我试图实施"首次发布"在用户第一次打开应用程序时打开特定活动的方法。
我有一个名为prefs的SharedPreferences:
public class MainActivity extends ActionBarActivity {
SharedPreferences prefs = null;
public static final String DEFAULT = "N/A";
@Override
protected void onCreate(Bundle savedInstanceState) { ...
现在,在我的onResume方法中:
@Override
protected void onResume() {
super.onResume();
if (prefs.getBoolean("firstrun", true)) {
// Do first run stuff here then set 'firstrun' as false
// using the following line to edit/commit prefs
Toast.makeText(this, "Hello", Toast.LENGTH_SHORT).show();
prefs.edit().putBoolean("firstrun", false).apply();
}
}
这在我想要拥有此功能的其他应用程序中完全正常。但在这个项目中,它失败了:
java.lang.RuntimeException: Unable to resume activity {com.heavyfork.partystarter/com.heavyfork.partystarter.MainActivity}: java.lang.NullPointerException: Attempt to invoke interface method 'boolean android.content.SharedPreferences.getBoolean(java.lang.String, boolean)' on a null object reference
at android.app.ActivityThread.performResumeActivity(ActivityThread.java:3224)
at android.app.ActivityThread.handleResumeActivity(ActivityThread.java:3257)
at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2479)
at android.app.ActivityThread.access$800(ActivityThread.java:144)
at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1359)
at android.os.Handler.dispatchMessage(Handler.java:102)
at android.os.Looper.loop(Looper.java:155)
at android.app.ActivityThread.main(ActivityThread.java:5696)
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:1028)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:823)
Caused by: java.lang.NullPointerException: Attempt to invoke interface method 'boolean android.content.SharedPreferences.getBoolean(java.lang.String, boolean)' on a null object reference
at com.heavyfork.partystarter.MainActivity.onResume(MainActivity.java:258)
at android.app.Instrumentation.callActivityOnResume(Instrumentation.java:1331)
at android.app.Activity.performResume(Activity.java:6044)
at android.app.ActivityThread.performResumeActivity(ActivityThread.java:3207)
at android.app.ActivityThread.handleResumeActivity(ActivityThread.java:3257)
at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2479)
at android.app.ActivityThread.access$800(ActivityThread.java:144)
at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1359)
at android.os.Handler.dispatchMessage(Handler.java:102)
at android.os.Looper.loop(Looper.java:155)
at android.app.ActivityThread.main(ActivityThread.java:5696)
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:1028)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:823)
很抱歉这种奇怪的代码格式。
有谁知道可能导致这种情况的原因,我认为它与onResume方法有关?
答案 0 :(得分:4)
您已将SharedPreferences prefs
初始化为null
,因此将NPE初始化。
要访问默认的SharedPreferences,请执行以下操作
SharedPreferences sharedPref = getActivity().getPreferences(Context.MODE_PRIVATE);
boolean first = prefs.getBoolean("firstrun", true)
然后,你可以继续做
if (first) {
// Do first run stuff here then set 'firstrun' as false
// using the following line to edit/commit prefs
Toast.makeText(this, "Hello", Toast.LENGTH_SHORT).show();
prefs.edit().putBoolean("firstrun", false).apply();
}
}
您的onCreate
方法中的
答案 1 :(得分:0)
你正在获得NPE。因为你还没有初始化共享的偏好,所以它是空的。
请在做任何偏好之前做这样的事。
pref = context.getSharedPreferences("pref_name", Context.MODE_PRIVATE);