using System;
using System.Collections.Generic;
using System.Text;

public class ObjectPool<T> where T : class
{
    private readonly Stack<T> _objects = new Stack<T>();
    private readonly Func<T> _objectGenerator;

    public ObjectPool(Func<T> objectGenerator)
    {
        if (objectGenerator == null)
        {
            throw new ArgumentNullException("objectGenerator");
        }
        _objectGenerator = objectGenerator;
    }

    public T Get()
    {
        lock (_objects)
        {
            if (_objects.Count > 0)
            {
                return _objects.Pop();
            }
        }
        return _objectGenerator();
    }

    public void Put(T item)
    {
        lock (_objects)
        {
            _objects.Push(item);
        }
    }

    public override string ToString()
    {
        lock (_objects)
        {
            StringBuilder sb = new StringBuilder();
            foreach (T o in _objects)
            {
                sb.Append(o.ToString()).Append('\n');
            }
            return sb.ToString();
        }
    }
}