测试了java对象的句柄，类A的两个实现a,b均调用了Data的实现dat，在a，b中分别修改dat的内容，a，b中均会发生相应的变化-Test the java object handle, class A two realize a, b are called Data realization dat, in a, b, respectively, to modify the contents of dat, a, b in the corresponding changes will oc