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Small extensions for the core Java language libraries.
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/*
 * Copyright 2016 Yetamine
 *
 * 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 net.yetamine.lang.containers.values;

import java.util.Objects;
import java.util.function.Supplier;

/**
 * A value supplier that defers the computation of the value.
 *
 * 
 * When {@link #get()} is called, this class uses the given supplier to compute
 * the value and stores the result; the result is returned always then, without
 * recomputing the value, until {@link #invalidate()} is invoked.
 *
 * @param 
 *            the type of the element
 */
public final class Deferred implements Value {

    /** Supplier to compute the result on demand. */
    private final Supplier supplier;
    /** Result to return with {@link #get()}. */
    private volatile Supplier value;

    /**
     * Creates a new instance.
     *
     * @param compute
     *            the supplier that shall compute the value. It must not be
     *            {@code null}.
     */
    public Deferred(Supplier compute) {
        supplier = Objects.requireNonNull(compute);
    }

    /**
     * @see java.lang.Object#toString()
     */
    @Override
    public String toString() {
        return String.format("deferred[supplier=%s, value=%s]", supplier, value);
    }

    /**
     * Returns the value supplied by the given supplier.
     *
     * 
     * The supplier is invoked only once until {@link #invalidate()} explicitly
     * request to invalidate the result and the invocation is guarded by this
     * instance, so that the invocation may be unsafe or have side effects. A
     * {@code null} result is accepted as well, exceptions are relayed to the
     * caller.
     *
     * @see java.util.function.Supplier#get()
     */
    public T get() {
        final Supplier guess = value;
        if (guess != null) {
            return guess.get();
        }

        final T result;
        synchronized (this) {
            final Supplier box = value;
            if (box != null) { // Double-checked locking applied here
                return box.get();
            }

            result = supplier.get();
            value = () -> result;
        }

        return result;
    }

    /**
     * Invalidates the value held by this instance, hence subsequent
     * {@link #get()} invocation triggers the computation again.
     *
     * @see net.yetamine.lang.concurrent.Invalidable#invalidate()
     */
    public void invalidate() {
        value = null;
    }
}