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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.nullness;

import com.sun.source.tree.ExpressionTree;
import com.sun.source.tree.NewClassTree;
import com.sun.source.tree.VariableTree;
import javax.lang.model.element.ExecutableElement;
import javax.lang.model.type.TypeKind;
import org.checkerframework.checker.nullness.KeyForPropagator.PropagationDirection;
import org.checkerframework.framework.type.AnnotatedTypeFactory;
import org.checkerframework.framework.type.AnnotatedTypeMirror;
import org.checkerframework.framework.type.AnnotatedTypeMirror.AnnotatedDeclaredType;
import org.checkerframework.framework.type.treeannotator.TreeAnnotator;
import org.checkerframework.javacutil.TreeUtils;

/**
 * For the following initializations we wish to propagate the annotations from the left-hand side to
 * the right-hand side or vice versa:
 *
 * 1. If a keySet is being saved to a newly declared set, we transfer the annotations from the
 * keySet to the lhs. e.g.,
 *
 * 
{@code
 * // The user is not required to explicitly annotate the LHS's type argument with @KeyFor("m")
 * Set keySet = m.keySet();
 * }
 *
 * 2. If a variable declaration contains type arguments with an @KeyFor annotation and its
 * initializer is a new class tree with corresponding type arguments that have an @UknownKeyFor
 * primary annotation, we transfer from the LHS to RHS. e.g.,
 *
 * {@code
 * // The user does not have to write @KeyFor("m") on both sides
 * List<@KeyFor("m") String> keys = new ArrayList();
 * }
 *
 * 3. IMPORTANT NOTE: The following case must be (and is) handled in KeyForAnnotatedTypeFactory. In
 * BaseTypeVisitor we check to make sure that the constructor called in a NewClassTree is actually
 * compatible with the annotations placed on the NewClassTree. This requires that, prior to this
 * check we also propagate the annotations to this constructor in constructorFromUse so that the
 * constructor call matches the type given to the NewClassTree.
 *
 * @see
 *     org.checkerframework.checker.nullness.KeyForAnnotatedTypeFactory#constructorFromUse(com.sun.source.tree.NewClassTree)
 *     Note propagation only occurs between two AnnotatedDeclaredTypes. If either side is not an
 *     AnnotatedDeclaredType then this class does nothing.
 */
public class KeyForPropagationTreeAnnotator extends TreeAnnotator {
  private final KeyForPropagator keyForPropagator;
  private final ExecutableElement keySetMethod;

  public KeyForPropagationTreeAnnotator(
      AnnotatedTypeFactory atypeFactory, KeyForPropagator propagationTreeAnnotator) {
    super(atypeFactory);
    this.keyForPropagator = propagationTreeAnnotator;
    keySetMethod =
        TreeUtils.getMethod("java.util.Map", "keySet", 0, atypeFactory.getProcessingEnv());
  }

  /**
   * Returns true iff expression is a call to java.util.Map.KeySet.
   *
   * @return true iff expression is a call to java.util.Map.KeySet
   */
  public boolean isCallToKeyset(ExpressionTree expression) {
    return TreeUtils.isMethodInvocation(expression, keySetMethod, atypeFactory.getProcessingEnv());
  }

  /**
   * Transfers annotations on type arguments from the initializer to the variableTree, if the
   * initializer is a call to java.util.Map.keySet.
   */
  @Override
  public Void visitVariable(VariableTree variableTree, AnnotatedTypeMirror type) {
    super.visitVariable(variableTree, type);

    // This should only happen on Map.keySet();
    if (type.getKind() == TypeKind.DECLARED) {
      final ExpressionTree initializer = variableTree.getInitializer();

      if (isCallToKeyset(initializer)) {
        final AnnotatedDeclaredType variableType = (AnnotatedDeclaredType) type;
        final AnnotatedTypeMirror initializerType = atypeFactory.getAnnotatedType(initializer);

        // Propagate just for declared (class) types, not for array types, boxed primitives, etc.
        if (variableType.getKind() == TypeKind.DECLARED) {
          keyForPropagator.propagate(
              (AnnotatedDeclaredType) initializerType,
              variableType,
              PropagationDirection.TO_SUPERTYPE,
              atypeFactory);
        }
      }
    }

    return null;
  }

  /** Transfers annotations to type if the left hand side is a variable declaration. */
  @Override
  public Void visitNewClass(NewClassTree node, AnnotatedTypeMirror type) {
    keyForPropagator.propagateNewClassTree(node, type, (KeyForAnnotatedTypeFactory) atypeFactory);
    return super.visitNewClass(node, type);
  }
}