java - 尽管初始化对象,但空对象引用错误
问题描述
我有 2 个活动,登录和选择语言。在选择特定语言时,选择的语言通过共享首选项保存,应用程序通过 Intent 进入 SignIn 活动。所有这一切都工作得很好。ChooseLanguage 类中有一个 loadLocale() 方法,我想从 SignIn 类中调用它。然而,这给出了一个空指针异常。注意:语言首选项已正确保存,我已将其打开并亲自查看。我哪里错了?
在选择语言活动中:
public void loadLocale(){
Log.i("TAG","CAME HERE");
SharedPreferences prefs = getSharedPreferences("LanguageSettings", Activity.MODE_PRIVATE);
String preferredLanguage = prefs.getString("PreferredLang","");
setLocale(preferredLanguage);
}
登录活动
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
ChooseLanguage chooseLanguage = new ChooseLanguage();
chooseLanguage.loadLocale();
setContentView(R.layout.sign_in);
.
.
.
}
我的日志
2019-06-13 11:58:35.302 10533-10533/com.example.gofresh I/TAG: CAME HERE
2019-06-13 11:58:35.304 10533-10533/com.example.gofresh E/AndroidRuntime: FATAL EXCEPTION: main
Process: com.example.gofresh, PID: 10533
java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.gofresh/com.example.gofresh.SignIn}: 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:3037)
at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:3172)
at android.app.servertransaction.LaunchActivityItem.execute(LaunchActivityItem.java:78)
at android.app.servertransaction.TransactionExecutor.executeCallbacks(TransactionExecutor.java:108)
at android.app.servertransaction.TransactionExecutor.execute(TransactionExecutor.java:68)
at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1906)
at android.os.Handler.dispatchMessage(Handler.java:106)
at android.os.Looper.loop(Looper.java:193)
at android.app.ActivityThread.main(ActivityThread.java:6863)
at java.lang.reflect.Method.invoke(Native Method)
at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:537)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:858)
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 android.content.ContextWrapper.getSharedPreferences(ContextWrapper.java:174)
at com.example.gofresh.ChooseLanguage.loadLocale(ChooseLanguage.java:103)
at com.example.gofresh.SignIn.onCreate(SignIn.java:64)
at android.app.Activity.performCreate(Activity.java:7149)
at android.app.Activity.performCreate(Activity.java:7140)
at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1288)
at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:3017)
此处添加的 setLocale() 方法
private void setLocale(String preferredLanguage){
Locale locale = new Locale(preferredLanguage);
Locale.setDefault(locale);
Configuration configuration = new Configuration();
configuration.locale = locale;
getBaseContext().getResources().updateConfiguration(configuration, getBaseContext().getResources().getDisplayMetrics());
if(!preferredLanguage.equals(""))
onConfigurationChanged(configuration);
SharedPreferences.Editor editor = getSharedPreferences("LanguageSettings",MODE_PRIVATE).edit();
editor.putString("PreferredLang",preferredLanguage);
editor.apply();
}
@Override
public void onConfigurationChanged(Configuration newConfig)
{
chooseLanguage.setText(R.string.chooseYourLanguage);
okButton.setText(R.string.ok);
super.onConfigurationChanged(newConfig);
}
解决方案
你不应该自己实例化活动。把它留给操作系统。否则Context
将不会被初始化并因此引发NullPointetException
. (Context为null,无法获取SharedPrefs,则SharedPrefs为null)
使其成为当前活动的方法或使其成为static
沿上下文传递的方法,如下所示:
public static void setPreferredLocale(Context ctx) {
SharedPreferences prefs = ctx.getSharedPreferences("LanguageSettings", Activity.MODE_PRIVATE);
String preferredLanguage = prefs.getString("PreferredLang","");
setLocale(preferredLanguage);
}
然后,从任何其他 Activity 调用它
ChooseLanguage.setPreferredLocale(this)
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