/*
    * Copyright (C) 2011 The Guava 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 com.google.common.base;
  
  import static com.google.common.base.Preconditions.checkNotNull;
  
  import com.google.common.annotations.GwtCompatible;
  
  import java.util.Collections;
  import java.util.Set;
  
  import javax.annotation.Nullable;
  
  /**
   * Implementation of an {@link Optional} not containing a reference.
   */
  @GwtCompatible
  final class Absent<T> extends Optional<T> {
    static final Absent<Object> INSTANCE = new Absent<Object>();
  
    @SuppressWarnings("unchecked") // implementation is "fully variant"
    static <T> Optional<T> withType() {
      return (Optional<T>) INSTANCE;
    }
  
    private Absent() {}
  
    @Override public boolean isPresent() {
      return false;
    }
  
    @Override public T get() {
      throw new IllegalStateException("Optional.get() cannot be called on an absent value");
    }
  
    @Override public T or(T defaultValue) {
      return checkNotNull(defaultValue, "use Optional.orNull() instead of Optional.or(null)");
    }
  
    @SuppressWarnings("unchecked") // safe covariant cast
    @Override public Optional<T> or(Optional<? extends T> secondChoice) {
      return (Optional<T>) checkNotNull(secondChoice);
    }
  
    @Override public T or(Supplier<? extends T> supplier) {
      return checkNotNull(supplier.get(),
          "use Optional.orNull() instead of a Supplier that returns null");
    }
  
    @Override @Nullable public T orNull() {
      return null;
    }
  
    @Override public Set<T> asSet() {
      return Collections.emptySet();
    }
  
    @Override public <V> Optional<V> transform(Function<? super T, V> function) {
      checkNotNull(function);
      return Optional.absent();
    }
  
    @Override public boolean equals(@Nullable Object object) {
      return object == this;
    }
  
    @Override public int hashCode() {
      return 0x598df91c;
    }
  
    @Override public String toString() {
      return "Optional.absent()";
    }
  
    private Object readResolve() {
      return INSTANCE;
    }
  
    private static final long serialVersionUID = 0;
  }