Question

我对Android开发很新，我正在开发一个应用程序，该应用程序应该在按下按钮时保存当前时间和日期，将其存储到SharedPreferences，然后在不同按钮时显示字符串按下。我得到了它的工作，但今天做了一些修改，现在我得到了一个我无法解决的NullPointerException，即使在谷歌和Stackoverflow花了大约45分钟尝试不同的代码片段之后。任何比这更好的人能告诉我我失踪的是什么吗？

以下是相关代码：

public class GetTimeStamp extends AppCompatActivity {

    private String date;
    public void setDate (View view){
        DateFormat df = new SimpleDateFormat("dd.MM.yyy 'om' HH:mm");
        date = df.format(Calendar.getInstance().getTime());
    }
    public String getDate(){
        return date;
    }
        SharedPreferences prefs = this.getSharedPreferences("SharedPrefs",       Context.MODE_PRIVATE);
    SharedPreferences.Editor editor = prefs.edit();
    protected void onCreate(Bundle savedInstanceState) {

        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_get_time_stamp);
        Toolbar toolbar = (Toolbar) findViewById(R.id.toolbar);
        setSupportActionBar(toolbar);

        FloatingActionButton fab = (FloatingActionButton) findViewById(R.id.fabSaveTimeStamp);
        fab.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                setDate(view);
                editor.putString("storedDate", date);
                editor.commit();
                Toast.makeText(getBaseContext(), "blablabla", Toast.LENGTH_SHORT).show();
            }
        })
FloatingActionButton fab3 = (FloatingActionButton) findViewById(R.id.fabShowTimeStamp);
        fab3.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                //retrieve the date from SharedPreferences and store it to a local String
                String retrievedDate = prefs.getString("storedDate", null);
                if (retrievedDate!=null){
                    AlertDialog alertDialog = new AlertDialog.Builder(GetTimeStamp.this).create(); //Read Update
                    alertDialog.setTitle("");
                    alertDialog.setMessage("somemessage" + retrievedDate);
                    alertDialog.setButton("OK", new DialogInterface.OnClickListener() {
                        public void onClick(DialogInterface dialog, int which) {
                        }
                    });
                    alertDialog.show();
                }else {
                    //create an alert dialog explaining to the user that no date has been saved yet
                    AlertDialog alertDialog = new AlertDialog.Builder(GetTimeStamp.this).create(); //Read Update
                    alertDialog.setTitle("Error");
                    alertDialog.setMessage("errorexplanation");
                    alertDialog.setButton("OK", new DialogInterface.OnClickListener() {
                        public void onClick(DialogInterface dialog, int which) {
                        }
                    });
                    alertDialog.show();
               }
            }
        });

以下是错误：

02-08 22:49:36.142 19735-19735/? W/dalvikvm: threadid=1: thread exiting with uncaught exception (group=0x41ed7ce0)
02-08 22:49:36.152 19735-19735/? E/AndroidRuntime: FATAL EXCEPTION: main
   Process: com.example.user.appname, PID: 19735
   java.lang.RuntimeException: Unable to instantiate activity ComponentInfo{com.example.user.appname/com.example.user.appname.GetTimeStamp}: java.lang.NullPointerException
       at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2131)
       at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2264)
       at android.app.ActivityThread.access$800(ActivityThread.java:144)
       at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1205)
       at android.os.Handler.dispatchMessage(Handler.java:102)
       at android.os.Looper.loop(Looper.java:136)
       at android.app.ActivityThread.main(ActivityThread.java:5139)
       at java.lang.reflect.Method.invokeNative(Native Method)
       at java.lang.reflect.Method.invoke(Method.java:515)
       at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:796)
       at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:612)
       at dalvik.system.NativeStart.main(Native Method)
    Caused by: java.lang.NullPointerException
       at android.content.ContextWrapper.getSharedPreferences(ContextWrapper.java:173)
       at com.example.user.appname.GetTimeStamp.<init>(GetTimeStamp.java:36)
       at java.lang.Class.newInstanceImpl(Native Method)
       at java.lang.Class.newInstance(Class.java:1208)
       at android.app.Instrumentation.newActivity(Instrumentation.java:1061)
       at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2122)
       at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2264) 
       at android.app.ActivityThread.access$800(ActivityThread.java:144) 
       at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1205) 
       at android.os.Handler.dispatchMessage(Handler.java:102) 
       at android.os.Looper.loop(Looper.java:136) 
       at android.app.ActivityThread.main(ActivityThread.java:5139) 
       at java.lang.reflect.Method.invokeNative(Native Method) 
       at java.lang.reflect.Method.invoke(Method.java:515) 
       at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:796) 
       at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:612) 
       at dalvik.system.NativeStart.main(Native Method)

Answer 1

您正在尝试初始化以下类字段，然后才允许：

SharedPreferences prefs = this.getSharedPreferences("SharedPrefs",       Context.MODE_PRIVATE);

仅声明字段：

SharedPreferences prefs;

然后在onCreate()方法中分配：

protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    prefs = getSharedPreferences("SharedPrefs", Context.MODE_PRIVATE);
    ...
}

究竟是什么导致了这个NullPointerException？

1 个答案: