单列模式是不让你创建对象，并且只让你访问一个地址。
class People {
private static People p = new People();
private People() {
}
public static People get() {
return p;
}
}
public class Demo {
public static void main(String[] args) {
System.out.println(People.get());
System.out.println(People.get());
}
}
