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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.index.searchindex;

import java.lang.annotation.Annotation;
import java.util.ArrayList;
import java.util.Arrays;
import java.util.HashSet;
import java.util.LinkedHashSet;
import java.util.List;
import java.util.Set;
import java.util.TreeSet;
import javax.lang.model.element.AnnotationMirror;
import javax.lang.model.element.ExecutableElement;
import javax.lang.model.util.Elements;
import org.checkerframework.checker.index.qual.NegativeIndexFor;
import org.checkerframework.checker.index.qual.SearchIndexBottom;
import org.checkerframework.checker.index.qual.SearchIndexFor;
import org.checkerframework.checker.index.qual.SearchIndexUnknown;
import org.checkerframework.common.basetype.BaseAnnotatedTypeFactory;
import org.checkerframework.common.basetype.BaseTypeChecker;
import org.checkerframework.common.value.ValueAnnotatedTypeFactory;
import org.checkerframework.common.value.ValueChecker;
import org.checkerframework.framework.type.ElementQualifierHierarchy;
import org.checkerframework.framework.type.QualifierHierarchy;
import org.checkerframework.javacutil.AnnotationBuilder;
import org.checkerframework.javacutil.AnnotationUtils;
import org.checkerframework.javacutil.TreeUtils;
import org.checkerframework.javacutil.TypeSystemError;

/**
 * The Search Index Checker is used to help type the results of calls to the JDK's binary search
 * methods. It is part of the Index Checker.
 */
public class SearchIndexAnnotatedTypeFactory extends BaseAnnotatedTypeFactory {

  /** The @{@link SearchIndexUnknown} annotation. */
  public final AnnotationMirror UNKNOWN =
      AnnotationBuilder.fromClass(elements, SearchIndexUnknown.class);
  /** The @{@link SearchIndexBottom} annotation. */
  public final AnnotationMirror BOTTOM =
      AnnotationBuilder.fromClass(elements, SearchIndexBottom.class);

  /** The NegativeIndexFor.value field/element. */
  protected final ExecutableElement negativeIndexForValueElement =
      TreeUtils.getMethod(NegativeIndexFor.class, "value", 0, processingEnv);
  /** The SearchIndexFor.value field/element. */
  protected final ExecutableElement searchIndexForValueElement =
      TreeUtils.getMethod(SearchIndexFor.class, "value", 0, processingEnv);

  /**
   * Create a new SearchIndexAnnotatedTypeFactory.
   *
   * @param checker the type-checker associated with this
   */
  public SearchIndexAnnotatedTypeFactory(BaseTypeChecker checker) {
    super(checker);

    this.postInit();
  }

  /**
   * Provides a way to query the Constant Value Checker, which computes the values of expressions
   * known at compile time (constant propagation and folding).
   */
  ValueAnnotatedTypeFactory getValueAnnotatedTypeFactory() {
    return getTypeFactoryOfSubchecker(ValueChecker.class);
  }

  @Override
  protected Set> createSupportedTypeQualifiers() {
    return new LinkedHashSet<>(
        Arrays.asList(
            SearchIndexFor.class,
            SearchIndexBottom.class,
            SearchIndexUnknown.class,
            NegativeIndexFor.class));
  }

  @Override
  protected QualifierHierarchy createQualifierHierarchy() {
    return new SearchIndexQualifierHierarchy(this.getSupportedTypeQualifiers(), elements);
  }

  /**
   * Returns the {@code value} field/element of the given annotation.
   *
   * @param am a @NegativeIndexFor or @SearchIndexFor annotation
   * @return the {@code value} field/element of the given annotation
   */
  private List getValueElement(AnnotationMirror am) {
    if (areSameByClass(am, NegativeIndexFor.class)) {
      return AnnotationUtils.getElementValueArray(am, negativeIndexForValueElement, String.class);
    } else if (areSameByClass(am, SearchIndexFor.class)) {
      return AnnotationUtils.getElementValueArray(am, searchIndexForValueElement, String.class);
    } else {
      throw new TypeSystemError("indexForValue(%s)", am);
    }
  }

  /** SearchIndexQualifierHierarchy. */
  private final class SearchIndexQualifierHierarchy extends ElementQualifierHierarchy {

    /**
     * Creates a SearchIndexQualifierHierarchy from the given classes.
     *
     * @param qualifierClasses classes of annotations that are the qualifiers
     * @param elements element utils
     */
    public SearchIndexQualifierHierarchy(
        Set> qualifierClasses, Elements elements) {
      super(qualifierClasses, elements);
    }

    @Override
    public AnnotationMirror greatestLowerBound(AnnotationMirror a1, AnnotationMirror a2) {
      if (AnnotationUtils.areSame(a1, UNKNOWN)) {
        return a2;
      }
      if (AnnotationUtils.areSame(a2, UNKNOWN)) {
        return a1;
      }
      if (AnnotationUtils.areSame(a1, BOTTOM)) {
        return a1;
      }
      if (AnnotationUtils.areSame(a2, BOTTOM)) {
        return a2;
      }
      if (isSubtype(a1, a2)) {
        return a1;
      }
      if (isSubtype(a2, a1)) {
        return a2;
      }
      // If neither is a subtype of the other, then create an
      // annotation that combines their values.

      // Each annotation is either NegativeIndexFor or SearchIndexFor.
      Set combinedArrays = new HashSet<>(getValueElement(a1));
      combinedArrays.addAll(getValueElement(a2));

      // NegativeIndexFor <: SearchIndexFor.
      if (areSameByClass(a1, NegativeIndexFor.class)
          || areSameByClass(a2, NegativeIndexFor.class)) {
        return createNegativeIndexFor(Arrays.asList(combinedArrays.toArray(new String[0])));
      } else {
        return createSearchIndexFor(Arrays.asList(combinedArrays.toArray(new String[0])));
      }
    }

    @Override
    public AnnotationMirror leastUpperBound(AnnotationMirror a1, AnnotationMirror a2) {
      if (AnnotationUtils.areSame(a1, UNKNOWN)) {
        return a1;
      }
      if (AnnotationUtils.areSame(a2, UNKNOWN)) {
        return a2;
      }
      if (AnnotationUtils.areSame(a1, BOTTOM)) {
        return a2;
      }
      if (AnnotationUtils.areSame(a2, BOTTOM)) {
        return a1;
      }
      if (isSubtype(a1, a2)) {
        return a2;
      }
      if (isSubtype(a2, a1)) {
        return a1;
      }
      // If neither is a subtype of the other, then create an
      // annotation that includes only their overlapping values.

      // Each annotation is either NegativeIndexFor or SearchIndexFor.
      List arrayIntersection = getValueElement(a1);
      arrayIntersection.retainAll(getValueElement(a2)); // intersection

      if (arrayIntersection.isEmpty()) {
        return UNKNOWN;
      }

      if (areSameByClass(a1, SearchIndexFor.class) || areSameByClass(a2, SearchIndexFor.class)) {
        return createSearchIndexFor(arrayIntersection);
      } else {
        return createNegativeIndexFor(arrayIntersection);
      }
    }

    @Override
    public boolean isSubtype(AnnotationMirror subAnno, AnnotationMirror superAnno) {
      if (areSameByClass(superAnno, SearchIndexUnknown.class)) {
        return true;
      }
      if (areSameByClass(subAnno, SearchIndexBottom.class)) {
        return true;
      }
      if (areSameByClass(subAnno, SearchIndexUnknown.class)) {
        return false;
      }
      if (areSameByClass(superAnno, SearchIndexBottom.class)) {
        return false;
      }

      // Each annotation is either NegativeIndexFor or SearchIndexFor.
      List superArrays = getValueElement(superAnno);
      List subArrays = getValueElement(subAnno);

      // Subtyping requires:
      //  * subtype is NegativeIndexFor or supertype is SearchIndexFor
      //  * subtype's arrays are a superset of supertype's arrays
      return ((areSameByClass(subAnno, NegativeIndexFor.class)
              || areSameByClass(superAnno, SearchIndexFor.class))
          && subArrays.containsAll(superArrays));
    }
  }

  /** Create a new {@code @NegativeIndexFor} annotation with the given arrays as its arguments. */
  AnnotationMirror createNegativeIndexFor(List arrays) {
    if (arrays.isEmpty()) {
      return UNKNOWN;
    }

    arrays = new ArrayList<>(new TreeSet<>(arrays)); // remove duplicates and sort

    AnnotationBuilder builder = new AnnotationBuilder(processingEnv, NegativeIndexFor.class);
    builder.setValue("value", arrays);
    return builder.build();
  }

  /** Create a new {@code @SearchIndexFor} annotation with the given arrays as its arguments. */
  AnnotationMirror createSearchIndexFor(List arrays) {
    if (arrays.isEmpty()) {
      return UNKNOWN;
    }

    arrays = new ArrayList<>(new TreeSet<>(arrays)); // remove duplicates and sort

    AnnotationBuilder builder = new AnnotationBuilder(processingEnv, SearchIndexFor.class);
    builder.setValue("value", arrays);
    return builder.build();
  }
}