Absent.java

package com.zhuinden.simplestackdemoexamplefragments.util.optional;

/*
 * 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.
 */

import android.support.annotation.Nullable;

import com.zhuinden.simplestackdemoexamplefragments.util.Preconditions;

import java.util.Collections;
import java.util.Set;


/**
 * Implementation of an {@link Optional} not containing a reference.
 */
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 Preconditions.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>) Preconditions.checkNotNull(secondChoice);
    }

    @Override
    @Nullable
    public T orNull() {
        return null;
    }

    @Override
    public Set<T> asSet() {
        return Collections.emptySet();
    }

    @Override
    public boolean equals(@Nullable Object object) {
        return object == this;
    }

    @Override
    public int hashCode() {
        return 0x79a31aac;
    }

    @Override
    public String toString() {
        return "Optional.absent()";
    }

    private Object readResolve() {
        return INSTANCE;
    }

    private static final long serialVersionUID = 0;
}