How to write a function passing in Class name in Java?

Question

I have the below piece of code.

    Fragment emptyViewFragment;
    Fragment songListingFragment;

    // .... and various codes ...

    if (emptyViewFragment == null) {
        emptyViewFragment =
                (EmptyViewFragment) getFragmentManager().findFragmentByTag(EmptyViewFragment.TAG);
        if (emptyViewFragment == null) {
            emptyViewFragment = new EmptyViewFragment();
        }
    }
    addFragment(emptyViewFragment, EmptyViewFragment.TAG);

    if (songListingFragment == null) {
        songListingFragment =
                (SongListingFragment) getFragmentManager().findFragmentByTag(SongListingFragment.TAG);
        if (songListingFragment == null) {
            songListingFragment = new SongListingFragment();
        }
    }
    addFragment(songListingFragment, SongListingFragment.TAG);

The two pieces of if-else codes looks very similar, and I'm thinking of making a function from it, so that I could just make my code as as below: -

    EmptyViewFragment emptyViewFragment;
    SongListingFragment songListingFragment;

    // .... and various codes ...

    createFragment(emptyViewFragment, EmptyViewFragment);
    createFragment(songListingFragment, SongListingFragment);

or even better (since the 2nd parameter is the 1st parameter pass in object's type).

    EmptyViewFragment emptyViewFragment;
    SongListingFragment songListingFragment;

    // .... and various codes ...

    createFragment(emptyViewFragment);
    createFragment(songListingFragment);

How should my createFragment function be written?

Which IDE are you using? If it is Eclipse, you can use the short-cut (Alt+Shift+M) to generate a common method and later you can put additional Class parameter accordingly. — coretechie
– coretechie, Commented Nov 20, 2015 at 10:14

Francisco Hernandez · Accepted Answer · 2015-11-20 10:15:16Z

3

Maybe something like this

private void a (Fragment frag, Class<? extends Fragment> clazz, String tag) {
    if (frag == null) {
        frag = getFragmentManager().findFragmentByTag(tag);
        if (frag == null) {
            try {
                frag = clazz.newInstance();
            } catch (InstantiationException e) {
                e.printStackTrace();
            } catch (IllegalAccessException e) {
                e.printStackTrace();
            }
        }
    }
    addFragment(frag);
}

You can access to the tag attribute inside and delete the tag parameter too

UPDATE

I can not test it now, but I think you can access to the static member TAG in this way

private void a (Fragment frag, Class<? extends Fragment> clazz) {

    try {
        if (frag == null) {

            frag = getFragmentManager().findFragmentByTag(clazz.getField("TAG").get(null).toString());

            if (frag == null) {
                frag = clazz.newInstance();
            }
        }
    } catch (InstantiationException e) {
        e.printStackTrace();
    } catch (IllegalAccessException e) {
        e.printStackTrace();
    } catch (NoSuchFieldException e) {
        e.printStackTrace();
    }
    addFragment(frag);
}

edited Nov 20, 2015 at 10:15

answered Nov 20, 2015 at 10:07

Francisco Hernandez

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4 Comments

Elye Over a year ago

Thanks Francisco! Can I not send the TAG in, but access through the class or the object? The TAG is a final static string in the class.

WilQu Over a year ago

frag.getClass() will throw NPE, since frag is null at this point.

Francisco Hernandez Over a year ago

Yessss. you are right. Pass the class instead, I will update the answer

Elye Over a year ago

Thanks Francisco, I finally ended using your first approach above, sending in the TAG directly. Tick for your answer.

WilQu · Accepted Answer · 2015-11-20 10:11:48Z

2

Instead of using the class, you can use an abstract factory:

Fragment createFragment(Fragment fragment, FragmentFactory factory) {
    if (fragment == null) {
        fragment = getFragmentManager().findFragmentByTag(factory.getTag());
        if(fragment == null) {
            fragment = factory.createFragment();
        }
    }
    addFragment(fragment, factory.getTag());
}

Where FragmentFactory is an interface which will have one implementation for each fragment type.

answered Nov 20, 2015 at 10:11

WilQu

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Comments

dotvav · Accepted Answer · 2015-11-20 10:00:33Z

1

Try something like this:

public addFragment(Fragment frag) throws IllegalAccessException, InstantiationException {
    Fragment newFrag = frag.getClass().newInstance();
    //...
}

This will not work if frag == null.

If this is your use case, then get the class by using Class<Fragment> clz = EmptyViewFragment.class, then call clz.newInstance().

answered Nov 20, 2015 at 10:00

dotvav

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1 Comment

Elye Over a year ago

Thanks dotvav! How should I handle the TAG? TAG is a static final string variable in the class.

coretechie · Accepted Answer · 2015-11-20 11:59:58Z

0

/**
 * @param fragment
 * @throws IllegalAccessException 
 * @throws InstantiationException 
 */
private static void createFragment(Fragment fragment, String tag, Class<? extends Fragment> clazz) throws InstantiationException, IllegalAccessException {
    if (fragment == null) {
        fragment =
                 getFragmentManager().findFragmentByTag(tag);
        if (fragment == null) {
            fragment = clazz.newInstance();
        }
    }
    addFragment(fragment, tag);
}

Assuming that Fragment is a Parent class of other to *Fragment classes and with following method defintion :

    /**
 * @return
 */
private static Fragment getFragmentManager() {
    // TODO Auto-generated method stub
    return null;
}

edited Nov 20, 2015 at 11:59

answered Nov 20, 2015 at 10:23

coretechie

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1 Comment

Elye Over a year ago

Hi Dishi, I think fragment.getClass().newInstance(); will crash, given fragment is null.

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