findFragmentById return always null

Question

my findFragmentById method return always null (v4). i wanto to call a fragment function from wrapper activity; somthing like Fragment.myFunc();

in Container activity import android.support.v4.app.FragmentManager; in fragment activity import android.support.v4.app.Fragment;

<?xml version="1.0" encoding="utf-8"?>
<FrameLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:adroid="http://schemas.android.com/apk/res-auto"
    android:id="@+id/firstFragmentWrapper"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:background="@color/transparent"
    android:orientation="vertical"
    android:layout_gravity="center_vertical"
    android:gravity="center_vertical"
    android:fromYScale="0.0"
    android:fillAfter="true"
    android:focusable="true"
    android:focusableInTouchMode="true"
    android:descendantFocusability="beforeDescendants"
    android:tag="firstFragmentWrapper">
    <LinearLayout
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        android:orientation="vertical">

        <RelativeLayout android:id="@+id/relativeWrapperRecoverText"
            android:layout_weight="1"
            android:layout_width="match_parent"
            android:layout_height="wrap_content"
            android:focusableInTouchMode="true"
            android:descendantFocusability="beforeDescendants"
            android:layout_gravity="bottom"
            android:gravity="bottom"
            android:paddingBottom="0dp">
            <design.ubris.myuni.UtilityAvenirNextFontTextView
                android:id="@+id/recoverPLabel"
                android:layout_width="match_parent"
                android:layout_height="wrap_content"
                android:gravity="bottom"
                android:text="@string/recover_password"
                android:lineSpacingExtra="4dp"
                android:textSize="16dp"
                android:textColor="#757575"
                android:paddingLeft="5dp"
                android:letterSpacing="0.2"/>
        </RelativeLayout>


        <RelativeLayout android:id="@+id/relativeWrapperRecover"
            android:layout_weight="1"
            android:layout_width="match_parent"
            android:layout_height="wrap_content"
            android:focusableInTouchMode="true"
            android:paddingTop="5dp"
            android:descendantFocusability="beforeDescendants">
            <design.ubris.myuni.UtilityAvenirNextFontTextView
                android:id="@+id/recoverLabel"
                android:layout_width="match_parent"
                android:layout_height="60dp"
                android:gravity="bottom"
                android:layout_marginBottom="30dp"
                android:text="Codice Fiscale"
                android:textSize="25dp"
                android:textColor="#757575"
                android:paddingLeft="5dp"
                android:letterSpacing="0.2"
                android:paddingBottom="10dp"
                android:clickable="false"
                android:longClickable="false"
                android:focusable="false"
                android:focusableInTouchMode="false"
                android:textCursorDrawable="@drawable/cursor_color"
                />

            <EditText
                android:id="@+id/recoverEditText"
                android:layout_width="match_parent"
                android:layout_height="70dp"
                android:backgroundTint="@color/colorPrimary"
                android:textColorHint="@color/colorPrimary"
                android:textColor="#fff"
                android:gravity="bottom"
                android:paddingBottom="20dp"
                android:focusableInTouchMode="true"
                android:focusable="true"
                android:singleLine="true"
                android:inputType="textVisiblePassword"
                android:imeOptions="actionDone"
                android:imeActionLabel="Login"
                android:layout_alignParentRight="true"
                android:textCursorDrawable="@drawable/cursor_color"/>

            <Button
                android:layout_width="match_parent"
                android:layout_height="wrap_content"
                android:layout_marginEnd="4dp"
                android:layout_marginStart="4dp"
                android:layout_marginTop="85dp"
                android:height="50dp"
                android:background="@color/colorPrimary"
                android:text="Invia richiesta"
                android:textColor="#fff"
                android:id="@+id/recovery_send"/>


        </RelativeLayout>

 </LinearLayout>

FragmentManager fm = getSupportFragmentManager();
FragmentLogin1 fragment =FragmentLogin1)fm.findFragmentById(R.id.firstFragmentWrapper);
Toast.makeText(getApplicationContext(), ""+fragment, Toast.LENGTH_LONG).show();


mPager.addOnPageChangeListener(new ViewPager.OnPageChangeListener() {
    public void onPageScrollStateChanged(int state) {
    }

    public void onPageScrolled(int position, float positionOffset, int positionOffsetPixels) {
    }

    public void onPageSelected(int position) {

        //>> HIDE / UNHIDE lost password
        //LOST PASSWORD


        switch(position){
            case 0:
                fSelected = 0;
                lostPassword.startAnimation(AnimationUtils.loadAnimation(getApplicationContext(), R.anim.fade_out));
                backButton.startAnimation(AnimationUtils.loadAnimation(getApplicationContext(), R.anim.fade_in));
                backButton.bringToFront();


                EditText recover = findViewById(R.id.recoverEditText);
                if(recover.getText().toString().equals("")) {

                    FragmentManager fm = getSupportFragmentManager();
                    FragmentLogin1 fragment = (FragmentLogin1) fm.findFragmentById(R.id.firstFragmentWrapper);
                    Toast.makeText(getApplicationContext(), ""+fragment, Toast.LENGTH_LONG).show();
                    //fragment.yourPublicMethod();
                }

                break;

            case 1:

                fSelected = 1;
                lostPassword.startAnimation(AnimationUtils.loadAnimation(getApplicationContext(), R.anim.fade_in));
                backButton.startAnimation(AnimationUtils.loadAnimation(getApplicationContext(), R.anim.fade_out));
                lostPassword.bringToFront();


                break;

            case 2:
                fSelected = 2;
                //>>
                break;
        }
        //Toast.makeText(getApplicationContext(),"swipe",Toast.LENGTH_SHORT).show();
        fAnimation(position);
    }
});

that result in a nullpointer exception; no error detected.

Answer 1

Fragment's in XML use the <fragment> XML tag. Right now, you're using just a normal FrameLayout. In order to use FragmentManager.findFragmentById(id), you have to provide the Fragment with a <fragment> tag, class name, and id.

There is a pretty extensive training article on this if you need additional info.