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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.Collections;
import java.util.IdentityHashMap;
import org.checkerframework.checker.interning.qual.FindDistinct;
import org.checkerframework.javacutil.BugInCF;

/**
 * An arbitrary-size set that is very efficient (more efficient than HashSet) for 0 and 1 elements.
 * Uses object identity for object comparison.
 */
public final class IdentityMostlySingleton extends AbstractMostlySingleton {

  /** Create an IdentityMostlySingleton. */
  public IdentityMostlySingleton() {
    super(State.EMPTY);
  }

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

  @Override
  public boolean add(@FindDistinct T e) {
    switch (state) {
      case EMPTY:
        state = State.SINGLETON;
        value = e;
        return true;
      case SINGLETON:
        if (value == e) {
          return false;
        }
        makeNonSingleton();
        // fall through
      case ANY:
        assert set != null : "@AssumeAssertion(nullness): set initialized before";
        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 = Collections.newSetFromMap(new IdentityHashMap<>(4));
    assert value != null : "@AssumeAssertion(nullness): previous add is non-null";
    set.add(value);
    value = null;
  }

  @SuppressWarnings("interning:not.interned") // this class uses object identity
  @Override
  public boolean contains(Object o) {
    switch (state) {
      case EMPTY:
        return false;
      case SINGLETON:
        return o == value;
      case ANY:
        assert set != null : "@AssumeAssertion(nullness): set initialized before";
        return set.contains(o);
      default:
        throw new BugInCF("Unhandled state " + state);
    }
  }
}