HOWTO · Java
How to Attempt to Invoke Virtual Method on a Null Object Reference Error in Java
This tutorial demonstrates how to solve the attempt to invoke a virtual method on a null object reference error in Java.
This tutorial demonstrates how to solve the Attempt to invoke virtual method on a null object reference error in Java.
the Attempt to invoke virtual method on a null object reference Error in Java
While working on an Android application, the error Attempt to invoke virtual method on a null object reference can occur, a type of NullPointerException. Whenever we try to invoke a method using a null object, it will throw this error.
Here is a system based on the Android platform, which throws the NullPointerException with the attempt to invoke virtual method on a null object reference description.
This example has three different Java classes.
Main_Player.java:
package delftstack;
import javax.naming.Context;
public class Main_Player {
private Context AppContext;
private SharedPreferences SharedPreferencesSettingsU;
private SharedPreferences.Editor PreferencesEditorU;
private static final int PREFERENCE_MODE = 0;
private static final String UNIQUE_PREFERENCES_FILE = "DemoApp";
public Player(Context AppContext, String Player_Name) {
this.AppContext = AppContext;
Save_Name(Player_Name);
}
public void Save_Name(String nValue) {
SharedPreferencesSettingsU =
AppContext.getSharedPreferences(UNIQUE_PREFERENCES_FILE, PREFERENCE_MODE);
PreferencesEditorU = SharedPreferencesSettingsU.edit();
PreferencesEditorU.putString("keyName", nValue);
PreferencesEditorU.commit();
}
public String Get_Name(Context DemoContext) {
SharedPreferencesSettingsU =
DemoContext.getSharedPreferences(UNIQUE_PREFERENCES_FILE, PREFERENCE_MODE);
String PlayerName = SharedPreferencesSettingsU.getString("keyName", "ANONYMOUS");
return PlayerName;
}
}
Player_Name.java:
package delftstack;
import javax.naming.Context;
public class Player_Name extends Activity {
private EditText Player_Name;
private Player M_Player;
public static Context AppContext;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.player_name);
AppContext = this;
Player_Name = (EditText) findViewById(R.id.etName);
}
public void onC_Confirm(View btnclick) {
M_Player = new Player(AppContext, String.valueOf(Player_Name.getText()));
// M_Player.saveName();
Intent intent = new Intent(Player_Name.this, Game_Play.class);
startActivity(intent);
}
public void onC_testShPref(View btnclick) {
Intent intent = new Intent(Player_Name.this, Game_Play.class);
startActivity(intent);
}
Game_Play.java:
public class Game_Play extends Activity {
private TextView Welcome_Player;
private ListView Games_Created;
private Player M_Player;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.play_game);
Welcome_Player = (TextView) findViewById(R.id.tvPlayer_Name);
Welcome_Player.setText("Welcome Back Player, " + String.valueOf(M_Player.getName(this)) + " !");
Games_Created = (ListView) findViewById(R.id.listCreatedGames);
Games_Created.setEmptyView(findViewById(R.id.tvNoGames));
}
}
We run the above system on an Android platform, which will throw an error caused by the java.lang.NullPointerException: Attempt to invoke virtual method.
Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'java.lang.String plp.cs4b.thesis.drawitapp.Main_Player.getName()' on a null object reference
at plp.cs4b.thesis.drawitapp.Game_Play.onCreate(Game_Play.java:20)
at android.app.Activity.performCreate(Activity.java:5933)
at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1105)
at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2251)
... 10 more
The error occurs in the below code line in the class Game_Play.java.
Welcome_Player.setText("Welcome Back Player, " + String.valueOf(M_Player.getName(this)) + " !");
This error occurs because the M_Player comes as Null. After all, it is not initialized. By initializing, we can solve that issue.
See the solution:
Game_Play.java:
package delftstack;
public class Game_Play extends Activity {
private TextView Welcome_Player;
private ListView Games_Created;
// initialize the M_Player
private Player M_Player = new Main_Player(AppContext, "");
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.play_game);
Welcome_Player = (TextView) findViewById(R.id.tvPlayer_Name);
Welcome_Player.setText("Welcome Back Player, " + String.valueOf(M_Player.getName(this)) + " !");
Games_Created = (ListView) findViewById(R.id.listCreatedGames);
Games_Created.setEmptyView(findViewById(R.id.tvNoGames));
}
}
So whenever working on a system like this, you must ensure that a method is not invoking any null value; otherwise, it will throw this exception.
