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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.dataflow.reachingdef;

import org.checkerframework.checker.nullness.qual.Nullable;
import org.checkerframework.dataflow.cfg.node.AssignmentNode;

/**
 * A ReachingDefinitionNode contains a CFG node, which can only be a AssignmentNode. It is used to
 * represent the estimate of a reaching definition at certain CFG block during dataflow analysis. We
 * override `.equals` in this class to compare Nodes by value equality rather than reference
 * equality. We want two different nodes with the same values (that is, the two nodes refer to the
 * same reaching definition in the program) to be regarded as the same here.
 */
public class ReachingDefinitionNode {

  /**
   * A reaching definition is represented by a node, which can only be a {@link
   * org.checkerframework.dataflow.cfg.node.AssignmentNode}.
   */
  protected final AssignmentNode def;

  /**
   * Create a new reaching definition.
   *
   * @param n an assignment node
   */
  public ReachingDefinitionNode(AssignmentNode n) {
    this.def = n;
  }

  @Override
  public int hashCode() {
    return this.def.hashCode();
  }

  @Override
  public boolean equals(@Nullable Object obj) {
    if (!(obj instanceof ReachingDefinitionNode)) {
      return false;
    }
    ReachingDefinitionNode other = (ReachingDefinitionNode) obj;
    // We use `.equals` instead of `==` here to compare value equality.
    return this.def.equals(other.def);
  }

  @Override
  public String toString() {
    return this.def.toString();
  }
}