/*
 * Copyright (C) 2014 The Dagger Authors.
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 * http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

package dagger.internal;

import static dagger.internal.Preconditions.checkNotNull;

import dagger.Lazy;

/**
 * A {@link Factory} implementation that returns a single instance for all invocations of {@link
 * #get}.
 *
 * <p>Note that while this is a {@link Factory} implementation, and thus unscoped, each call to
 * {@link #get} will always return the same instance. As such, any scoping applied to this factory
 * is redundant and unnecessary. However, using this with {@link DoubleCheck#provider} is valid and
 * may be desired for testing or contractual guarantees.
 */
public final class InstanceFactory<T> implements Factory<T>, Lazy<T> {
  public static <T> Factory<T> create(T instance) {
    return new InstanceFactory<T>(checkNotNull(instance, "instance cannot be null"));
  }

  public static <T> Factory<T> createNullable(T instance) {
    return instance == null
        ? InstanceFactory.<T>nullInstanceFactory()
        : new InstanceFactory<T>(instance);
  }

  @SuppressWarnings("unchecked") // bivariant implementation
  private static <T> InstanceFactory<T> nullInstanceFactory() {
    return (InstanceFactory<T>) NULL_INSTANCE_FACTORY;
  }

  private static final InstanceFactory<Object> NULL_INSTANCE_FACTORY =
      new InstanceFactory<Object>(null);

  private final T instance;

  private InstanceFactory(T instance) {
    this.instance = instance;
  }

  @Override
  public T get() {
    return instance;
  }
}