if one does not exist
   */
  @Nullable QualifierKind getPolymorphic();

  /**
   * Returns true if this is polymorphic.
   *
   * @return true if this is polymorphic
   */
  @Pure
  boolean isPoly();

  /**
   * Returns true if the annotation class this qualifier kind represents has annotation
   * elements/arguments.
   *
   * @return true if the annotation class this qualifier kind represents has elements/arguments
   */
  boolean hasElements();

  /**
   * All the qualifier kinds that are a strict super qualifier of this qualifier. Does not include
   * this qualifier kind itself.
   *
   * @return all the qualifier kinds that are a strict super qualifier of this qualifier
   */
  Set getStrictSuperTypes();

  /**
   * Returns true if this and {@code other} are in the same hierarchy.
   *
   * @param other a qualifier kind
   * @return true if this and {@code other} are in the same hierarchy
   */
  boolean isInSameHierarchyAs(QualifierKind other);

  /**
   * Returns true if this qualifier kind is a subtype of or equal to {@code superQualKind}.
   *
   * @param superQualKind other qualifier kind
   * @return true if this qualifier kind is a subtype of or equal to {@code superQualKind}
   */
  boolean isSubtypeOf(QualifierKind superQualKind);

  @Override
  default int compareTo(QualifierKind o) {
    return this.getName().compareTo(o.getName());
  }
}