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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.util;

import java.util.LinkedHashSet;
import org.checkerframework.javacutil.BugInCF;

/**
 * A set that is more efficient than HashSet for 0 and 1 elements. Uses {@code Objects.equals} for
 * object comparison and a {@link LinkedHashSet} for backing storage.
 */
public final class MostlySingleton extends AbstractMostlySingleton {

  /** Create a MostlySingleton. */
  public MostlySingleton() {
    super(State.EMPTY);
  }

  /** Create a MostlySingleton. */
  public MostlySingleton(T value) {
    super(State.SINGLETON, value);
  }

  @Override
  public boolean add(T e) {
    switch (state) {
      case EMPTY:
        state = State.SINGLETON;
        value = e;
        return true;
      case SINGLETON:
        assert value != null : "@AssumeAssertion(nullness): SINGLETON => value != null";
        if (value.equals(e)) {
          return false;
        }
        makeNonSingleton();
        // fall through
      case ANY:
        assert set != null : "@AssumeAssertion(nullness): ANY => value != null";
        return set.add(e);
      default:
        throw new BugInCF("Unhandled state " + state);
    }
  }

  /** Switch the representation of this from SINGLETON to ANY. */
  private void makeNonSingleton() {
    state = State.ANY;
    set = new LinkedHashSet<>();
    assert value != null : "@AssumeAssertion(nullness): SINGLETON => value != null";
    set.add(value);
    value = null;
  }

  @Override
  public boolean contains(Object o) {
    switch (state) {
      case EMPTY:
        return false;
      case SINGLETON:
        assert value != null : "@AssumeAssertion(nullness): SINGLETON => value != null";
        return value.equals(o);
      case ANY:
        assert set != null : "@AssumeAssertion(nullness): set initialized before";
        return set.contains(o);
      default:
        throw new BugInCF("Unhandled state " + state);
    }
  }
}