Update #0 - First Release

sources/main/java/com/google/common/base/Absent.java

@@ -0,0 +1,106 @@
+/*
+ * 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 java.util.Collections;
+import java.util.Set;
+
+import javax.annotation.Nullable;
+
+import com.google.common.annotations.GwtCompatible;
+
+/**
+ * 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;
+}