java - 从另一个类中获取值(value)

Question

我想从另一个类获取值，但是当我在第二类中获取它时，它是空的。我认为这是由于 bcs 当向此字符串添加某些内容时导致代码无法运行。

protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_maps);
    // Obtain the SupportMapFragment and get notified when the map is ready to be used.
    SupportMapFragment mapFragment = (SupportMapFragment) getSupportFragmentManager()
            .findFragmentById(R.id.map);
    mapFragment.getMapAsync(this);

很多不重要的代码.................................................

FirebaseDatabase database5 = FirebaseDatabase.getInstance();
    DatabaseReference myRef5 = database5.getReference("userdata");

    myRef5.addValueEventListener(new ValueEventListener() {
        @Override
        public void onDataChange(DataSnapshot dataSnapshot) {
            // This method is called once with the initial value and again
            // whenever data at this location is updated.

很多不重要的代码...................................................... ...

mMap.setOnMarkerClickListener(new GoogleMap.OnMarkerClickListener() {
                    @Override
                    public boolean onMarkerClick(final Marker marker) {

很多不重要的代码........................................ ..

new CountDownTimer(2000, 1000) {

           public void onTick(long millisUntilFinished) {

           }
           public void onFinish() {

               progressBar.setVisibility(View.GONE);
               textViewPleaseWait.setVisibility(View.INVISIBLE);

               recipientEmail = marker.getTitle();
               Log.i("omg", recipientEmail);

               dialogBuilder.setMessage("Username: " + usernameAlert + "\n" + "Gender: " + genderAlert
                                       + "\n" + "Age: " + ageAlert + "\n" + marker.getSnippet() + "\n" + marker.getTitle());

               dialogBuilder.setPositiveButton("Send message", new DialogInterface.OnClickListener() {
                   public void onClick(DialogInterface arg0, int arg1) {
                       Intent intent = new Intent(getApplicationContext(), SendMessage.class);
                       startActivity(intent);

                       recipientEmail = marker.getTitle();
                   }
               });

               dialogBuilder.setNegativeButton("Close", null);
               dialogBuilder.show();


           }
       }.start();

}

这是......当计时器完成时，我将值设置为recipientEmail，但我必须通过单击 map 上的标记来运行计时器。我无法在此计时器之外设置收件人电子邮件的值，因为 bcs 值不会相同。所以当我在另一个类中调用它时，通常:

MapsActivity mapsActivity = new MapsActivity();

    recipientUsername2 = mapsActivity.getRecipientEmail();
    Log.i("values2", mapsActivity.recipientEmail);

运行第二个 Activity 后应用程序崩溃。

08-28 21:34:25.943 4609-4609/com.samo.facedatefb E/AndroidRuntime: FATAL EXCEPTION: main
Process: com.samo.facedatefb, PID: 4609
java.lang.RuntimeException: Unable to start activity ComponentInfo{com.samo.facedatefb/com.samo.facedatefb.SendMessage}: java.lang.NullPointerException: println needs a message
    at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2464)
    at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2526)
    at android.app.ActivityThread.access$800(ActivityThread.java:169)
    at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1421)
    at android.os.Handler.dispatchMessage(Handler.java:111)
    at android.os.Looper.loop(Looper.java:194)
    at android.app.ActivityThread.main(ActivityThread.java:5549)
    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:964)
    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:759)
 Caused by: java.lang.NullPointerException: println needs a message
    at android.util.Log.println_native(Native Method)
    at android.util.Log.i(Log.java:160)
    at com.samo.facedatefb.SendMessage.onCreate(SendMessage.java:79)
    at android.app.Activity.performCreate(Activity.java:5975)
    at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1111)
    at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2417)
    at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2526) 
    at android.app.ActivityThread.access$800(ActivityThread.java:169) 
    at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1421) 
    at android.os.Handler.dispatchMessage(Handler.java:111) 
    at android.os.Looper.loop(Looper.java:194) 
    at android.app.ActivityThread.main(ActivityThread.java:5549) 
    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:964) 
    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:759) 

你能帮我一下吗？

Answer 1

在这里，您创建一个新(可能是空)MapsActivity实例:

MapsActivity mapsActivity = new MapsActivity();

下面两行，你尝试从中读取:

recipientUsername2 = mapsActivity.getRecipientEmail();

难怪它是空的。 这个 MapsActivity 不是您正在寻找的 MapsActivity...(只是这一次它不是绝地诡计。)

尝试这样理解:当您在 MapsActivity 中设置 recipientEmail 字段时，您将其写在一张纸上。通过 new MapsActivity() 构造函数调用，您从抽屉中取出一张新纸 - 一张不同的纸，现在您尝试从纸上读取一些内容，其中你在另一张上写了...

为了能够获得该信息，您必须将其放在某个地方 - 将其设置为您传递的类的实例，将其设置为您接下来计划使用的类中的字段，等等...

但是，这一次，我认为将 recipientEmail 字符串设置为 SendMail Activity 的额外内容将是正确的方法:

Intent intent = new Intent(getApplicationContext(), SendMessage.class);
intent.putExtra("RECIPIENT_EMAIL", marker.getTitle());
startActivity(intent); //this has to be after the putExtra call

当您尝试在 SendMail Activity 中访问它时:

String recipientEmail = null;
Bundle extras = getIntent().getExtras();
if(extras == null) {
    throw new IllegalStateException("No email address found at SendMail activity!");
} else {
    recipientEmail = extras.getString("RECIPIENT_EMAIL");
}

另外，请阅读 this question 的答案我通常在需要处理 Extras 时求助。

另一件事:访问对象成员的推荐方法是将它们设置为私有(private) vland 添加 getter 方法以及适用的 setter 方法。仅仅让变量可访问是有问题的。