Question

经典＆＃34; null对象引用＆＃34;我认为这是因为我已经创建了sharedPreferences，但我不确定解决方案，任何人都可以提供帮助，这将是非常好的。高分类（下）＆＃34; sharedPreferences prefs = ...＆＃34;是错误的地方，我现在在MainActivity的onCreate方法中得到错误 - displayHighscore（）调用highscore类的highscoreSetorGet方法。和之前一样，我第二次调用函数时只得到了这个错误。

高分等级

public class highscore {
static highscore hs = new highscore();
static int currentHighscore[] = new int[4];

static void highscoreSetorGet(int min, int sec, int centi, int counter, boolean write) {
    SharedPreferences pref = ma.context.getSharedPreferences("myPrefsKey", Context.MODE_PRIVATE);
    SharedPreferences.Editor editor = pref.edit();

    if(write) {
        editor.putInt("min", min);
        editor.putInt("sec", sec);
        editor.putInt("centi", centi);
        editor.putInt("counter", counter);

        editor.commit();
    }
    if(!write) {
        currentHighscore[0] = pref.getInt("min", 0);
        currentHighscore[1] = pref.getInt("sec", 0);
        currentHighscore[2] = pref.getInt("centi", 0);
        currentHighscore[3] = pref.getInt("counter", 0);
    }
}

}

用于在崩溃时调用highscoreSetorGet的代码

public class MainActivity extends AppCompatActivity {
static MainActivity ma = new MainActivity();
Context context;

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);

    context = this.getApplicationContext();

    Music.playMusic(this.getApplicationContext(), 1, 0, 3, 0);
    highscore.highscoreSetorGet(0, 0, 0, 0, false);
    displayHighscore();
}

public void displayHighscore() {
    highscore.highscoreSetorGet(0, 0, 0, 0, false);

    TextView tv = (TextView)findViewById(R.id.highscoreView);
    tv.setText("Current Highscore: " + Integer.toString(highscore.currentHighscore[0]) + ":" + Integer.toString(highscore.currentHighscore[1]) + "." + Integer.toString(highscore.currentHighscore[2]));
}

错误

E/AndroidRuntime: FATAL EXCEPTION: main
                  Process: com.example.sam.myfirstapp, PID: 18807
                  java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.sam.myfirstapp/com.example.sam.myfirstapp.MainActivity}: java.lang.NullPointerException: Attempt to invoke virtual method 'android.content.SharedPreferences android.content.Context.getSharedPreferences(java.lang.String, int)' on a null object reference
                      at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2646)
                      at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2707)
                      at android.app.ActivityThread.-wrap12(ActivityThread.java)
                      at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1460)
                      at android.os.Handler.dispatchMessage(Handler.java:102)
                      at android.os.Looper.loop(Looper.java:154)
                      at android.app.ActivityThread.main(ActivityThread.java:6077)
                      at java.lang.reflect.Method.invoke(Native Method)
                      at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:865)
                      at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:755)
                   Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'android.content.SharedPreferences android.content.Context.getSharedPreferences(java.lang.String, int)' on a null object reference
                      at com.example.sam.myfirstapp.highscore.highscoreSetorGet(highscore.java:20)
                      at com.example.sam.myfirstapp.MainActivity.displayHighscore(MainActivity.java:61)
                      at com.example.sam.myfirstapp.MainActivity.onCreate(MainActivity.java:31)
                      at android.app.Activity.performCreate(Activity.java:6664)
                      at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1118)
                      at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2599)
                      at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2707) 
                      at android.app.ActivityThread.-wrap12(ActivityThread.java) 
                      at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1460) 
                      at android.os.Handler.dispatchMessage(Handler.java:102) 
                      at android.os.Looper.loop(Looper.java:154) 
                      at android.app.ActivityThread.main(ActivityThread.java:6077) 
                      at java.lang.reflect.Method.invoke(Native Method) 
                      at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:865) 
                      at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:755)

Answer 1

尝试将highscore.highscoreSetorGet(this, 0, 0, 0, 0, false);更改为 highscore.highscoreSetorGet(MainActivity.this, 0, 0, 0, 0, false);。

如果它没有帮助，请尝试使用： SharedPreferences pref = context.getSharedPreferences("myPrefsKey", Context.MODE_PRIVATE);

另外，当使用类中的常量时，你应该从它自己的类中调用它。而不是hs.ctx.MODE_PRIVATE 使用Context.MODE_PRIVATE。

编辑：几条评论，您不应该使用static MainActivity ma = new MainActivity();，而是使用this。请阅读android代码风格指南。 https://source.android.com/source/code-style.html。例如mContext而不是上下文等等。如果您正在尝试创建一个高分的Singelton课程，那不是那样，请阅读相关指南。

至于崩溃，高分类的变化：

static void highscoreSetorGet(Context context, int min, int sec, int centi, int counter, boolean write) {
SharedPreferences pref = context.getSharedPreferences("myPrefsKey", Context.MODE_PRIVATE);

并在MainActivity中使用：

highscore.highscoreSetorGet(context, 0, 0, 0, 0, false);

Android应用程序崩溃了。我已经尝试过明显的解决方案，不知道从哪里开始

1 个答案: