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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.List;
import java.util.Set;
import javax.lang.model.element.AnnotationMirror;
import org.checkerframework.checker.nullness.qual.Nullable;
import org.checkerframework.checker.signature.qual.CanonicalName;
import org.checkerframework.framework.qual.AnnotatedFor;
import org.checkerframework.framework.type.ElementQualifierHierarchy;
import org.checkerframework.framework.type.MostlyNoElementQualifierHierarchy;
import org.checkerframework.framework.type.NoElementQualifierHierarchy;
import org.checkerframework.javacutil.TypeSystemError;

/**
 * This interface holds information about the subtyping relationships between kinds of qualifiers. A
 * "kind" of qualifier is its annotation class and is represented by the {@link QualifierKind}
 * class. If a type system has more than one hierarchy, information about all hierarchies is stored
 * in this class.
 *
 * The qualifier kind subtyping relationship may be an over-approximation of the qualifier
 * subtyping relationship, for qualifiers that have elements/arguments. In other words, if a
 * qualifier kind is a subtype of another qualifier kind, then qualifiers of those kinds may or may
 * not be subtypes, depending on the values of any elements of the qualifiers. If qualifier kinds
 * are not subtypes, then qualifiers of those kinds are never subtypes.
 *
 * This interface is used by {@link NoElementQualifierHierarchy} and {@link
 * ElementQualifierHierarchy} (but not {@link MostlyNoElementQualifierHierarchy}) to
 * implement methods that compare {@link javax.lang.model.element.AnnotationMirror}s, such as {@link
 * org.checkerframework.framework.type.QualifierHierarchy#isSubtype(AnnotationMirror,
 * AnnotationMirror)}.
 *
 * @see DefaultQualifierKindHierarchy
 * @see org.checkerframework.framework.util.DefaultQualifierKindHierarchy.DefaultQualifierKind
 */
@AnnotatedFor("nullness")
public interface QualifierKindHierarchy {

  /**
   * Returns the qualifier kinds that are the top qualifier in their hierarchies.
   *
   * @return the qualifier kinds that are the top qualifier in their hierarchies
   */
  Set getTops();

  /**
   * Returns the qualifier kinds that are the bottom qualifier in their hierarchies.
   *
   * @return the qualifier kinds that are the bottom qualifier in their hierarchies
   */
  Set getBottoms();

  /**
   * Returns the least upper bound of {@code q1} and {@code q2}, or {@code null} if the qualifier
   * kinds are not in the same hierarchy. Ignores elements/arguments (as QualifierKind always does).
   *
   * @param q1 a qualifier kind
   * @param q2 a qualifier kind
   * @return the least upper bound of {@code q1} and {@code q2}, or {@code null} if the qualifier
   *     kinds are not in the same hierarchy
   */
  @Nullable QualifierKind leastUpperBound(QualifierKind q1, QualifierKind q2);

  /**
   * Returns the greatest lower bound of {@code q1} and {@code q2}, or {@code null} if the qualifier
   * kinds are not in the same hierarchy. Ignores elements/arguments (as QualifierKind always does).
   *
   * @param q1 a qualifier kind
   * @param q2 a qualifier kind
   * @return the greatest lower bound of {@code q1} and {@code q2}, or {@code null} if the qualifier
   *     kinds are not in the same hierarchy
   */
  @Nullable QualifierKind greatestLowerBound(QualifierKind q1, QualifierKind q2);

  /**
   * Returns a list of all {@link QualifierKind}s sorted in ascending order.
   *
   * @return a list of all {@link QualifierKind}s sorted in ascending order
   */
  List allQualifierKinds();

  /**
   * Returns the {@link QualifierKind} for the given annotation class name, or null if one does not
   * exist.
   *
   * @param name canonical name of an annotation class
   * @return the {@link QualifierKind} for the given annotation class name, or null if one does not
   *     exist
   */
  @Nullable QualifierKind getQualifierKind(@CanonicalName String name);

  /**
   * Returns the canonical name of {@code clazz}. Throws a {@link TypeSystemError} if {@code clazz}
   * is anonymous or otherwise does not have a name.
   *
   * @param clazz annotation class
   * @return the canonical name of {@code clazz}
   */
  static @CanonicalName String annotationClassName(Class clazz) {
    String name = clazz.getCanonicalName();
    if (name == null) {
      throw new TypeSystemError("Qualifier classes must not be anonymous.");
    }
    return name;
  }
}