First:Elevator NorthElevator = null; Elevator SouthELevator = null; ......Then in the constructor:NorthElevator = new Elevator(); SouthElevator = new Elevator();
你说的并不直接等于, 正像我上面说的,首先N, S = null(Field Member 默认为 null, 0, 0.0, '\u0000' false ....), 然后才再new FrontDesk()中N=new Elevator(), S=new Elevator(); 假设你还有一个FrontDesk构建器,FrontDesk(int x);而其中并没有对N,S付值的话,两者仍然是null,如果你要调用NorthElevator.shutDown()的话,会得到NullPointerException,而这一点在编译的时候是永远不会被检查出来的
Elevator SouthELevator = null;
......Then in the constructor:NorthElevator = new Elevator();
SouthElevator = new Elevator();
正像我上面说的,首先N, S = null(Field Member 默认为 null, 0, 0.0, '\u0000' false ....),
然后才再new FrontDesk()中N=new Elevator(), S=new Elevator();
假设你还有一个FrontDesk构建器,FrontDesk(int x);而其中并没有对N,S付值的话,两者仍然是null,如果你要调用NorthElevator.shutDown()的话,会得到NullPointerException,而这一点在编译的时候是永远不会被检查出来的