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POListView: Null pointer exception
 

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    <p>Good day. I'm having some issues with my android project specifically listview. I tried searching for other information here in this site, and implemented some of the answers. However, it is still not working.</p> <p>The error specifically is </p> <blockquote> <p>NullPointerException at line 76 at MainActivity</p> </blockquote> <p>Here is the code of my MainActivity</p> <pre><code>import java.util.ArrayList; import android.os.Bundle; import android.app.Activity; import android.content.Intent; import android.view.Menu; import android.view.View; import android.view.View.OnClickListener; import android.widget.ArrayAdapter; import android.widget.Button; import android.widget.ListView; public class MainActivity extends Activity { final ArrayList&lt;String&gt; studentName = new ArrayList&lt;String&gt;(); ArrayAdapter&lt;String&gt; aa; @Override protected void onCreate(Bundle savedInstanceState) { super.onCreate(savedInstanceState); setContentView(R.layout.activity_main); ListView myList = (ListView) findViewById(R.id.listName); aa = new ArrayAdapter&lt;String&gt;(this, android.R.layout.simple_list_item_1, studentName); myList.setAdapter(aa); //droid.R.id.list; //add Button bAdd = (Button) findViewById(R.id.addstudent); bAdd.setOnClickListener(new View.OnClickListener(){ @Override public void onClick(View v) { startActivity(new Intent("android.intent.action.ADDSTUDENTS")); } }); //edit Button bEdit = (Button) findViewById(R.id.editstudent); bEdit.setOnClickListener(new View.OnClickListener(){ @Override public void onClick(View x) { startActivity(new Intent("android.intent.action.EDITSTUDENTS")); } }); //edit Button bDelete = (Button) findViewById(R.id.deletestudent); bDelete.setOnClickListener(new View.OnClickListener(){ @Override public void onClick(View x) { startActivity(new Intent("android.intent.action.DELETESTUDENTS")); } }); } public ArrayList&lt;String&gt; getArray(){ return studentName; } public void notifyArray(){ aa.notifyDataSetChanged(); } @Override public boolean onCreateOptionsMenu(Menu menu) { // Inflate the menu; this adds items to the action bar if it is present. getMenuInflater().inflate(R.menu.activity_main, menu); return true; } } </code></pre> <p>and line 76 by the way is</p> <pre><code>aa.notifyDataSetChanged(); </code></pre> <p>Here is my code for the AddStudents class</p> <pre><code>import android.app.Activity; import android.content.Intent; import android.os.Bundle; import android.view.View; import android.widget.Button; import android.widget.EditText; public class AddStudents extends Activity{ @Override protected void onCreate(Bundle savedInstanceState) { // TODO Auto-generated method stub super.onCreate(savedInstanceState); setContentView(R.layout.add_student); Button bAddStudents = (Button) findViewById(R.id.add); final EditText et = (EditText) findViewById(R.id.student_name); bAddStudents.setOnClickListener(new View.OnClickListener(){ @Override public void onClick(View v) { MainActivity as = new MainActivity(); as.getArray().add(et.getText().toString()); as.notifyArray(); finish(); } }); Button bBack = (Button) findViewById(R.id.backadd); bBack.setOnClickListener(new View.OnClickListener(){ @Override public void onClick(View v) { finish(); } }); } } </code></pre> <p>and the xml part with the list view is </p> <pre><code> &lt;ListView android:id="@+id/listName" android:layout_width="match_parent" android:layout_height="wrap_content" android:layout_weight="1" &gt; &lt;/ListView&gt; </code></pre> <p>I hope you can help me cause I want to also learn what my mistakes are. I can add other information if you want. </p>
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    1. COWhat are the values of your `studentName` arraylist? Have you added any values in your arraylist?
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    2. COcheck this two link http://stackoverflow.com/questions/5817202/null-pointer-exception-during-android-listview-refresh and http://stackoverflow.com/questions/6894613/array-adapter-notifydatasetchanged-nullpointerexception-android
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    3. CO@Grishu I didn't add any values in the studentName array list. Cuase in my understanding, what im planning to do is that, when ever there is a new entry in the AddStudent class (when adding studens) , it automatically goes in the studentName arraylist. Please correct me if I'm wrong.
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