类实例作为 Java 中的私有成员

Question

I have an assignment where I need to use inheritance and polymorphism:

For this assignment, we will subclass the author's Time2 class as Time3 and subclass your previous Date2 to include a Time3 instance as a private member. This new class will be called TimeStamp3 (subclassing Date2 and having a Time3 instance).

I don't entirely understand the part:

include a Time3 instance as a private member

Although I do understand that Time3 extends Time2 and TimeStamp3 extends Date2 , I just can't figure out what I need to do to allow Date2 to have access to Time3.

Answer 1

After creating the Time3 class, create another class called TimeStamp3 (silly names...)

public class TimeStamp3 extends Date2 {

    private Time3 time;

    //methods, other instance variables etc.

}

Answer 2

A class member is also known as an instance variable. Therefore, your Time3 instance in your Date2 , would be private or protected, rather than public. It might look like:

class TimeStamp3 extends Date2{
...
   private Time3 timeThree; 
...

}

You don't have to worry about accessing the Time3 instance inside Date2 because it is subclassed. As you should know, when an instance variable is private , it is only accessible to members inside the class. This will not make the Time3 class private, just the specific instance of that object inside the Date2 class.

Even when the instance member is private, any methods inside the Date2 class will be able to access that member variable. It's the classes outside of Date2 that won't be able to access that instance. Other classes could potentially access Time3 , just not the the instance of Time3 inside Date2 .