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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.livevariable;

import org.checkerframework.checker.nullness.qual.Nullable;
import org.checkerframework.dataflow.cfg.node.FieldAccessNode;
import org.checkerframework.dataflow.cfg.node.LocalVariableNode;
import org.checkerframework.dataflow.cfg.node.Node;

/**
 * A LiveVarNode contains a CFG node, which can only be a LocalVariableNode or FieldAccessNode. It
 * is used to represent the estimate of live variables 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 value (that is, two nodes refer to
 * the same live variable in the program) to be regarded as the same.
 */
public class LiveVarNode {

  /**
   * A live variable is represented by a node, which can be a {@link
   * org.checkerframework.dataflow.cfg.node.LocalVariableNode} or {@link
   * org.checkerframework.dataflow.cfg.node.FieldAccessNode}.
   */
  protected final Node liveVariable;

  /**
   * Create a new live variable.
   *
   * @param n a node
   */
  public LiveVarNode(Node n) {
    assert n instanceof FieldAccessNode || n instanceof LocalVariableNode;
    this.liveVariable = n;
  }

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

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

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