SharedPreferences 空指针异常

Question

我正在尝试保存一个整数，该整数将存储用户使用 SharedPreferences 界面点击 ImageView 的次数。但是，当我运行应用程序时，它会在我声明 sharedPreferences 的行上出现空指针异常，如下所示：

SharedPreferences mSharedPreferences = this.getPreferences(Context.MODE_PRIVATE);

这是我第一次使用这个界面，对我来说有点困惑。我不知道为什么会这样。 这是日志：

java.lang.RuntimeException: Unable to instantiate activity ComponentInfo{koemdzhiev.com.eggyegg/koemdzhiev.com.eggyegg.MainActivity}: java.lang.NullPointerException: Attempt to invoke virtual method 'java.lang.String android.content.Context.getPackageName()' on a null object reference
        at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2236)
        at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2387)
        at android.app.ActivityThread.access$800(ActivityThread.java:151)
        at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1303)
        at android.os.Handler.dispatchMessage(Handler.java:102)
        at android.os.Looper.loop(Looper.java:135)
        at android.app.ActivityThread.main(ActivityThread.java:5254)
        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:903)
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:698)
 Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'java.lang.String android.content.Context.getPackageName()' on a null object reference
        at android.content.ContextWrapper.getPackageName(ContextWrapper.java:132)
        at android.app.Activity.getLocalClassName(Activity.java:4987)
        at android.app.Activity.getPreferences(Activity.java:5021)
        at koemdzhiev.com.eggyegg.MainActivity.<init>(MainActivity.java:20)
        at java.lang.reflect.Constructor.newInstance(Native Method)
        at java.lang.Class.newInstance(Class.java:1606)
        at android.app.Instrumentation.newActivity(Instrumentation.java:1066)
        at   

这是我在从 Application 类扩展的类中的代码：

public class EggyEggApplication extends Application {

@Override
public void onCreate() {
    SharedPreferences mSharedPreferences = this.getSharedPreferences(getString(R.string.koemdzhiev_eggyegg_PREFERENCE_FILE_KEY), Context.MODE_PRIVATE);
    SharedPreferences.Editor mEditor = mSharedPreferences.edit();
    if(mSharedPreferences.contains(getString(R.string.koemdzhiev_eggyegg_PREFERENCE_FILE_KEY))== false) {
        mEditor.putInt(getString(R.string.koemdzhiev_eggyegg_PREFERENCE_FILE_KEY), 0).apply();
    }
}

这是我在主要活动中的代码：

public class MainActivity extends AppCompatActivity {
private static final String TAG = MainActivity.class.getSimpleName();
private int defaultValue;
private int i;
ImageView tapImage;
SharedPreferences mSharedPreferences = this.getPreferences(Context.MODE_PRIVATE);
SharedPreferences.Editor mEditor = mSharedPreferences.edit();

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


    tapImage = (ImageView)findViewById(R.id.tapImage);
    tapImage.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View v) {
            i++;
            mEditor.putInt(getString(R.string.koemdzhiev_eggyegg_PREFERENCE_FILE_KEY),i);
            mEditor.commit();
        }
    });
    defaultValue = getResources().getInteger(R.string.koemdzhiev_eggyegg_PREFERENCE_FILE_KEY);
    i = mSharedPreferences.getInt(getString(R.string.koemdzhiev_eggyegg_PREFERENCE_FILE_KEY),defaultValue);



    int defaultValue = getResources().getInteger(R.string.koemdzhiev_eggyegg_PREFERENCE_FILE_KEY);
    long lastTapNumber = mSharedPreferences.getInt(getString(R.string.koemdzhiev_eggyegg_PREFERENCE_FILE_KEY),defaultValue);
    i = (int) lastTapNumber;

}
private boolean isNetworkAvailable() {
    ConnectivityManager connectivityManager
            = (ConnectivityManager) getSystemService(Context.CONNECTIVITY_SERVICE);
    NetworkInfo activeNetworkInfo = connectivityManager.getActiveNetworkInfo();
    return activeNetworkInfo != null && activeNetworkInfo.isConnected();
}
@Override
public boolean onCreateOptionsMenu(Menu menu) {
    // Inflate the menu; this adds items to the action bar if it is present.
    getMenuInflater().inflate(R.menu.menu_main, menu);
    return true;
}

Answer 1

你说你保存整数，所以你要查找的关键字是

       "java.lang.String android.content.Context.getPackageName()"

编辑：我的错！创建方法可能有问题

Answer 2

在Activity中，移动

SharedPreferences mSharedPreferences = this.getPreferences(Context.MODE_PRIVATE);
SharedPreferences.Editor mEditor = mSharedPreferences.edit();

在onCreate 内。你需要Context 才能访问SharedPreferences。内部getPreferences(Context.MODE_PRIVATE)，致电getPackageName。

编辑

您应该避免使用本地化字符串作为 SharedPreferences 的键

Answer 3

问题在于您的 MainActivity 中的这一行：

SharedPreferences mSharedPreferences = this.getPreferences(Context.MODE_PRIVATE);

异常说，你得到一个空指针，

这个空指针发生是因为上下文为空， 发生这种情况是因为上面的行是在活动完成创建之前执行的，

所以将这一行移到你的 onCreate 方法中应该可以解决问题

Answer 4

试试 -

public class MainActivity extends AppCompatActivity {
private static final String TAG = MainActivity.class.getSimpleName();
private int defaultValue;
private int i;
ImageView tapImage;
SharedPreferences mSharedPreferences;

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

    mSharedPreferences = this.getPreferences(Context.MODE_PRIVATE);
    this.editor = mSharedPreferences.edit();
    this.editor.commit();


    tapImage = (ImageView)findViewById(R.id.tapImage);
    tapImage.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View v) {
            i++;
                setValueToSharedPref("TAP_NUMBER",i);
        }
    });


long lastTapNumber = mSharedPreferences.getInt("TAP_NUMBER",0);
i = (int) lastTapNumber;

}


    public void setValueToSharedPref(String key, boolean value) {
        this.editor.putBoolean(key, value);
        this.editor.commit();

    }


private boolean isNetworkAvailable() {
    ConnectivityManager connectivityManager
            = (ConnectivityManager) getSystemService(Context.CONNECTIVITY_SERVICE);
    NetworkInfo activeNetworkInfo = connectivityManager.getActiveNetworkInfo();
    return activeNetworkInfo != null && activeNetworkInfo.isConnected();
}
@Override
public boolean onCreateOptionsMenu(Menu menu) {
    // Inflate the menu; this adds items to the action bar if it is present.
    getMenuInflater().inflate(R.menu.menu_main, menu);
    return true;
}