参考来源：《java编程思想（第四版）》 见第三章3.4节
class Tank {
int level;
}
public class Assignment {
public static void main(String[] args) {
Tank t1 = new Tank();
Tank t2 = new Tank();
t1.level = 9;
t2.level = 47;
System.out.println("1: t1.level: " + t1.level + ", t2.level: " t2.level);
t1 = t2;
System.out.println("2: t1.level: " + t1.level + ", t2.level: " t2.level);
t1.level = 27;
System.out.println("3: t1.level: " + t1.level + ", t2.level: " t2.level);
}
/* 输出：
* 1：t1.level：9, t2.level：47
* 2：t1.level：47, t2.level：47
* 3：t1.level：27, t2.level：27
*/
}