使用Enum和Bill Pugh方法的通用Singleton

Question

以下是我的通用Singleton，它为任何给定的类提供了Singleton对象。基本上是Bill Pugh和Singleton枚举模式的混合体。还能改善吗？请提出建议。

public enum GSingleton {

    INSTANCE;
    private GSingleton() {
    }

    private static class SingletonHelper {
        private static Map<String, Object> instances = new HashMap<String, Object>();

        private static <T> T Instance(Class<T> c) throws InstantiationException, IllegalAccessException {
            T newInstance = null;
            if (instances.containsKey(c.getName())) {
                newInstance = (T) instances.get(c.getName());
            } else {
                newInstance = c.newInstance();
                instances.put(c.getName(), newInstance);
            }
            return newInstance;
        }

    }

    public static <T> T getInstance(Class<T> c) throws InstantiationException, IllegalAccessException {
       synchronized (c) {
        return SingletonHelper.Instance(c);
       }
    }
}