I am trying to add a click listener for each cardview in a grid layout, but I get the following error.
java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.uspherejda/com.example.uspherejda.HomeScreen}: java.lang.NullPointerException: Attempt to invoke virtual method 'void androidx.cardview.widget.CardView.setOnClickListener(android.view.View$OnClickListener)' on a null object reference
Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'void androidx.cardview.widget.CardView.setOnClickListener(android.view.View$OnClickListener)' on a null object reference
public class HomeScreen extends AppCompatActivity implements View.OnClickListener{
//SQL
private SatFromHelper dbHelper;
private SQLiteDatabase db;
private CardView satList, addSat, satType, satCountries, aboutMe, goCrazy;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.home_screen);
dbHelper = new SatFromHelper(this);
db = dbHelper.getWritableDatabase();
//call the Home fragment
getSupportFragmentManager().beginTransaction().replace(R.id.fragment_container, new HomeFragment()).commit();
//Set up a custom bar using a custom
ActionBar actionBar = getSupportActionBar();
getSupportActionBar().setBackgroundDrawable(new ColorDrawable(getResources().getColor(R.color.start_dark_blue)));
actionBar.setDisplayShowCustomEnabled(true);
LayoutInflater layoutInflater = (LayoutInflater) this.getSystemService(Context.LAYOUT_INFLATER_SERVICE);
View view = layoutInflater.inflate(R.layout.custom_bar_image, null);
actionBar.setCustomView(view);
//Card views.
satList = (CardView) findViewById(R.id.crdList);
addSat = (CardView) findViewById(R.id.crdForm);
satType = (CardView) findViewById(R.id.crdSatTypes);
satCountries = (CardView) findViewById(R.id.crdCountries);
aboutMe = (CardView) findViewById(R.id.crdAboutMe);
goCrazy = (CardView) findViewById(R.id.crdGoCrayCraaaaay);
satList.setOnClickListener(this);
addSat.setOnClickListener(this);
satType.setOnClickListener(this);
satCountries.setOnClickListener(this);
aboutMe.setOnClickListener(this);
goCrazy.setOnClickListener(this);
//Bottom Navigation
BottomNavigationView bottomNav = findViewById(R.id.nav_menu);
//Item listener when one of hte items below from the nav_bar is selected
bottomNav.setOnItemSelectedListener(item -> {
Fragment selectedFragment = null;
switch(item.getItemId()){
case R.id.nav_home:
selectedFragment = new HomeFragment();
break;
case R.id.nav_list:
selectedFragment = new ListFragment(dbHelper, db);
break;
case R.id.nav_add:
selectedFragment = new AddFragment(dbHelper, db);
break;
}
getSupportFragmentManager().beginTransaction().replace(R.id.fragment_container, selectedFragment).commit();
return true;
});
}
@Override
public void onClick(View v){
switch (v.getId()){
case R.id.crdList: startActivity(new Intent(getApplicationContext(), ListFragment.class));
break;
}
}
//Close the db when the activity onDestroy
@Override
public void onDestroy() {
super.onDestroy();
dbHelper.close();
db.close();
}
}
XML code:
<?xml version="1.0" encoding="utf-8"?>
<ScrollView xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:app="http://schemas.android.com/apk/res-auto"
xmlns:tools="http://schemas.android.com/tools"
android:id="@ id/frameLayout"
android:layout_width="match_parent"
android:layout_height="match_parent"
tools:context=".HomeFragment">
<LinearLayout
android:layout_width="match_parent"
android:layout_height="match_parent"
android:orientation="vertical">
<TextView
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:text="My Dashboard"
android:textSize="40sp"
android:textStyle="bold"
android:textColor="@color/white"
android:layout_margin="20dp" />
<GridLayout
android:layout_width="match_parent"
android:layout_height="match_parent"
android:rowCount="4"
android:columnCount="2"
android:layout_margin="5dp"
android:alignmentMode="alignMargins"
android:layout_gravity="center_horizontal"
android:columnOrderPreserved="false">
<androidx.cardview.widget.CardView
android:id="@ id/crdList"
android:layout_width="170dp"
android:layout_height="170dp"
app:cardCornerRadius="12dp"
app:cardElevation="5dp"
app:cardBackgroundColor="@color/center_light_blue"
android:layout_margin="10dp">
<LinearLayout
android:layout_width="match_parent"
android:layout_height="match_parent"
android:orientation="vertical"
android:gravity="center">
<ImageView
android:layout_width="84dp"
android:layout_height="84dp"
android:src="@drawable/satdashboard"/>
<TextView
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:text="Satellite List"
android:textSize="16sp"
android:layout_marginTop="20dp"
android:textColor="@color/white"/>
</LinearLayout>
</androidx.cardview.widget.CardView>
<androidx.cardview.widget.CardView
android:id="@ id/crdForm"
android:layout_width="170dp"
android:layout_height="170dp"
app:cardCornerRadius="12dp"
app:cardElevation="5dp"
app:cardBackgroundColor="@color/center_light_blue"
android:layout_margin="10dp">
<LinearLayout
android:layout_width="match_parent"
android:layout_height="match_parent"
android:orientation="vertical"
android:gravity="center">
<ImageView
android:layout_marginLeft="10dp"
android:layout_width="84dp"
android:layout_height="84dp"
android:src="@drawable/formdashboard"/>
<TextView
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:text="Add a Satellite"
android:textSize="16sp"
android:layout_marginTop="20dp"
android:textColor="@color/white"/>
</LinearLayout>
</androidx.cardview.widget.CardView>
<androidx.cardview.widget.CardView
android:id="@ id/crdSatTypes"
android:layout_width="170dp"
android:layout_height="170dp"
app:cardCornerRadius="12dp"
app:cardElevation="5dp"
app:cardBackgroundColor="@color/center_light_blue"
android:layout_margin="10dp">
<LinearLayout
android:layout_width="match_parent"
android:layout_height="match_parent"
android:orientation="vertical"
android:gravity="center">
<ImageView
android:layout_width="84dp"
android:layout_height="84dp"
android:src="@drawable/sattypes"/>
<TextView
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:text="Types of Satellites"
android:textSize="16sp"
android:layout_marginTop="20dp"
android:textColor="@color/white"/>
</LinearLayout>
</androidx.cardview.widget.CardView>
<androidx.cardview.widget.CardView
android:id="@ id/crdCountries"
android:layout_width="170dp"
android:layout_height="170dp"
android:layout_margin="10dp"
app:cardBackgroundColor="@color/center_light_blue"
app:cardCornerRadius="12dp"
app:cardElevation="5dp">
<LinearLayout
android:layout_width="match_parent"
android:layout_height="match_parent"
android:gravity="center"
android:orientation="vertical">
<ImageView
android:layout_width="84dp"
android:layout_height="84dp"
android:src="@drawable/countrysats" />
<TextView
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_marginTop="20dp"
android:text="Satellite Countries"
android:textColor="@color/white"
android:textSize="16sp" />
</LinearLayout>
</androidx.cardview.widget.CardView>
<androidx.cardview.widget.CardView
android:id="@ id/crdAboutMe"
android:layout_width="170dp"
android:layout_height="170dp"
android:layout_margin="10dp"
app:cardBackgroundColor="@color/center_light_blue"
app:cardCornerRadius="12dp"
app:cardElevation="5dp">
<LinearLayout
android:layout_width="match_parent"
android:layout_height="match_parent"
android:gravity="center"
android:orientation="vertical">
<ImageView
android:layout_width="84dp"
android:layout_height="84dp"
android:src="@drawable/aboutme" />
<TextView
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_marginTop="20dp"
android:text="About Me"
android:textColor="@color/white"
android:textSize="16sp" />
</LinearLayout>
</androidx.cardview.widget.CardView>
<androidx.cardview.widget.CardView
android:id="@ id/crdGoCrayCraaaaay"
android:layout_width="170dp"
android:layout_height="170dp"
android:layout_margin="10dp"
app:cardBackgroundColor="@color/center_light_blue"
app:cardCornerRadius="12dp"
app:cardElevation="5dp">
<LinearLayout
android:layout_width="match_parent"
android:layout_height="match_parent"
android:gravity="center"
android:orientation="vertical">
<ImageView
android:layout_width="84dp"
android:layout_height="84dp"
android:src="@drawable/gocrazy" />
<TextView
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_marginTop="20dp"
android:text="Go Crazy!"
android:textColor="@color/white"
android:textSize="16sp" />
</LinearLayout>
</androidx.cardview.widget.CardView>
</GridLayout>
</LinearLayout>
</ScrollView>
CodePudding user response：
Is listfragment is a fragment or activity.
If listfragment is a fragment you can't call fragment like that.
CodePudding user response：
I am almost certain what error is in the reference of your xml:
tools:context=".HomeFragment"
Should to be:
tools:context=".HomeScreen"
Are you sure this xml belongs to your activity HomeScreen?
The java code belongs to an activity and its xml belongs to a Fragment.