How to instantiate fragment class using class name instead of index

Question

I have two fragment class named SessionTab and BillingTaband i am trying to create instance of those class using

SessionTab sessionTab = (SessionTab) getSupportFragmentManager().getFragments().get(1);

but sometimes index for those classes are reversed and then it causes ClassCastException

How can i get instance of those fragment class by passing class name instead of index or any way to make sure that index of those class stays the same everytime so it doesn't cause ClassCastException

Answer 1

Use one of this methods : findFragmentById() and findFragmentByTag() methods.

Reference : https://developer.android.com/reference/android/app/FragmentManager.html#findFragmentById(int)

Update :

ClassCastException is invoked when you are not casting the appropriate classes to one another. In your case, Your FragmentManager is returning different fragment than SessionTab, so the exception is thrown.

If you use findFragmentById() or findFragmentByTag() , then it will return the fragment exactly what you want, and exception will not be thrown.

Answer 2

Define a 'tag' for the Fragment while adding it like

 getFragmentManager().beginTransaction().add(new Fragment(),"your_tag");

And while referencing it use

getFragmentManager().findFragmentByTag("your_tag");

In most cases, you would like to use YourFragment.class.getSimpleName() as your tag.

Answer 3

First of all, if you should understand that instance for any Fragment you can take from Java class api. Like below:

Class<?> class = Class.forName("example.package.BillingFragment");
Constructor<?> cons = class.getConstructor(BillingFragment.class);
BillingFragment object = (BillingFragment) cons.newInstance();

Code example show, how get Instance from any class in Java. But you talking a little bit other things. If I understand correct, you want to get Fragment from FragmentManager.

You can do it, in case if you already defined Fragment before! For example, you have base application flow, and then you want added Fragment. You can check FragmentManager if there are Fragments in stack. But in case of empty stack, you should manually add them:

String billingFragmentTag = BillingFragment.class.getSimpleName();

......

if (getFragmentManager.findFragmentByTag(billingFragmentTag) == null) {
  BillingFragment fragment = new BillingFragment();
  String billingFragmentTag = BillingFragment.class.getSimpleName();

  FragmentTransaction fragTrans = getFragmentManager().beginTransaction();
           fragTrans.add(fragment, billingFragmentTag).commit();
}

......

So after this, you can check if there your Fragment in stack and hook this active instance. This is correct and standard flow for using Fragments.

......

if (getFragmentManager.findFragmentByTag(billingFragmentTag) != null) {
  BillingFragment fragment = getFragmentManager.findFragmentByTag(billingFragmentTag);
  String billingFragmentTag = BillingFragment.class.getSimpleName();

  FragmentTransaction fragTrans = getFragmentManager().beginTransaction();
           fragTrans.add(fragment, billingFragmentTag).commit();
}

....

Welcome!