Contents of ListView not showing - Android

Question

I am building a ListView in my MainActivity, which contains a list of users. I initialize the ArrayAdapter and ListView using this:

adapter = new ArrayAdapter<String>(this, R.layout.activity_main_component, array);

listView = (ListView) findViewById(R.id.list);
listView.setAdapter(adapter);

I am filling the contents of an ArrayList using the following function:

public void usersUpdated(Message mes) {
    try {
        JSONObject jsonObj = new JSONObject(mes.body.getField(Fields.BODY).toString());
        JSONArray jsonArr = (JSONArray) jsonObj.get("listOfUsers");
        array = new ArrayList<>();
        for (int i = 0; i < jsonArr.length(); i++) {
            System.out.println("Added : " + jsonArr.get(i).toString());
            System.out.println(jsonArr.get(i).toString().getClass().getName());
            array.add(jsonArr.get(i).toString());
        }
        System.out.println(array);
        runOnUiThread(new Runnable() {
            public void run() {
                adapter.notifyDataSetChanged();
            }
        });
    } catch (Exception e) {
        e.printStackTrace();
    }
}

Here is the relevant portion of my activity_main.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"
android:weightSum="1">

<ListView
    android:layout_width="match_parent"
    android:layout_height="332dp"
    android:id="@+id/list"
    android:layout_weight="0.70" />

</LinearLayout>

And here is my activity_main_component.xml:

<?xml version="1.0" encoding="utf-8"?>
<TextView xmlns:android="http://schemas.android.com/apk/res/android"
android:id="@+id/label"
android:layout_width="fill_parent"
android:layout_height="fill_parent"
android:padding="10dip"
android:textSize="16dip"
android:textStyle="bold" >
</TextView>

The ArrayList is being properly filled, as the print statement just before the runOnUiThread in the usersUpdated() function is telling me. However, the list shows up blank. Any ideas on what could be going wrong?

notifyDataSetChanged in not a magic wand you need to modify underlying data.. And, no, you didn't... You had create new array — Selvin, Feb 26 '17 at 15:57

score 2 · Answer 1 · edited May 23 '17 at 11:53

Either rename your TextView id as android:id="@android:id/text1" because the adapter will look for the TextView id as android:id="@android:id/text1" to set data

<TextView xmlns:android="http://schemas.android.com/apk/res/android"      
    android:id="@android:id/text1"
    android:layout_width="fill_parent"
    android:layout_height="fill_parent"
    android:padding="10dip"
    android:textSize="16dip"
    android:textStyle="bold" >
</TextView>

or you can mention TextView id to tell the adapter to look for the TextView

mention your TextView id while creating adapter

ArrayAdapter (Context context,int resource,int textViewResourceId,List<T> objects)

   new ArrayAdapter<String>(this, R.layout.activity_main_component,R.id.label, array);

also follow the advice of @Cricket and @Selvin to complete the answer and use array.clear() instead of creating new ArrayList object

For clarity read Is Java “pass-by-reference” or “pass-by-value”?

score 2 · Accepted Answer · edited Feb 26 '17 at 16:29

2

Instead of this

array = new ArrayList<>();

You need to clear the list instead.

array.clear();

Otherwise, the Adapter loses the reference to its list

(you can also add directly to an ArrayAdapter rather than through the list; adapter.add(String) as well as adapter.clear())

And also replace

android:id="@+id/label"

With

android:id="@android:id/text1"

Because this is the id that ArrayAdapter of String looks for by default

Or use this constructor

ArrayAdapter (Context context, 
            int resource,   // set to R.layout.activity_main_component
            int textViewResourceId,  // set to R.id.label 
            List<T> objects)

edited Feb 26 '17 at 16:29

Pavneet_Singh

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answered Feb 26 '17 at 15:57

OneCricketeer

179,855
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Thank you, this solved the issue (specifically the `array.clear()` did). Will accept this answer in 5 minutes – Avery Vine Feb 26 '17 at 16:00
@Pavneet Try it. Making a new ArrayList will de-reference the one in the adapter constructor, so notifying will have no affect – OneCricketeer Feb 26 '17 at 16:17
@PavneetSingh as cricket_007 mentioned in his post, the Adapter loses the reference to the ArrayList that I created (which makes sense in hindsight). I did implement your solution as well, however, as I believe that it is a better practice than what I had. – Avery Vine Feb 26 '17 at 16:18

Contents of ListView not showing - Android

2 Answers2