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我正在做一個來自在線教程的實時跟蹤應用程序，在這裏我使用firebase設置存在系統。DataSnapShot對象的值在getvalue上返回null（Boolean.class）
/java.lang.NullPointerException：嘗試在空對象引用調用虛方法「布爾java.lang.Boolean.booleanValue（）」
我不明白什麼，但它與崩潰編碼這個人的人錯了，它的工作很完美。
該異常正在發生在這條線：如果（dataSnapshot.getValue（Boolean.class））{
當我記錄此屏幕上的datasnapshot對象具有鍵但沒有值
幫助！
ListOnline類
//firebase
DatabaseReference onlineRef,currentUserRef,counterRef;
FirebaseRecyclerAdapter<User,ListOnlineViewHolder> adapter;
//View
RecyclerView listOnline;
RecyclerView.LayoutManager layoutManager;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_list_online);
//setting the recyclerview
listOnline = (RecyclerView)findViewById(R.id.listOnlineRecyclerview);
listOnline.setHasFixedSize(true);
layoutManager = new LinearLayoutManager(this);
listOnline.setLayoutManager(layoutManager);
//set toolbar and menu/join,logout
Toolbar toolbar = (Toolbar)findViewById(R.id.toolbarID);
toolbar.setTitle("Presence System");
setSupportActionBar(toolbar);
//firebase
onlineRef = FirebaseDatabase.getInstance().getReference().child("info/connected");
counterRef = FirebaseDatabase.getInstance().getReference("lastOnline"); //create new child name lastOnline
currentUserRef = FirebaseDatabase.getInstance().getReference().child(FirebaseAuth.getInstance().getCurrentUser().getUid());
setupSystem();
//after setup we load all users and display in recyclerview
//this is online list
updateList();
}
private void updateList() {
adapter = new FirebaseRecyclerAdapter<User, ListOnlineViewHolder>(
User.class,R.layout.user_layout,ListOnlineViewHolder.class,counterRef
) {
@Override
protected void populateViewHolder(ListOnlineViewHolder viewHolder, User model, int position) {
viewHolder.emailTextView.setText(model.getEmail());
}
};
adapter.notifyDataSetChanged();
listOnline.setAdapter(adapter);
}
private void setupSystem() {
onlineRef.addValueEventListener(new ValueEventListener() {
@Override
public void onDataChange(DataSnapshot dataSnapshot) {
if(dataSnapshot.getValue(Boolean.class)){
currentUserRef.onDisconnect().removeValue();
//set online user in list
counterRef.child(FirebaseAuth.getInstance().getCurrentUser().getUid())
.setValue(FirebaseAuth.getInstance().getCurrentUser().getEmail(),"Online");
adapter.notifyDataSetChanged();
}
}
@Override
public void onCancelled(DatabaseError databaseError) {
}
});
counterRef.addValueEventListener(new ValueEventListener() {
@Override
public void onDataChange(DataSnapshot dataSnapshot) {
for(DataSnapshot postSnapshot:dataSnapshot.getChildren()){
User user = postSnapshot.getValue(User.class);
Log.d("LOG",""+user.getEmail()+"is "+user.getStatus());
}
}
@Override
public void onCancelled(DatabaseError databaseError) {
}
});
}
@Override
public boolean onCreateOptionsMenu(Menu menu) {
MenuInflater menuInflater = getMenuInflater();
menuInflater.inflate(R.menu.main_menu,menu);
return true;
}
@Override
public boolean onOptionsItemSelected(MenuItem item) {
switch (item.getItemId()){
case R.id.action_join:
counterRef.child(FirebaseAuth.getInstance().getCurrentUser().getUid())
.setValue(FirebaseAuth.getInstance().getCurrentUser().getEmail(),"Online");
break;
case R.id.action_logout:
currentUserRef.removeValue();
}
return super.onOptionsItemSelected(item);
}
}
用戶類
public class User {
private String email,status;
public User(String email, String status) {
this.email = email;
this.status = status;
}
public User() {
}
public String getEmail() {
return email;
}
public void setEmail(String email) {
this.email = email;
}
public String getStatus() {
return status;
}
public void setStatus(String status) {
this.status = status;
}}
MainActivity
public class MainActivity extends AppCompatActivity {
Button signInButton;
private final static int LOGIN_PERMISSION = 1000;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
signInButton = (Button) findViewById(R.id.signInButton);
signInButton.setOnClickListener(new View.OnClickListener() {
@Override
public void onClick(View view) {
startActivityForResult(AuthUI.getInstance().createSignInIntentBuilder().setAllowNewEmailAccounts(true).build(),LOGIN_PERMISSION);
}
});
}
@Override
protected void onActivityResult(int requestCode, int resultCode, Intent data) {
if(requestCode == LOGIN_PERMISSION){
startNewActivity(resultCode,data);
}
}
private void startNewActivity(int resultcode, Intent data) {
if(resultcode == RESULT_OK){
Intent intent = new Intent(MainActivity.this,ListOnline.class);
startActivity(intent);
finish();
}
else{
Toast.makeText(this,"login failed!!",Toast.LENGTH_SHORT).show();
}
}}
[？什麼是空指針異常，以及如何解決呢（https://stackoverflow.com/questions/218384/what-is-a-nullpointerexception-and如何做我修復它） – Grimthorr
@Grimthorr謝謝，但沒有幫助 – Akay