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Durian - Guava's spikier (unofficial) cousin

There is a newer version: 3.4.0

/**
 * Copyright 2015 DiffPlug
 *
 * 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.diffplug.common.base;

import java.util.Objects;

/**
 * Simple class for creating boxed variables. Useful when you
 * need to make a mutable variable available to a closure.
 */
public class Box implements GetterSetter {
	/** The (possibly-null) object being held. */
	private volatile T obj;

	protected Box(T init) {
		this.set(init);
	}

	/** Creates a Holder of the given object. */
	public static  Box of(T init) {
		return new Box(init);
	}

	/** Creates an empty Holder object. */
	public static  Box empty() {
		return new Box(null);
	}

	@Override
	public T get() {
		return obj;
	}

	@Override
	public void set(T obj) {
		this.obj = obj;
	}

	@Override
	public String toString() {
		return get() == null ? "(null)" : get().toString();
	}

	/** A Box which guarantees to never be null (by disallowing null values in its setter). */
	public static class NonNull extends Box {
		public static  NonNull of(T value) {
			return new NonNull(value);
		}

		protected NonNull(T value) {
			super(Objects.requireNonNull(value));
		}

		@Override
		public void set(T obj) {
			super.set(Objects.requireNonNull(obj));
		}
	}
}