1 /* 2 * Copyright (C) 2011 The Guava Authors 3 * 4 * Licensed under the Apache License, Version 2.0 (the "License"); 5 * you may not use this file except in compliance with the License. 6 * You may obtain a copy of the License at 7 * 8 * http://www.apache.org/licenses/LICENSE-2.0 9 * 10 * Unless required by applicable law or agreed to in writing, software 11 * distributed under the License is distributed on an "AS IS" BASIS, 12 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. 13 * See the License for the specific language governing permissions and 14 * limitations under the License. 15 */ 16 17 package com.google.common.base; 18 19 import static com.google.common.base.Preconditions.checkNotNull; 20 21 import com.google.common.annotations.GwtCompatible; 22 23 import java.util.Collections; 24 import java.util.Set; 25 26 import javax.annotation.Nullable; 27 28 /** 29 * Implementation of an {@link Optional} not containing a reference. 30 */ 31 @GwtCompatible 32 final class Absent<T> extends Optional<T> { 33 static final Absent<Object> INSTANCE = new Absent<Object>(); 34 35 @SuppressWarnings("unchecked") // implementation is "fully variant" 36 static <T> Optional<T> withType() { 37 return (Optional<T>) INSTANCE; 38 } 39 40 private Absent() {} 41 42 @Override public boolean isPresent() { 43 return false; 44 } 45 46 @Override public T get() { 47 throw new IllegalStateException("Optional.get() cannot be called on an absent value"); 48 } 49 50 @Override public T or(T defaultValue) { 51 return checkNotNull(defaultValue, "use Optional.orNull() instead of Optional.or(null)"); 52 } 53 54 @SuppressWarnings("unchecked") // safe covariant cast 55 @Override public Optional<T> or(Optional<? extends T> secondChoice) { 56 return (Optional<T>) checkNotNull(secondChoice); 57 } 58 59 @Override public T or(Supplier<? extends T> supplier) { 60 return checkNotNull(supplier.get(), 61 "use Optional.orNull() instead of a Supplier that returns null"); 62 } 63 64 @Override @Nullable public T orNull() { 65 return null; 66 } 67 68 @Override public Set<T> asSet() { 69 return Collections.emptySet(); 70 } 71 72 @Override public <V> Optional<V> transform(Function<? super T, V> function) { 73 checkNotNull(function); 74 return Optional.absent(); 75 } 76 77 @Override public boolean equals(@Nullable Object object) { 78 return object == this; 79 } 80 81 @Override public int hashCode() { 82 return 0x598df91c; 83 } 84 85 @Override public String toString() { 86 return "Optional.absent()"; 87 } 88 89 private Object readResolve() { 90 return INSTANCE; 91 } 92 93 private static final long serialVersionUID = 0; 94 } 95
