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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;

import com.sun.source.tree.MemberSelectTree;
import org.checkerframework.checker.nullness.qual.Nullable;
import org.checkerframework.common.value.ValueCheckerUtils;
import org.checkerframework.dataflow.expression.JavaExpression;
import org.checkerframework.framework.type.AnnotatedTypeFactory;
import org.checkerframework.framework.type.AnnotatedTypeMirror;
import org.checkerframework.framework.type.treeannotator.TreeAnnotator;
import org.checkerframework.framework.util.dependenttypes.DependentTypesHelper;
import org.checkerframework.framework.util.dependenttypes.DependentTypesTreeAnnotator;
import org.checkerframework.javacutil.TreeUtils;

/**
 * Dependent type helper for array offset expressions. Each array offset expression may be the
 * addition or subtraction of several Java expressions. For example, {@code array.length - 1}.
 */
public class OffsetDependentTypesHelper extends DependentTypesHelper {
  public OffsetDependentTypesHelper(AnnotatedTypeFactory factory) {
    super(factory);
  }

  @Override
  protected @Nullable JavaExpression transform(JavaExpression javaExpr) {
    return ValueCheckerUtils.optimize(javaExpr, factory);
  }

  @Override
  public TreeAnnotator createDependentTypesTreeAnnotator() {
    return new DependentTypesTreeAnnotator(factory, this) {
      @Override
      public Void visitMemberSelect(MemberSelectTree tree, AnnotatedTypeMirror type) {
        // UpperBoundTreeAnnotator changes the type of array.length to @LTEL("array").
        // If the DependentTypesTreeAnnotator tries to viewpoint-adapt it based on the
        // declaration of length, it will fail.
        if (TreeUtils.isArrayLengthAccess(tree)) {
          return null;
        }
        return super.visitMemberSelect(tree, type);
      }
    };
  }
}