如何在泛型类型变量上调用方法

Question

I'm writing a method that pretty straightforward and easy: I want it to get an object and work on it's starting and ending date. All of the object I want it to work on have dates to do stuff with, but of course eclipse won't allow me to write such a method. Can it be done? How?

Here's the frame I'm working on

public <T> void updateStuff(T stuffObject, Calendar startDate, Integer delta) {
    stuffObject.setStartDate(startDate);
    Calendar calendarEnd = Calendar.getInstance();
    calendarEnd = startDate;
    calendarEnd.add(Calendar.MONTH, delta);
    stuffObject.setEndDate(calendarEnd);
}

Answer 1

A bit of topic but you could simply remove the genericity and directly use an interface as parameter.

Using OldCurmudgeon interface for the example :

interface Stuff {
    void setStartDate(Calendar startDate);
    void setEndDate(Calendar startDate);
}

public void updateStuff(Stuff stuffObject, Calendar startDate, Integer delta) {
    stuffObject.setStartDate(startDate);
    Calendar calendarEnd = Calendar.getInstance();
    calendarEnd = startDate;
    calendarEnd.add(Calendar.MONTH, delta);
    stuffObject.setEndDate(calendarEnd);
}

This is not necessary to use generic here and this give a method more verbose and, for non initiate, more obscure.

Generic could be useful if you returned the instance, to chained the method for example. Because with my example, you would need to cast the return instance.

Answer 2

You have to tell Java that there is a setStartDate and a setEndDate method in the T objects.

interface Stuff {
    void setStartDate(Calendar startDate);
    void setEndDate(Calendar startDate);
}

public <T extends Stuff> void updateStuff(T stuffObject, Calendar startDate, Integer delta) {
    stuffObject.setStartDate(startDate);
    Calendar calendarEnd = Calendar.getInstance();
    calendarEnd = startDate;
    calendarEnd.add(Calendar.MONTH, delta);
    stuffObject.setEndDate(calendarEnd);
}

Then your objects need to implement this interface.

class SomeStuff implements Stuff {
    Calendar startDate;
    Calendar endDate;

    @Override
    public void setStartDate(Calendar startDate) {
        this.startDate = startDate;
    }

    @Override
    public void setEndDate(Calendar startDate) {
        this.endDate = endDate;
    }
}

Incidentally, the way you are working out the end date is dangerous, you are actually modifying the Calendar you are passed as a parameter.

    Calendar calendarEnd = Calendar.getInstance();
    // Throw away the new Calendar and point calendarEnd at the parameter.
    calendarEnd = startDate;
    // change the parameter.
    calendarEnd.add(Calendar.MONTH, delta);

See Defensive copy of Calendar for how to do this right. In particular this answer .

Answer 3

The objects you want to pass to this method must implement a common interface or extend a common super class that contains the setStartDate() and setEndDate() methods.

Then you can define a type bound on T :

public <T extends SomeInterface> void updateStuff(T stuffObject, Calendar startDate, Integer delta) {
    stuffObject.setStartDate(startDate);
    Calendar calendarEnd = Calendar.getInstance();
    calendarEnd = startDate;
    calendarEnd.add(Calendar.MONTH, delta);
    stuffObject.setEndDate(calendarEnd);
}

where SomeInterface is the common interface.

Answer 4

Your can defined the methods to be exposed by the generic type into an interface:

    public <T extends MyInterface> void updateStuff(T stuffObject, Calendar startDate, Integer delta) {
        stuffObject.setStartDate(startDate);
        Calendar calendarEnd = Calendar.getInstance();
        calendarEnd = startDate;
        calendarEnd.add(Calendar.MONTH, delta);
        stuffObject.setEndDate(calendarEnd);
    }

    interface MyInterface{
        void setStartDate(Calendar c);
        void setEndDate(Calendar c);
    }