Reputation: 23283
I'm working my way through this tutorial and am stuck adding names to a list, and having the list populate.
Here's what I'm trying to get, a list of names that I type in in the input box:
(The underlined "Darryl" is the input box, and it shows that name (after hitting the button) on top, then adds it to the list).
I'm getting this error though:
08-24 18:17:50.903 21935-21935/com.example.batman.myapplication E/AndroidRuntime﹕ FATAL EXCEPTION: main
Process: com.example.batman.myapplication, PID: 21935
java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.ArrayAdapter.notifyDataSetChanged()' on a null object reference
at com.example.batman.myapplication.MainActivity.onClick(MainActivity.java:65)
at android.view.View.performClick(View.java:5217)
at android.view.View$PerformClick.run(View.java:20983)
at android.os.Handler.handleCallback(Handler.java:739)
at android.os.Handler.dispatchMessage(Handler.java:95)
at android.os.Looper.loop(Looper.java:145)
at android.app.ActivityThread.main(ActivityThread.java:6141)
at java.lang.reflect.Method.invoke(Native Method)
at java.lang.reflect.Method.invoke(Method.java:372)
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1399)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1194)
Here's my MainActivity.java
:
package com.example.batman.myapplication;
import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.view.Menu;
import android.view.View;
import android.widget.ArrayAdapter;
import android.widget.Button;
import android.widget.EditText;
import android.widget.ListView;
import android.widget.TextView;
import java.util.ArrayList;
public class MainActivity extends AppCompatActivity implements View.OnClickListener {
TextView mainTextView;
EditText mainEditText;
ListView mainListView;
ArrayAdapter mArrayAdapter;
// ArrayList<String> mNameList = new ArrayList<String>();
ArrayList mNameList = new ArrayList();
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
// 1. Access the TextView defined in layout XML
// and then set its text
mainTextView = (TextView) findViewById(R.id.main_textview);
mainTextView.setText("Set in Java!");
Button mainButton;
mainButton = (Button) findViewById(R.id.main_button);
mainButton.setOnClickListener(this);
// 3. Access the EditText defined in layout XML
mainEditText = (EditText) findViewById(R.id.main_edittext);
// 4. Access the ListView
mainListView = (ListView) findViewById(R.id.main_listview);
// Create an ArrayAdapter for the ListView
mArrayAdapter = new ArrayAdapter(this, // removed ArrayAdapter before mArrayAdapter
android.R.layout.simple_list_item_1,
mNameList);
// Set the ListView to use the ArrayAdapter
mainListView.setAdapter(mArrayAdapter);
}
@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 void onClick(View v) {
// Take what was typed into the EditText
// and use in TextView
mainTextView.setText(mainEditText.getText().toString() + ".");
// Also add that value to the list shown in the ListView
mNameList.add(mainEditText.getText().toString());
mArrayAdapter.notifyDataSetChanged();
}
} // end class
Here's ActivityMain.XML
:
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:tools="http://schemas.android.com/tools"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:orientation="vertical"
tools:context=".MainActivity">
<TextView
android:id="@+id/main_textview"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_marginLeft="20dp"
android:layout_marginTop="20dp"
android:text="textView"/>
<!-- Displays keyboard when touched -->
<LinearLayout
android:layout_width="match_parent"
android:layout_height="wrap_content">
<!-- List whose dataset is defined in code with an adapter -->
<ListView
android:id="@+id/main_listview"
android:layout_width="match_parent"
android:layout_height="0dp"
android:layout_weight="1"
android:layout_marginTop="20dp"/>
<!-- Set OnClickListener to trigger results when pressed -->
<Button
android:id="@+id/main_button"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_marginTop="20dp"
android:layout_marginLeft="20dp"
android:text="button" />
<!-- Shows an image from your drawable resources -->
<ImageView
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_marginTop="20dp"
android:layout_marginLeft="20dp"
android:src="@drawable/ic_launcher" />
<!-- Closing tag for the horizontal nested layout -->
</LinearLayout>
<EditText
android:id="@+id/main_edittext"
android:inputType="textCapSentences"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_marginTop="20dp"
android:layout_marginLeft="20dp"
android:hint="hint"/>
<!-- Closing tag for the horizontal nested layout -->
</LinearLayout>
I also get some highlighting (Android Studio) on the new ArrayAdapter(this,
android.R.layout.simple_list_item_1,
mNameList);
line, saying there's an "Unchecked call" to it.
As you can see by my one comment, do I need to make the mNameList
a string by something like ArrayList<String> mNameList = new ArrayList<String>();
?
(If you need the manifest or my MainActivity.xml
, let me know).
Thanks for any ideas!
Edit: Thanks to the comments, I removed ArrayAdapter
from the onClick part. The final part of issue is to get the list to display. It now displays the input, but doesn't show up on a list below. I've attached the XML too.
Upvotes: 0
Views: 9139
Reputation: 1421
Your mArrayAdapter is always null since you're creating a local mArrayAdapter with this:
ArrayAdapter mArrayAdapter = new ArrayAdapter(this,
android.R.layout.simple_list_item_1,
mNameList);
So... just remove ArrayAdapter
so you will initialize the global version of the adapter and you should be fine.
Upvotes: 2
Reputation: 387
You did not initialized the global variable mArrayAdapter
change :
ArrayAdapter mArrayAdapter = new ArrayAdapter(this,
android.R.layout.simple_list_item_1,
mNameList);
to :
mArrayAdapter = new ArrayAdapter(this,
android.R.layout.simple_list_item_1,
mNameList);
in the onCreate Method of your activity
Upvotes: 6