How can I avoid using instanceof in this case?

Question

I've had to use instanceofseveral times here while checking what subclass a Fragment is, and I feel like I'm going wrong somewhere:

@Override
public void onBackPressed() {

    Fragment frag = mManager.findFragmentByTag("Fragment");

    if(frag instanceof CustFragmentOne || 
       frag instanceof CustFragmentTwo || 
       frag instanceof CustFragmentThree || 
       frag instanceof CustFragmentFour) {

            //This method displays an instance of "CustFragmentFive"
            displayView(0);
    }
    else if(frag instanceof CustFragmentFive)
         super.onBackPressed();

    else if(mManager.getBackStackEntryCount() > 0)
        getFragmentManager().popBackStack();

    else
       super.onBackPressed();

}

Is there any way around using it here? How else can I identify the fragment's type and accordingly invoke the right method?

The typical way of bypassing the need for the `instanceof` operator is the `Visitor pattern` — EpicPandaForce, Jul 03 '14 at 12:51
Looks to me like `CustFragmentOne`, `CustFragmentTwo`, `CustFragmentThree` and `CustFragmentFour` should all extend an interface that has some method to identify if the object needs to display the value. — Dan Temple, Jul 03 '14 at 12:53

score 2 · Accepted Answer · edited May 23 '17 at 11:55

2

The simplest way is to have CustFragmentOne, CustFragmentTwo, etc. implement a common interface. That way you just need to do a single instanceof check against that interface, instead of needing to check each concrete class individually.

Another way would be something like along these lines: How to implement onBackPressed() in Fragments?

edited May 23 '17 at 11:55

Community

1
1

answered Jul 03 '14 at 12:48

vaughandroid

4,315
1
27
33

Got it... Also I've looked at that link before, but I find it pretty confusing, is there any simpler explanation you know of? – u3l Jul 03 '14 at 12:54

score 2 · Answer 2 · answered Jul 03 '14 at 12:52

Use an interface. For example:

public interface ICustomFragment {

      public boolean shouldNavigateBack();

And then you can do something like this:

@Override
public void onBackPressed() {

CustomFragment frag = (CustomFragment)mManager.findFragmentByTag("Fragment");

    if(frag.shouldNavigateBack()) {

            //This method displays an instance of "CustFragmentFive"
            displayView(0);
    }
    else {
         super.onBackPressed();
}

score 0 · Answer 3 · answered Jul 03 '14 at 12:40

0

if(frag.getClass() == CustFragmentOne.class) not sure which approach is better tho

answered Jul 03 '14 at 12:40

j.con

879
7
19

score 0 · Answer 4 · answered Jul 03 '14 at 12:50

Why dont you keep your Fragment objects into your Activity?

CustFragmentOne fragment1 = (FragmentOne)getFragmentManager().findFragmentById(R.id.fragment1);  
 CustFragmentTwo fragment2 = (FragmentOne)getFragmentManager().findFragmentById(R.id.fragment2);

and in your layout xml file you will have smth like this:

<FrameLayout
    ...>
    <fragment
        android:id="@+id/fragment1"
        android:name="com.smartmirror.FragmentOne"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"/>
    <fragment
        android:id="@+id/fragment2"
        android:name="com.smartmirror.FragmentTwo"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"/>
</FrameLayout>

score 0 · Answer 5 · answered Jul 03 '14 at 13:05

I suggest you to use something like this to have control of the back button on the fragments:

Create a base fragment abstract class:

public abstract class BaseFragment extends Fragment {

    public void onBackPressed() {

    }

}

Extend all your fragments from BaseFragment

public class CustFragmentOne extends BaseFragment { ... }

Change your activity

@Override
public void onBackPressed() {

    Fragment frag = mManager.findFragmentByTag("Fragment");

    if(frag instanceof BaseFragment)
        frag.onBackPressed();
    else if(mManager.getBackStackEntryCount() > 0)
        getFragmentManager().popBackStack();
    else
       super.onBackPressed();

}

override in your fragments the onBackPressed() method with the logic you want.

score 0 · Answer 6 · edited May 23 '17 at 10:27

You could always create a BaseFragment that extends Fragment to be extend by all fragments. Then you can let each fragment handle it as necessary making it a bit cleaner. e.g.

public class BaseFragment extends Fragment { public void onBackPressed() { // Override this function in your fragment classes as needed } }

or you could do something like (Android Fragment handle back button press)

How can I avoid using instanceof in this case?

6 Answers6