Empty constructor of Fragment called instead of newInstance

Question

I experience some weird behaviour in an android app I am developing:

I have a class HintsMenu (snippet):

public class HintsMenu extends Fragment implements AbsListView.OnItemClickListener {

...


/**
 * Returns a new instance of this fragment for the given section
 * number.
 */
public static HintsMenu newInstance() {
    System.out.println("HintsMenu.newInstance()");
    HintsMenu fragment = new HintsMenu();
    Bundle args=new Bundle();
    args.putString(ARG_TITLE,"Hints");
    fragment.setArguments(args);
    return fragment;
}

/**
 * Mandatory empty constructor for the fragment manager to instantiate the
 * fragment (e.g. upon screen orientation changes).
 */
public HintsMenu() {
    System.out.println("Hintsmenu constructor");
}

This Fragment is dynamically used in an activity:

public class MainActivity extends ActionBarActivity implements MainMenu.NavigationDrawerCallbacks {

....

@Override
public void onNavigationDrawerItemSelected(MenuEntry item) {
    // update the main content by replacing fragments
    if (item.hasActivity()){
        ...
    } else if (item.hasFragment()){
        try {
            FragmentManager fragmentManager = getSupportFragmentManager();
            Class<? extends Fragment> cl=item.getFragmentClass();
            System.out.println("Calling " + cl.getSimpleName() + ".newInstance()...");
            Fragment newFragment=cl.newInstance();
            fragmentManager.beginTransaction().replace(R.id.container, newFragment).commit();
        } catch (InstantiationException e) {
            e.printStackTrace();
        } catch (IllegalAccessException e) {
            e.printStackTrace();
        }
    }
}

The idea is, that item.activityClass (code not shown) is set by the expression

item.fragmentClass=HintsMenu.class;

at another position in the code, and item.getFragmentClass() returns this value inside onNavigationDrawerItemSelected(). In "onNavigationDrawerItemSelected", a new HintsMenu is created using the cl.newInstance() call. However, HintsMenu's newInstance method is NOT called, but the empty Constructor is.

So insted of the expected output

Calling HintsMenu.newInstance()...
HintsMenu.newInstance()
Hintsmenu constructor

I only get

Calling HintsMenu.newInstance()...
Hintsmenu constructor

If i do

HintsMenu.newInstance()

I get the expected output. Can anyone tell me, what I am doing wrong?

score 2 · Accepted Answer · answered Aug 13 '15 at 17:10

2

The static newInstance() on HintsMenu is not the newInstance() method on Class. You are calling the latter. For example, if you were to rename newInstance() on HintsMenu to be newMenu(), and you changed your onNavigationDrawerItemSelected() implementation to try calling newMenu(), you will get a compile error, as the compiler will complain that it cannot find a newMenu() method on Class.

answered Aug 13 '15 at 17:10

CommonsWare

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Also read this link if you still want to call static `newInstance` by reflection http://stackoverflow.com/questions/8836299/in-java-how-to-use-reflection-to-get-a-static-method-and-execute-it – justHooman Aug 13 '15 at 17:13
Ok, but is there a way to achieve, that HintsMenu.newInstance() is called? Or shall I move the desired code (Bundle creation) to the constructor (called by HintsMenu.newInstance). (I know the latter is disregarded by android guidelines)? – Stephan Richter Aug 13 '15 at 17:15

Empty constructor of Fragment called instead of newInstance

1 Answers1