每次我开始上一个活动时Android应用程序崩溃

Question

I have 2 activities let's say activity Alpha and activity Beta.

in my AlphaActivity.class i have the code below:

...
@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);

    setContentView(R.layout.activity_profile);

    cDatabase.addValueEventListener(new ValueEventListener() {
        public static final String TAG = "XD";

        @Override
        public void onDataChange(@NonNull DataSnapshot dataSnapshot) {
            for(DataSnapshot snapshot : dataSnapshot.getChildren()) {
                String u = snapshot.child("username").getValue().toString(),
                        p = snapshot.child("phone").getValue().toString(),
                        ad = snapshot.child("address").getValue().toString(),
                        f = snapshot.child("floor").getValue().toString(),
                        ns = snapshot.child("notes").getValue().toString();
                Address a = new Address(u, p, ad, f, ns);
                infoArray.add(a.address);
                nameArray.add(a.name);
                phoneArray.add(a.phone);
                writeListView();
            }
        }

        @Override
        public void onCancelled(@NonNull DatabaseError databaseError) {
            Toast.makeText(ProfileActivity.this, "Action canceled!", Toast.LENGTH_SHORT).show();
        }
    });
}
...

Whenever i run that activity works like a charm. I have a button in this one where i start the Beta activity onClick like so:

public void betaMethod(View view) {
    finish();
    Intent intent = new Intent(this, BetaActivity.class);
    startActivity(intent);
}

In BetaActivity.class i do something and then again i have a button where onClick acts like below:

public void addToDatabase() {
    ...

    finish();

    Intent intent = new Intent(this, AlphaActivity.class);
    startActivity(intent);
}

When that codes executes, my app crashes and the error i get is on the AlphaActivity.class down there where i have p = snapshot.child("phone").getValue().toString() . I bet it has to do with the firebase method onDataChange but i can't figure it out. Any suggestion please ?

The error i get is:

06-04 16:59:16.885 13978-13978/com.example.johng.assosfood E/AndroidRuntime: FATAL EXCEPTION: main
Process: com.example.johng.assosfood, PID: 13978
java.lang.NullPointerException
    at com.example.johng.assosfood.ProfileActivity$1.onDataChange(ProfileActivity.java:48)
    at com.google.android.gms.internal.firebase_database.zzfc.zza(Unknown Source)
    at com.google.android.gms.internal.firebase_database.zzgx.zzdr(Unknown Source)
    at com.google.android.gms.internal.firebase_database.zzhd.run(Unknown Source)
    at android.os.Handler.handleCallback(Handler.java:808)
    at android.os.Handler.dispatchMessage(Handler.java:103)
    at android.os.Looper.loop(Looper.java:193)
    at android.app.ActivityThread.main(ActivityThread.java:5333)
    at java.lang.reflect.Method.invokeNative(Native Method)
    at java.lang.reflect.Method.invoke(Method.java:515)
    at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:829)
    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:645)
    at dalvik.system.NativeStart.main(Native Method)

where ProfileActivity.java = AlphaActivity.java

Answer 1

When you starting a new activtiy your code is

public void betaMethod(View view) {
    finish(); //here
    Intent intent = new Intent(this, BetaActivity.class);
    startActivity(intent);
}

Why you finish your activity when launching intent it means when you came back it can not find your last activity and in that case it can not load your last activity because it finished before launching intent . and if you want to remove backstack from activity then use

intent.addFlags(Intent.FLAG_ACTIVITY_CLEAR_TOP | Intent.FLAG_ACTIVITY_NEW_TASK); 

so remove finish from here and check your code works fine.

Let me know if it works for you.

Answer 2

I don't see any line looking like cDatabase = FireabseDatabase.getInstance().getReference();

phone child must contain value in numbers and you're retrieving it as a String

Instead of p = snapshot.child("phone").getValue().toString() .

Always use p = snapshot.child("phone").getValue(String.class); or

p = String.valueOf(snapshot.child("phone").getValue(String.class)); do this for all snapshots.

Also in public void betaMethod(View view) use finish(); after startActivity() it's just good practice

Answer 3

Actually there is nothing wrong with calling finish() ! in fact if you go back and forth too many times without calling finish() this can cause an OOM exception as the stack fills up with instances of each activity. I think you just need to call the finish() method after startActivity() not before.

Answer 4

When you store long numbers in Firebase Database, it probably stores it in Float data type. So maybe to get value of "phone" you should try

String.valueOf((Float)snapshot.child("phone").getValue());