编写Java 代码实现一个计数器类Counter，其中包含：

　　域counterValue　　　　用来保存计数器的当前数值；

　　方法increment（）　　计数器加1；

　　方法decrement（）　　计数器减1；

　　方法reset（）　　　　清零；

　　思考：这个类应该定义几个构造函数？为什么？

利用该计数器类，编写一测试类，用以管理会议室投影仪的出借情况。每新购入一台投影仪或收回一台投影仪，计数器加1；每淘汰一台投影仪或借出一台投影仪，计数器减 1。-Write Java code to achieve a counter class Counter, which includes: the domain counterValue used to save the current value of the counter　method increment () counter is incremented by 1　method decrement () the counter is decremented by one　method reset () clear　thinking: This category should be defining several constructor? Why? The counter class, write a test class to manage the conference room projector lending situation. Each new purchase of a projector or withdraw a projector, the counter is incremented　out of a projector, or lend a projector, the counter is decremented by 1.