我想通过MessageActivity显示消息，但是应用程序不断崩溃，即使我更改代码，调试器也会向我显示相同的错误。 我的数据库如下所示： ！[1] https://imgur.com/a/SWAxkWV
我尝试断言聊天！=空。
我从数据库中读取并在其中放置内容的MessageActivity。
public class MessageActivity extends AppCompatActivity {
TextView username;
FirebaseUser fuser;
DatabaseReference reference;
Intent intent;
ImageButton btn_send;
EditText text_send;
MessageAdapter messageAdapter;
List<Chat> mchat;
RecyclerView recyclerView;
@Override
protected void onCreate (Bundle savedInstanceState) {
super.onCreate ( savedInstanceState );
setContentView ( R.layout.activity_message );
btn_send=findViewById ( R.id.btn_send );
text_send=findViewById ( R.id.editText );
recyclerView=findViewById ( R.id.recycler_view );
recyclerView.setHasFixedSize ( true );
LinearLayoutManager linearLayoutManager=new LinearLayoutManager ( getApplicationContext () );
linearLayoutManager.setStackFromEnd ( true );
recyclerView.setLayoutManager ( linearLayoutManager );
username=findViewById ( R.id.username6 );
intent=getIntent ();
final String userid=intent.getStringExtra ( "FIRST NAME " );
fuser= FirebaseAuth.getInstance ().getCurrentUser ();
reference= FirebaseDatabase.getInstance ().getReference ("users");
reference.addValueEventListener ( new ValueEventListener () {
@Override
public void onDataChange (@NonNull DataSnapshot dataSnapshot) {
for(DataSnapshot snapshot:dataSnapshot.getChildren ()){
User user=snapshot.getValue (User.class);
assert user!=null;
assert fuser!=null;
if(!fuser.getUid ().equals ( user.getFirstName () )){
username.setText ( user.getFirstName () );
}
}
readMessages ( fuser.getUid (),userid );
}
@Override
public void onCancelled (@NonNull DatabaseError databaseError) {
}
} );
btn_send.setOnClickListener ( new View.OnClickListener () {
@Override
public void onClick (View view) {
String msg=text_send.getText ().toString ();
if(!msg.equals ( "" )){
sendMessage ( fuser.getUid (),userid,msg );
} else{
Toast.makeText ( MessageActivity.this,"You can't send empty",Toast.LENGTH_SHORT ).show ();
}
text_send.setText ( "" );
}
} );
}
private void sendMessage(String sender,String receiver,String message)
{
DatabaseReference reference=FirebaseDatabase.getInstance ().getReference ();
HashMap<String,Object> hashMap=new HashMap<> ( );
hashMap.put ( "sender",sender );
hashMap.put ( "receiver",receiver );
hashMap.put ( "message",message );
reference.child ( "Chats" ).push ().setValue ( hashMap );
}
private void readMessages(final String myid, final String userId)
{
mchat=new ArrayList<> ( );
reference=FirebaseDatabase.getInstance ().getReference ("Chats");
reference.addValueEventListener ( new ValueEventListener () {
@Override
public void onDataChange (@NonNull DataSnapshot dataSnapshot) {
mchat.clear ();
for(DataSnapshot snapshot:dataSnapshot.getChildren ()){
Chat chat=snapshot.getValue (Chat.class);
assert chat != null;
if(chat.equals ( chat.getReceiver ())&& !fuser.getUid ().equals ( chat.getSender ()) || !fuser.getUid ().equals ( chat.getSender () )&&!fuser.getUid ().equals ( chat.getReceiver () ) ) {
mchat.add ( chat );
}
messageAdapter=new MessageAdapter ( MessageActivity.this,mchat );
recyclerView.setAdapter ( messageAdapter );
}
}
@Override
public void onCancelled (@NonNull DatabaseError databaseError) {
}
} );
}
}
出现错误的地方：
private void readMessages(final String myid, final String userId)
{
mchat=new ArrayList<> ( );
reference=FirebaseDatabase.getInstance ().getReference ("Chats");
reference.addValueEventListener ( new ValueEventListener () {
@Override
public void onDataChange (@NonNull DataSnapshot dataSnapshot) {
mchat.clear ();
for(DataSnapshot snapshot:dataSnapshot.getChildren ()){
Chat chat=snapshot.getValue (Chat.class);
assert chat != null;
if(chat.getReceiver ().equals ( myid )&& chat.getSender ().equals ( userId ) || chat.getReceiver ().equals ( userId )&& chat.getSender ().equals ( myid )) {
mchat.add ( chat );
}
messageAdapter=new MessageAdapter ( MessageActivity.this,mchat );
recyclerView.setAdapter ( messageAdapter );
}
}
@Override
public void onCancelled (@NonNull DatabaseError databaseError) {
}
} );
}
聊天活动：
public class Chat {
private String sender;
private String receiver;
private String message;
public Chat (String sender, String receiver, String message) {
this.sender = sender;
this.receiver = receiver;
this.message = message;
}
public Chat(){
}
public String getSender () {
return sender;
}
public void setSender (String sender) {
this.sender = sender;
}
public String getReceiver () {
return receiver;
}
public void setReceiver (String receiver) {
this.receiver = receiver;
}
public String getMessage () {
return message;
}
public void setMessage (String message) {
this.message = message;
}
}
调试器向我显示：
E/AndroidRuntime: FATAL EXCEPTION: main
Process: com.example.sportsbuddy, PID: 8531
java.lang.NullPointerException: Attempt to invoke virtual method 'boolean java.lang.String.equals(java.lang.Object)' on a null object reference
at com.example.sportsbuddy.MessageActivity$3.onDataChange(MessageActivity.java:135)
at com.google.firebase.database.core.ValueEventRegistration.fireEvent(com.google.firebase:firebase-database@@17.0.0:75)
at com.google.firebase.database.core.view.DataEvent.fire(com.google.firebase:firebase-database@@17.0.0:63)
at com.google.firebase.database.core.view.EventRaiser$1.run(com.google.firebase:firebase-database@@17.0.0:55)
at android.os.Handler.handleCallback(Handler.java:873)
at android.os.Handler.dispatchMessage(Handler.java:99)
at android.os.Looper.loop(Looper.java:214)
at android.app.ActivityThread.main(ActivityThread.java:6986)
at java.lang.reflect.Method.invoke(Native Method)
at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:493)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1445)
我希望在数据库中显示来自接收者和发送者的消息。