In an abstract class does the “this” keyword refrerence the parent or child class?

问题

I have an abstract class Flight. Flight contains the method schedule() which calls the private method schedule(final Flight f)

public void schedule()
{
    schedule(this);
}

private void schedule(final Flight f)
{
    new Timer().schedule(new TimerTask()
    {
        @Override
        public void run()
        {
            f.checkIn();
            updateList();
        }
    }, this.getDate());
}

Lets now say I have a class SouthWestFlight that extends Flight

Flight f = new SouthWestFlight(); //ignore the missing params doesn't matter for example
f.schedule();

Would this pass the instance of the Flight or the instance of SouthWestFlight as a parameter in the schedule method?

回答1:

Would this pass the instance of the Flight or the instance of SouthWestFlight as a parameter in the schedule method?

The instance of Flight it's exactly the instance of SouthWestFlight. You don't have 2 different instances. Your SoutWestFlight instance IS A Flight. Actually you can't have a Flight instance itself cause it's abstract.

回答2:

The instance of Flight and SouthhWestFlight are actually the same instance. There is only one instance to pass, and it will be passed.

回答3:

I've prepared a simple test app.

public class TEST {

    static abstract class Flight {
        String type = "Flight";
        abstract String getDirection();
        public String getType() {
            return this.type;
        }
        public void schedule() {
            System.out.printf("Scheduled a %s towards %s",this.getType(),this.getDirection());
            System.out.println();
            System.out.printf("ReadyToGo: %s",this.isReadyToGo());
        }
        public boolean isReadyToGo() {
            return false;
        }
    }

    static class SouthWestFlight extends Flight {
        String type = "SouthWestFlight";
        @Override
        String getDirection() {
            return "SouthWest";
        }

        @Override
        public boolean isReadyToGo() {
            return true;
        }

    }

    public static void main(String... args) {
        new SouthWestFlight().schedule();
    }
}

Output:
Scheduled a Flight towards SouthWest
ReadyToGo: true

Conclusions
Here the SouthWestFlight object is still a Flight Object.
But when you extend a class the child class overrides all the methods of parent class.
That is why isReadyToGo() of SouthWestFlight() returned true.
None of the properties are overriden.
That is why this.getType() returns Flight.
But if you override the getType() method with a new method in SouthWestFlight like this:

@Override
public String getType() {
   return this.type;
}

It will return type field of SouthWestFlight. Wondering why?
It is because, the first getType() method is defined in parent class Flight. So it returns type field of class Flight. The second getType() method is defined in SouthWestFlight so it returns type field of SouthWestFlight.

Hope you've got it clear. Please comment if you found any mistake or got any doubt.

来源：https://stackoverflow.com/questions/18939597/in-an-abstract-class-does-the-this-keyword-refrerence-the-parent-or-child-clas

标签

java

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abstract