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JavaUtil is a utility library to speed the development of Java software. We developed it over multiple years during our internal development efforts, and it has reached a point of stability where we have decided to make it available to the outside world. The JavaUtil package aims to fill in the gaps of the Apache Commons and similar utility libraries out there. Features include convenience classes for string, collections, arrays, dates, iterators, colors, logging, unit testing, a little bit of basic statistics, database queries, caching and more.

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package org.arp.javautil.collections;

/*-
 * #%L
 * JavaUtil
 * %%
 * Copyright (C) 2012 - 2017 Emory University
 * %%
 * 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.
 * #L%
 */

import java.util.HashMap;

/**
 *
 * @author Andrew Post
 */
public class HashMapFactory {
    public  HashMap getInstance(int expectedSize) {
        return new HashMap<>(hashMapCapacity(expectedSize));
    }
    
    private static int hashMapCapacity(int expectedSize) {
        if (expectedSize < 0) {
            throw new IllegalArgumentException("expectedSize cannot be < 0");
        }
        if (expectedSize < 3) {
            return expectedSize + 1;
        } else if (expectedSize < 1 << (Integer.SIZE - 2)) { //largest power of two
            return expectedSize + expectedSize / 3;
        } else {
            return Integer.MAX_VALUE;
        }
    }
}