/**
 * The MIT License
 * Copyright (c) 2014-2016 Ilkka Seppälä
 *
 * Permission is hereby granted, free of charge, to any person obtaining a copy
 * of this software and associated documentation files (the "Software"), to deal
 * in the Software without restriction, including without limitation the rights
 * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
 * copies of the Software, and to permit persons to whom the Software is
 * furnished to do so, subject to the following conditions:
 *
 * The above copyright notice and this permission notice shall be included in
 * all copies or substantial portions of the Software.
 *
 * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
 * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
 * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
 * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
 * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
 * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
 * THE SOFTWARE.
 */
package com.iluwatar.object.pool;

import java.util.HashSet;
import java.util.Set;

/**
 * Generic object pool
 * @param <T> Type T of Object in the Pool
 */
public abstract class ObjectPool<T> {

  private Set<T> available = new HashSet<>();
  private Set<T> inUse = new HashSet<>();

  protected abstract T create();

  /**
   * Checkout object from pool
   */
  public synchronized T checkOut() {
    if (available.size() <= 0) {
      available.add(create());
    }
    T instance = available.iterator().next();
    available.remove(instance);
    inUse.add(instance);
    return instance;
  }

  public synchronized void checkIn(T instance) {
    inUse.remove(instance);
    available.add(instance);
  }

  @Override
  public String toString() {
    return String.format("Pool available=%d inUse=%d", available.size(), inUse.size());
  }
}

/**
 * The MIT License
 * Copyright (c) 2014-2016 Ilkka Seppälä
 *
 * Permission is hereby granted, free of charge, to any person obtaining a copy
 * of this software and associated documentation files (the "Software"), to deal
 * in the Software without restriction, including without limitation the rights
 * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
 * copies of the Software, and to permit persons to whom the Software is
 * furnished to do so, subject to the following conditions:
 *
 * The above copyright notice and this permission notice shall be included in
 * all copies or substantial portions of the Software.
 *
 * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
 * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
 * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
 * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
 * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
 * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
 * THE SOFTWARE.
 */
package com.iluwatar.object.pool;

import java.util.HashSet;
import java.util.Set;

/**
 * Generic object pool
 * @param <T> Type T of Object in the Pool
 */
public abstract class ObjectPool<T> {

  private Set<T> available = new HashSet<>();
  private Set<T> inUse = new HashSet<>();

  protected abstract T create();

  /**
   * Checkout object from pool
   */
  public synchronized T checkOut() {
    if (available.isEmpty()) {
      available.add(create());
    }
    T instance = available.iterator().next();
    available.remove(instance);
    inUse.add(instance);
    return instance;
  }

  public synchronized void checkIn(T instance) {
    inUse.remove(instance);
    available.add(instance);
  }

  @Override
  public String toString() {
    return String.format("Pool available=%d inUse=%d", available.size(), inUse.size());
  }
}

Update ObjectPool.java