Errol Green Errol Green - 10 days ago 4
Android Question

Attempt to invoke virtual method 'void android.widget.ListView.setAdapter

I am trying to make a list view that contains pictures and text within a fragment that is on a viewpager with tabs, so I can swipe left or right to bring up another fragment.

I'm relatively new to android so what I may be doing could be completely wrong. I can build the project perfectly fine but then when the emulator runs it crashes (log displayed at bottom of page).

I created a separate application and successfully was able to create a list view, but when I try to combine it within a fragment like so, it doesn't work.
Here is my following code


MainActivity.java


import java.util.ArrayList;
import java.util.List;
import java.util.Locale;

import android.app.ListFragment;
import android.content.Intent;
import android.support.v7.app.ActionBarActivity;
import android.support.v7.app.ActionBar;
import android.support.v4.app.Fragment;
import android.support.v4.app.FragmentManager;
import android.support.v4.app.FragmentTransaction;
import android.support.v4.app.FragmentPagerAdapter;
import android.os.Bundle;
import android.support.v4.view.ViewPager;
import android.view.Gravity;
import android.view.LayoutInflater;
import android.view.Menu;
import android.view.MenuItem;
import android.view.View;
import android.view.ViewGroup;
import android.widget.AdapterView;
import android.widget.ArrayAdapter;
import android.widget.ImageView;
import android.widget.ListView;
import android.widget.TextView;
import android.widget.Toast;


public class MainActivity extends ActionBarActivity implements ActionBar.TabListener {

/**
* The {@link android.support.v4.view.PagerAdapter} that will provide
* fragments for each of the sections. We use a
* {@link FragmentPagerAdapter} derivative, which will keep every
* loaded fragment in memory. If this becomes too memory intensive, it
* may be best to switch to a
* {@link android.support.v4.app.FragmentStatePagerAdapter}.
*/
SectionsPagerAdapter mSectionsPagerAdapter;

/**
* The {@link ViewPager} that will host the section contents.
*/
ViewPager mViewPager;
private List<Posts> myPosts = new ArrayList<Posts>();
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
// Set up the action bar.
final ActionBar actionBar = getSupportActionBar();
actionBar.setNavigationMode(ActionBar.NAVIGATION_MODE_TABS);

// Create the adapter that will return a fragment for each of the three
// primary sections of the activity.
mSectionsPagerAdapter = new SectionsPagerAdapter(getSupportFragmentManager());

// Set up the ViewPager with the sections adapter.
mViewPager = (ViewPager) findViewById(R.id.pager);
mViewPager.setAdapter(mSectionsPagerAdapter);

// When swiping between different sections, select the corresponding
// tab. We can also use ActionBar.Tab#select() to do this if we have
// a reference to the Tab.
mViewPager.setOnPageChangeListener(new ViewPager.SimpleOnPageChangeListener() {
@Override
public void onPageSelected(int position) {
actionBar.setSelectedNavigationItem(position);
}
});

// For each of the sections in the app, add a tab to the action bar.
for (int i = 0; i < mSectionsPagerAdapter.getCount(); i++) {
// Create a tab with text corresponding to the page title defined by
// the adapter. Also specify this Activity object, which implements
// the TabListener interface, as the callback (listener) for when
// this tab is selected.
actionBar.addTab(
actionBar.newTab()
.setText(mSectionsPagerAdapter.getPageTitle(i))
.setTabListener(this));

}

Intent intent = new Intent(MainActivity.this, NotMainActivity.class);
startActivity(intent);
finish();
}

@Override
public boolean onCreateOptionsMenu(Menu menu) {
// Inflate the menu; this adds items to the action bar if it is present.
getMenuInflater().inflate(R.menu.menu_main, menu);
return true;
}

@Override
public boolean onOptionsItemSelected(MenuItem item) {
// Handle action bar item clicks here. The action bar will
// automatically handle clicks on the Home/Up button, so long
// as you specify a parent activity in AndroidManifest.xml.
int id = item.getItemId();

//noinspection SimplifiableIfStatement
if (id == R.id.action_settings) {
return true;
}

return super.onOptionsItemSelected(item);
}

@Override
public void onTabSelected(ActionBar.Tab tab, FragmentTransaction fragmentTransaction) {
// When the given tab is selected, switch to the corresponding page in
// the ViewPager.
mViewPager.setCurrentItem(tab.getPosition());
}

@Override
public void onTabUnselected(ActionBar.Tab tab, FragmentTransaction fragmentTransaction) {
}

@Override
public void onTabReselected(ActionBar.Tab tab, FragmentTransaction fragmentTransaction) {
}
/**
* A {@link FragmentPagerAdapter} that returns a fragment corresponding to
* one of the sections/tabs/pages.
*/
public class SectionsPagerAdapter extends FragmentPagerAdapter {

public SectionsPagerAdapter(FragmentManager fm) {
super(fm);
}

@Override
public Fragment getItem(int position) {
// getItem is called to instantiate the fragment for the given page.
// Return a PlaceholderFragment (defined as a static inner class below).
switch (position){
case 0:
return new TheWallFragment();
case 1:
return new PeekFragment();
case 2:
return new CameraFragment();
default:
break;

}

return PlaceholderFragment.newInstance(position + 1);
}




@Override
public int getCount() {
// Show 3 total pages.
return 3;
}

@Override
public CharSequence getPageTitle(int position) {
Locale l = Locale.getDefault();
switch (position) {
case 0:
return getString(R.string.title_section1).toUpperCase(l);
case 1:
return getString(R.string.title_section2).toUpperCase(l);
case 2:
return getString(R.string.title_section3).toUpperCase(l);
}
return null;
}
}

/**
* A placeholder fragment containing a simple view.
*/
public static class PlaceholderFragment extends Fragment {
/**
* The fragment argument representing the section number for this
* fragment.
*/
private static final String ARG_SECTION_NUMBER = "section_number";

/**
* Returns a new instance of this fragment for the given section
* number.
*/
public static PlaceholderFragment newInstance(int sectionNumber) {
PlaceholderFragment fragment = new PlaceholderFragment();
Bundle args = new Bundle();
args.putInt(ARG_SECTION_NUMBER, sectionNumber);
fragment.setArguments(args);
return fragment;
}

public PlaceholderFragment() {
}

@Override
public View onCreateView(LayoutInflater inflater, ViewGroup container,
Bundle savedInstanceState) {
View rootView = inflater.inflate(R.layout.wall_layout, container, false);

return rootView;
}
}

}



NotMainActivity.java


public class NotMainActivity extends FragmentActivity {

private List<Posts> myPosts = new ArrayList<Posts>();

@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
populatePostList();
populateListView();

registerClickCallBack();
System.out.print("Main Activity Started");



}


private void populatePostList() {
myPosts.add(new Posts("Ottawa, Ontario", "787", "Picture of my Dog!", R.drawable.dog));
myPosts.add(new Posts("Ottawa, Ontario", "787", "Picture of my Dog!", R.drawable.dog));
myPosts.add(new Posts("Ottawa, Ontario", "787", "Picture of my Dog!", R.drawable.dog));
myPosts.add(new Posts("Ottawa, Ontario", "787", "Picture of my Dog!", R.drawable.dog));
myPosts.add(new Posts("Ottawa, Ontario", "787", "Picture of my Dog!", R.drawable.dog));
myPosts.add(new Posts("Ottawa, Ontario", "787", "Picture of my Dog!", R.drawable.dog));
myPosts.add(new Posts("Ottawa, Ontario", "787", "Picture of my Dog!", R.drawable.dog));
myPosts.add(new Posts("Ottawa, Ontario", "787", "Picture of my Dog!", R.drawable.dog));



}

private void populateListView() {
ArrayAdapter<Posts> adapter = new MyListAdapter();
ListView list = (ListView) findViewById(android.R.id.list);
list.setAdapter(adapter);

}


//Handles clicks on the list items
private void registerClickCallBack(){
ListView list = (ListView) findViewById(android.R.id.list);
list.setOnItemClickListener(new AdapterView.OnItemClickListener() {
@Override
public void onItemClick(AdapterView<?> parent, View view, int position, long id) {
Posts clickedPost = myPosts.get(position);
String message = "You clicked" + position
+"Location of post is" + clickedPost.getPostlocation();
Toast.makeText(NotMainActivity.this, message, Toast.LENGTH_LONG).show();
}
});
}


private class MyListAdapter extends ArrayAdapter<Posts> {
public MyListAdapter() {
super(NotMainActivity.this, R.layout.item_view, myPosts);
}


@Override
public View getView(int position, View convertView, ViewGroup parent) {
// Make sure we have a view to work with {may have given null}
View itemView = convertView;
if (itemView != null) {
itemView = getLayoutInflater().inflate(R.layout.item_view, parent, false);
}
// Find the post to work with.
Posts currentPost = myPosts.get(position);


//Fill the view
ImageView imageView = (ImageView)itemView.findViewById(R.id.item_postImage);
imageView.setImageResource(currentPost.getIconID());

//Fill in Title

TextView titleText = (TextView) itemView.findViewById(R.id.item_postText);
titleText.setText(currentPost.getPosttitle());

//Set Vote Number
TextView voteText = (TextView) itemView.findViewById(R.id.item_postVoteText);
voteText.setText(currentPost.getPostvote());



return itemView;
}

}

}



Posts.java


public class Posts {
private String postlocation;
private String postvote;
private String posttitle;
private int iconID;

public Posts(String postlocation, String postvote, String posttitle, int iconID) {
this.postlocation = postlocation;
this.postvote = postvote;
this.posttitle = posttitle;
this.iconID = iconID;
}







public String getPostlocation() {
return postlocation;
}

public String getPostvote() {
return postvote;
}

public String getPosttitle() {
return posttitle;
}
public int getIconID() {
return iconID;
}



TheWallFragment.java


public class TheWallFragment extends android.support.v4.app.ListFragment {
List<Posts> myPosts = new ArrayList<Posts>();
private String[] strListView;
private ListView myListView;
private int number;
public View onCreateView(LayoutInflater inflater, ViewGroup container,
Bundle savedInstanceState) {
View rootView = inflater.inflate(R.layout.wall_layout, container, false);
return rootView;
}


}



activity_main.xml


<android.support.v4.view.ViewPager xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:tools="http://schemas.android.com/tools" android:id="@+id/pager"
android:layout_width="match_parent" android:layout_height="match_parent"
tools:context=".MainActivity" />



AndroidManifest.xml


<?xml version="1.0" encoding="utf-8"?>
<manifest xmlns:android="http://schemas.android.com/apk/res/android"
package="com.gnumbu.errolgreen.testing" >

<application
android:allowBackup="true"
android:icon="@mipmap/ic_launcher"
android:label="@string/app_name"
android:theme="@style/AppTheme" >
<activity
android:name=".MainActivity"
android:label="@string/app_name" >
<intent-filter>
<action android:name="android.intent.action.MAIN" />

<category android:name="android.intent.category.LAUNCHER" />
</intent-filter>
</activity>
<activity
android:name=".NotMainActivity">
</activity>
</application>

</manifest>



item_view.xml


<?xml version="1.0" encoding="utf-8"?>
<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
android:layout_width="match_parent" android:layout_height="match_parent">

<ImageView
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:id="@+id/item_postImage"
android:src="@drawable/dog"
android:maxWidth="80dp"
android:minHeight="80dp"
android:adjustViewBounds="true"
android:layout_marginLeft="14dp"
android:layout_marginStart="14dp"
android:layout_alignParentTop="true"
android:layout_toRightOf="@+id/item_upvoteArrow"
android:layout_toEndOf="@+id/item_upvoteArrow"
android:layout_alignBottom="@+id/item_downvotearrow" />

<TextView
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:text="This is my dog! My Dog is amazing, give it a lick!"
android:id="@+id/item_postText"
android:paddingStart="5dp"
android:layout_above="@+id/item_downvotearrow"
android:layout_toRightOf="@+id/item_postImage"
android:layout_toEndOf="@+id/item_postImage" />

<ImageView
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:id="@+id/item_upvoteArrow"
android:src="@drawable/upvotearrow"
android:maxHeight="24dp"
android:maxWidth="24dp"
android:adjustViewBounds="true"
android:layout_alignParentTop="true"
android:layout_alignLeft="@+id/item_downvotearrow"
android:layout_alignStart="@+id/item_downvotearrow"
android:clickable="true" />

<ImageView
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:id="@+id/item_downvotearrow"
android:src="@drawable/downvotearrow"
android:maxWidth="24dp"
android:maxHeight="24dp"
android:adjustViewBounds="true"
android:layout_marginLeft="9dp"
android:layout_marginStart="9dp"
android:layout_below="@+id/item_postVoteText"
android:layout_alignParentLeft="true"
android:layout_alignParentStart="true"
android:longClickable="false"
android:clickable="true" />

<TextView
android:layout_height="wrap_content"
android:text="22"
android:id="@+id/item_postVoteText"
android:maxHeight="20dp"
android:focusableInTouchMode="false"
android:visibility="visible"
android:singleLine="true"
android:gravity="center"
android:enabled="true"
android:layout_below="@+id/item_upvoteArrow"
android:layout_alignLeft="@+id/item_upvoteArrow"
android:layout_alignStart="@+id/item_upvoteArrow"
android:nestedScrollingEnabled="false"
android:layout_width="wrap_content"
android:layout_toStartOf="@+id/item_postImage" />

</RelativeLayout>



wall_layout.xml


<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
android:orientation="vertical" android:layout_width="match_parent"
android:layout_height="match_parent">

<TextView
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:text="New Text"
android:id="@+id/textView"
android:layout_gravity="center_horizontal" />

<ListView
android:id="@android:id/list"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
/>
</LinearLayout>



Error Log


java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.ListView.setAdapter(android.widget.ListAdapter)' on a null object reference
at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2298)
at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2360)
at android.app.ActivityThread.access$800(ActivityThread.java:144)
at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1278)
at android.os.Handler.dispatchMessage(Handler.java:102)
at android.os.Looper.loop(Looper.java:135)
at android.app.ActivityThread.main(ActivityThread.java:5223)
at java.lang.reflect.Method.invoke(Native Method)
at java.lang.reflect.Method.invoke(Method.java:372)
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:899)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:694)
Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.ListView.setAdapter(android.widget.ListAdapter)' on a null object reference
at com.gnumbu.errolgreen.testing.NotMainActivity.populateListView(NotMainActivity.java:70)
at com.gnumbu.errolgreen.testing.NotMainActivity.onCreate(NotMainActivity.java:43)
at android.app.Activity.performCreate(Activity.java:5937)
at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1105)
at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2251)
            at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2360)
            at android.app.ActivityThread.access$800(ActivityThread.java:144)
            at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1278)
            at android.os.Handler.dispatchMessage(Handler.java:102)
            at android.os.Looper.loop(Looper.java:135)
            at android.app.ActivityThread.main(ActivityThread.java:5223)
            at java.lang.reflect.Method.invoke(Native Method)
            at java.lang.reflect.Method.invoke(Method.java:372)
            at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:899)
            at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:694)



New Error


Process: com.gnumbu.errolgreen.testing, PID: 5304
java.lang.NullPointerException: Attempt to invoke virtual method 'int android.view.View.getImportantForAccessibility()' on a null object reference
at android.widget.AbsListView.obtainView(AbsListView.java:2360)
at android.widget.ListView.measureHeightOfChildren(ListView.java:1270)
at android.widget.ListView.onMeasure(ListView.java:1182)
at android.view.View.measure(View.java:17430)
at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:5463)
at android.widget.LinearLayout.measureChildBeforeLayout(LinearLayout.java:1436)
at android.widget.LinearLayout.measureVertical(LinearLayout.java:722)
at android.widget.LinearLayout.onMeasure(LinearLayout.java:613)
at android.view.View.measure(View.java:17430)
at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:5463)
at android.widget.FrameLayout.onMeasure(FrameLayout.java:430)
at android.view.View.measure(View.java:17430)
at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:5463)
at android.widget.LinearLayout.measureChildBeforeLayout(LinearLayout.java:1436)
at android.widget.LinearLayout.measureVertical(LinearLayout.java:722)
at android.widget.LinearLayout.onMeasure(LinearLayout.java:613)
at android.view.View.measure(View.java:17430)
at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:5463)
at android.widget.FrameLayout.onMeasure(FrameLayout.java:430)
at com.android.internal.policy.impl.PhoneWindow$DecorView.onMeasure(PhoneWindow.java:2560)
at android.view.View.measure(View.java:17430)
at android.view.ViewRootImpl.performMeasure(ViewRootImpl.java:2001)
at android.view.ViewRootImpl.measureHierarchy(ViewRootImpl.java:1166)
at android.view.ViewRootImpl.performTraversals(ViewRootImpl.java:1372)
at android.view.ViewRootImpl.doTraversal(ViewRootImpl.java:1054)
at android.view.ViewRootImpl$TraversalRunnable.run(ViewRootImpl.java:5779)
at android.view.Choreographer$CallbackRecord.run(Choreographer.java:767)
at android.view.Choreographer.doCallbacks(Choreographer.java:580)
at android.view.Choreographer.doFrame(Choreographer.java:550)
at android.view.Choreographer$FrameDisplayEventReceiver.run(Choreographer.java:753)
at android.os.Handler.handleCallback(Handler.java:739)
at android.os.Handler.dispatchMessage(Handler.java:95)
at android.os.Looper.loop(Looper.java:135)
at android.app.ActivityThread.main(ActivityThread.java:5221)
at java.lang.reflect.Method.invoke(Native Method)
at java.lang.reflect.Method.invoke(Method.java:372)
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:899)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:694)

Answer

NotMainActivity calls setContentView with R.layout.activity_main as parameter that contains a ViewPager and not a ListView. Since you don't have a ListView declare in to that layout, findViewById(android.R.id.list) returns null, and when you try access it (calling setAdapter) the NullPointerException is thrown.

in your MyAdapter, change from

if (itemView != null) {
      itemView = getLayoutInflater().inflate(R.layout.item_view, parent, false);
}

to

if (itemView == null) {
      itemView = getLayoutInflater().inflate(R.layout.item_view, parent, false);
}
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