创建一个connection类，该类管理一个connection的对象的固定数组，客户端程序员不能直接创建connection对象，因为把它的对象定义为一个private类型而只能通过ConnectionManager中的某个static的方法去获取他们，当connectionManager没有对象时候，他就还回null的对象，在main（）中检验这些类的实现。-Create a connection class, the fixed array object of the class management of a connection, the client programmer cannot create connection objects directly, because the object is defined as a private type, but only through a static in ConnectionManager to obtain them, when connectionManager did not object when the object, he is back to null in the main (test), in realizing these classes.