java.lang.NullPointerException:尝试在空对象引用上调用虚方法

我试图以共享偏好保存播放器的名称,并通过在共享偏好中再次获取它来使其显示在另一个活动中但我的应用程序崩溃。

致命exception:主要

Process: plp.cs4b.thesis.drawitapp, PID: 1970 java.lang.RuntimeException: Unable to start activity ComponentInfo{plp.cs4b.thesis.drawitapp/plp.cs4b.thesis.drawitapp.PlayGame}: java.lang.NullPointerException: Attempt to invoke virtual method 'java.lang.String plp.cs4b.thesis.drawitapp.Player.getName()' on a null object reference at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2298) at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2360) at android.app.ActivityThread.access$800(ActivityThread.java:144) at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1278) at android.os.Handler.dispatchMessage(Handler.java:102) at android.os.Looper.loop(Looper.java:135) at android.app.ActivityThread.main(ActivityThread.java:5221) 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:899) at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:694) Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'java.lang.String plp.cs4b.thesis.drawitapp.Player.getName()' on a null object reference at plp.cs4b.thesis.drawitapp.PlayGame.onCreate(PlayGame.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 

代码:

Player.java

 public class Player { private Context context; private SharedPreferences prefSettingsU; private SharedPreferences.Editor prefEditorU; private static final int PREFERENCE_MODE_PRIVATE = 0; private static final String MY_UNIQUE_PREF_FILE = "DrawItApp"; public Player(Context context, String name) { this.context = context; saveName(name); } public void saveName(String n) { prefSettingsU = context.getSharedPreferences(MY_UNIQUE_PREF_FILE, PREFERENCE_MODE_PRIVATE); prefEditorU = prefSettingsU.edit(); prefEditorU.putString("keyName", n); prefEditorU.commit(); } public String getName(Context ctx) { prefSettingsU = ctx.getSharedPreferences(MY_UNIQUE_PREF_FILE, PREFERENCE_MODE_PRIVATE); String name = prefSettingsU.getString("keyName", "ANONYMOUS"); return name; } 

PlayGame.java

 public class PlayGame extends Activity { private TextView welcomePlayer; private ListView createdGames; private Player mPlayer; @Override protected void onCreate(Bundle savedInstanceState) { super.onCreate(savedInstanceState); setContentView(R.layout.play_game); welcomePlayer = (TextView) findViewById (R.id.tvPlayerName); welcomePlayer.setText("Welcome Back, " + String.valueOf(mPlayer.getName(this)) + " !"); createdGames = (ListView) findViewById (R.id.listCreatedGames); // adapter etc createdGames.setEmptyView(findViewById (R.id.tvNoGames)); } 

PlayerName.java

 public class PlayerName extends Activity { private EditText playerName; private Player mPlayer; public static Context context; @Override protected void onCreate(Bundle savedInstanceState) { super.onCreate(savedInstanceState); setContentView(R.layout.player_name); context = this; playerName = (EditText) findViewById (R.id.etName); } public void onC_Confirm(View btnclick) { mPlayer = new Player(context, String.valueOf(playerName.getText())); //mPlayer.saveName(); Intent intent = new Intent(PlayerName.this, PlayGame.class); startActivity(intent); } public void onC_testShPref(View btnclick) { Intent intent = new Intent(PlayerName.this, PlayGame.class); startActivity(intent); } 

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  • 您的应用崩溃发生在:

     welcomePlayer.setText("Welcome Back, " + String.valueOf(mPlayer.getName(this)) + " !"); 

    因为mPlayer=null

    您忘记在PlayGame活动中初始化Player mPlayer

     mPlayer = new Player(context,"");