Java 单例类的实现！

servlet就是一个单例类，所以单例类还是很重要的：

public class SingleClass
{
    public static void main(String[] args)
    {
        Single s1 = Single.getInstance();
        Single s2 = Single.getInstance();
        System.out.println(s1 == s2);
    }
}

class Single
{
    private static Single s;
    private Single()
    {
    
    }
    public static Single getInstance()
    {
        if (s == null)
        {
            s = new Single();
        }
        return s;
    }
}