引用传递

class Tank {
      int level;
      public Tank(int level){
          this.level = level;
      }
}    

public class Assignment {
          public static void main(String[] args) {
                Tank t1 = new Tank(9);
                Tank t2 = new Tank(47);
                System.out.println("t1.level: " + t1.level +
                                              ", t2.level: " + t2.level);
                //t1现在指向t2对象
                t1 = t2;
                System.out.println("t1.level: " + t1.level +
                                            ", t2.level: " + t2.level);
                
                //修改t1的同时也修改了t2
                t1.level = 36;
                System.out.println("t1.level: " + t1.level +
                                            ", t2.level: " + t2.level);
                
                 //传递给f()函数的是t1的引用，而不是t1的副本                
                f(t1);
                System.out.println("t1.level:"+t1.level +
                                            ", t2.level: " + t2.level);
          }

         public static void f(Tank tank) {
             tank.level = 5;
          }
}
//如果只想改变t1的level值而不影响t2的值，可以这样调用 
// t1.level = t2.level
//但直接操作对象的属性违背面向对象程序设计的原则