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Guava library, with additional type annotations for verification

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package com.google.common.graph;

import static com.google.common.base.Preconditions.checkNotNull;

import com.google.common.base.Supplier;
import com.google.common.collect.ForwardingSet;
import java.util.Set;

/**
 * A subclass of `ForwardingSet` that throws `IllegalStateException` on invocation of any method
 * (except `hashCode` and `equals`) if the provided `Supplier` returns false.
 */
@ElementTypesAreNonnullByDefault
final class InvalidatableSet extends ForwardingSet {
  private final Supplier validator;
  private final Set delegate;
  private final Supplier errorMessage;

  public static final  InvalidatableSet of(
      Set delegate, Supplier validator, Supplier errorMessage) {
    return new InvalidatableSet<>(
        checkNotNull(delegate), checkNotNull(validator), checkNotNull(errorMessage));
  }

  @Override
  protected Set delegate() {
    validate();
    return delegate;
  }

  private InvalidatableSet(
      Set delegate, Supplier validator, Supplier errorMessage) {
    this.delegate = delegate;
    this.validator = validator;
    this.errorMessage = errorMessage;
  }

  // Override hashCode() to access delegate directly (so that it doesn't trigger the validate() call
  // via delegate()); it seems inappropriate to throw ISE on this method.
  @Override
  public int hashCode() {
    return delegate.hashCode();
  }

  private void validate() {
    // Don't use checkState(), because we don't want the overhead of generating the error message
    // unless it's actually going to be used; validate() is called for all set method calls, so it
    // needs to be fast.
    // (We could instead generate the message once, when the set is created, but zero is better.)
    if (!validator.get()) {
      throw new IllegalStateException(errorMessage.get());
    }
  }
}