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

Question

我正在尝试将播放器的名称保存为共享的首选项，并通过再次以共享的首选项获取它的名称来使其显示在另一个活动中，但是我的应用程序崩溃了。

致命异常：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：

播放器

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);
}

Answer 1

您的应用在以下位置崩溃：

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

因为mPlayer=null。

您忘记Player mPlayer在PlayGame活动中进行初始化。

mPlayer = new Player(context,"");