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Functional programming library

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/*
 * Copyright 2014 Shape Security, Inc.
 *
 * 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.shapesecurity.functional;

import javax.annotation.Nonnull;
import javax.annotation.Nullable;
import java.util.function.Supplier;

// Used for lazy evaluation.
// @FunctionalInterface
public final class Thunk {
    @Nonnull
    private final Supplier supplier;
    // Exception on style: private nullable.
    @Nullable
    private volatile A value = null;

    private Thunk(@Nonnull Supplier supplier) {
        this.supplier = supplier;
    }

    @Nonnull
    public static  Thunk constant(@Nonnull final A value) {
        Thunk t = new Thunk<>(() -> value);
        t.value = value;
        return t;
    }

    @Nonnull
    public static  Thunk from(@Nonnull Supplier supplier) {
        return new Thunk<>(supplier);
    }

    @Nonnull
    public final A get() {
        // Double locked.
        A value = this.value;
        if (value == null) {
            synchronized (this) {
                value = this.value;
                if (value == null) {
                    value = this.supplier.get();
                    this.value = value;
                }
            }
        }
        return value;
    }
}

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