Declaring a string array in the UI thread then populating it from the background thread

Question

Hello I am currently developing an android bank app project.

I have two threads the UI and the bankThread running as the background thread to do all the necessary bank operations.

In this activity I am using string arrays to inflate a listview so the user can view transactions made on their account.

I have declared these string arrays in the Ui thread and then used the handler from the bank thread to populate them however after I have done this these arrays all return null and the app crashes due to a null pointer exception.

I've used Localbroadcastmanager to send these string arrays as a broadcast in another class and its worked perfectly, yet in this class the string arrays still remain null.

Any ideas of how to solve this?

The code:

public class ViewTransactionsActivity extends AppCompatActivity {
ListView transactionsListView;
public static volatile String[] tranDates;
static volatile String[] tranTypes;
static volatile String[] tranDescs;
static volatile String[] tranAmounts;
BankThread bankThread;
Context mContext;




@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_view_transactions);
    bankThread = MainActivity.getBankThread();
    transactionsListView = (ListView) findViewById(R.id.transaction_list_view);
    final TextView errorTextView = (TextView) findViewById(R.id.view_transactions_error_text);


    bankThread.handler.post(new Runnable() {
        @Override
        public void run() {
            tranDates = bank.getAccTranDate();
            tranTypes = bank.getAccTranTypes();
            tranAmounts = bank.getAccTranAmount();
            tranDescs = bank.getAccTranDesc();

            Intent tranArraysIntent = new Intent("getTransactionInfo");
            tranArraysIntent.putExtra("theTypes", tranTypes);
            tranArraysIntent.putExtra("theDates", tranDates);
            tranArraysIntent.putExtra("theAmounts", tranAmounts);
            tranArraysIntent.putExtra("theDesc", tranDescs);
            if (tranAmounts != null || tranDates != null || tranDescs != null || tranTypes != null)
                errorTextView.setText("Alls grand.");
            LocalBroadcastManager.getInstance(mContext).sendBroadcast(tranArraysIntent);

        }
    });

    LocalBroadcastManager.getInstance(this).registerReceiver(mReceiver, new IntentFilter("getTransactionInfo"));


    if(tranAmounts != null || tranDates != null || tranDescs != null || tranTypes != null) {
        TransactionAdapter transactionAdapter = new TransactionAdapter(this, tranTypes, tranAmounts, tranDescs, tranDates);
        transactionsListView.setAdapter(transactionAdapter);
        errorTextView.setText("Complete.");
    }else{
        errorTextView.setText("A boo-boo has occurred.");
    }

}

BroadcastReceiver mReceiver = new BroadcastReceiver() {
    @Override
    public void onReceive(Context context, Intent intent) {
        tranTypes = intent.getStringArrayExtra("theTypes");
        tranAmounts = intent.getStringArrayExtra("theAmounts");
        tranDates = intent.getStringArrayExtra("theDates");
        tranDescs = intent.getStringArrayExtra("theDesc");

    }
};

}

Here's the error stack trace:

02-18 21:35:44.198 28081-28081/com.example.mk.bankapptest E/AndroidRuntime: FATAL EXCEPTION: main
Process: com.example.mk.bankapptest, PID: 28081
java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.mk.bankapptest/com.example.mk.bankapptest.ViewTransactionsActivity}: java.lang.NullPointerException: Attempt to read from null array
    at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2957)
    at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:3032)
    at android.app.ActivityThread.-wrap11(Unknown Source:0)
    at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1696)
    at android.os.Handler.dispatchMessage(Handler.java:105)
    at android.os.Looper.loop(Looper.java:164)
    at android.app.ActivityThread.main(ActivityThread.java:6944)
    at java.lang.reflect.Method.invoke(Native Method)
    at com.android.internal.os.Zygote$MethodAndArgsCaller.run(Zygote.java:327)
    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1374)
 Caused by: java.lang.NullPointerException: Attempt to read from null array
    at com.example.mk.bankapptest.ViewTransactionsActivity.onCreate(ViewTransactionsActivity.java:56)
    at android.app.Activity.performCreate(Activity.java:7183)
    at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1220)
    at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2910)
    at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:3032) 
    at android.app.ActivityThread.-wrap11(Unknown Source:0) 
    at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1696) 
    at android.os.Handler.dispatchMessage(Handler.java:105) 
    at android.os.Looper.loop(Looper.java:164) 
    at android.app.ActivityThread.main(ActivityThread.java:6944) 
    at java.lang.reflect.Method.invoke(Native Method) 
    at com.android.internal.os.Zygote$MethodAndArgsCaller.run(Zygote.java:327) 
    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1374) 

Answer 1

Your problem is here:

  errorTextView.setText(tranDates[0]);

At this moment, tranDates is null.

You should put this line in your Runnable (or directly remove the line).

Regards