content: /*
   * 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 org.whispersystems.libsignal.util.guava;
  
  import java.util.Collections;
  import java.util.Set;
  
  import static org.whispersystems.libsignal.util.guava.Preconditions.checkNotNull;
  
  
  /**
   * Implementation of an {@link Optional} not containing a reference.
   */
  
  final class Absent extends Optional<Object> {
    static final Absent INSTANCE = new Absent();
  
    @Override public boolean isPresent() {
      return false;
    }
  
    @Override public Object get() {
      throw new IllegalStateException("value is absent");
    }
  
    @Override public Object or(Object defaultValue) {
      return checkNotNull(defaultValue, "use orNull() instead of or(null)");
    }
  
    @SuppressWarnings("unchecked") // safe covariant cast
    @Override public Optional<Object> or(Optional<?> secondChoice) {
      return (Optional) checkNotNull(secondChoice);
    }
  
    @Override public Object or(Supplier<?> supplier) {
      return checkNotNull(supplier.get(),
          "use orNull() instead of a Supplier that returns null");
    }
  
    @Override public Object orNull() {
      return null;
    }
  
    @Override public Set<Object> asSet() {
      return Collections.emptySet();
    }
  
    @Override
    public <V> Optional<V> transform(Function<? super Object, V> function) {
      checkNotNull(function);
      return Optional.absent();
    }
  
    @Override public boolean equals(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;
  }
  