Listview not loading properly

Question

I am trying to save a link from webview. From my webview class, I am saving the page url as below.

public void AddUrl(String page_url){
    SaveUrlActivity urlactivity = new SaveUrlActivity();
    urlactivity.saveurl(page_url);
}

My SaveUrlActivity class is as below:

public class SaveUrlActivity extends Activity {

public String url;
public ListView lv;

@Override
public void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_savedurl);
    lv = (ListView) findViewById(R.id.list);
    saveurl(url);
    lv.setOnItemClickListener(new AdapterView.OnItemClickListener() {

        public void onItemClick(AdapterView<?> parent, View view,
                                int position, long id) {
            Intent in = new Intent(SaveUrlActivity.this, DisPlayWebPageActivity.class);

            in.putExtra("page_url", url);
            startActivity(in);
        }
    });
}

public void saveurl(String url1){
    url = url1;
    final List<RowItem> ri = new ArrayList<RowItem>();
    RowItem item = new RowItem(url);
    ri.add(item);
    SavedUrlAdapter adapter = new SavedUrlAdapter(SaveUrlActivity.this, ri);
    lv.setAdapter(adapter);
  }
}

Whenever I run my program, I am getting the following error.

 FATAL EXCEPTION: main
Process: com.example.smarthelp, PID: 16284
java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.ListView.setAdapter(android.widget.ListAdapter)' on a null object reference
        at com.example.smarthelp.SaveUrlActivity.saveurl(SaveUrlActivity.java:46)
        at com.example.smarthelp.DisPlayWebPageActivity.AddUrl(DisPlayWebPageActivity.java:76)
        at com.example.smarthelp.DisPlayWebPageActivity.onOptionsItemSelected(DisPlayWebPageActivity.java:66)

Can anyone help me figure out where I am making the mistake?

Answer 1

You're using reference to ListView lv before it has been initialized, and you can't do that without first starting the SaveUrlActivity, only after it's setContentView(R.layout.activity_savedurl); you can initialize the lv.

public void saveurl(String url1){
    url = url1;
    final List<RowItem> ri = new ArrayList<RowItem>();
    RowItem item = new RowItem(url);
    ri.add(item);
    SavedUrlAdapter adapter = new SavedUrlAdapter(SaveUrlActivity.this, ri);
    lv.setAdapter(adapter);
  }

It would be easier if you make a static var in your SaveUrlActivity class and assign a value to it before you start the Activity, and the make call to the saveurl() from inside of that class when you have all your views initialized.

Something like:

public void AddUrl(String page_url){
    SaveUrlActivity.page_url = page_url;
    Intent i = new Intent(this, SaveUrlActivity.class);
    startActivity(i);
}

And in your SaveUrlActivity.java:

public class SaveUrlActivity extends Activity {

public String url;
public ListView lv;
static page_url;

@Override
public void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_savedurl);
    lv = (ListView) findViewById(R.id.list);
    saveurl();
    lv.setOnItemClickListener(new AdapterView.OnItemClickListener() {

        public void onItemClick(AdapterView<?> parent, View view,
                                int position, long id) {
            Intent in = new Intent(SaveUrlActivity.this, DisPlayWebPageActivity.class);

            in.putExtra("page_url", url);
            startActivity(in);
        }
    });
}

public void saveurl(){
    url = page_url;
    final List<RowItem> ri = new ArrayList<RowItem>();
    RowItem item = new RowItem(url);
    ri.add(item);
    SavedUrlAdapter adapter = new SavedUrlAdapter(SaveUrlActivity.this, ri);
    lv.setAdapter(adapter);
  }
}

It's accurate if you're planning to start this activity, if you need to update data in already started one, you better use the Handler