字符串空对象引用 [英] String null object reference
问题描述
我想在我的聊天应用程序中向我显示所有消息，但是每次我打开活动时，该应用程序都会崩溃.如果我删除 readMessage()上的if方法，它将向我显示消息，但是它会在每次用户聊天中向我显示针对不同用户的每条消息.我认为这不是最好的解决方案.我可以修改什么? 我的数据库看起来像此
I want in my chat app to show me all the messages but every time i open the activity the app crashes.If i delete the if method on readMessage() it will show me the messages but it'll show me every message that i have for a different user in every user chat. I don't think it's the best solution. What I can modify ? My database looks like this
我试图断言聊天！=空.
I tried to asser chat!=null.
这是我的MessageAcitvity，其中有RecyclerView:
Here is My MessageAcitvity where i have the RecyclerView :
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.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) {
}
} );
}
}
我的MessageAdapter类:
My MessageAdapter class :
public class MessageAdapter extends RecyclerView.Adapter<MessageAdapter.ViewHolder> {
public static final int MSG_TYPE_LEFT=0;
public static final int MSH_TYPE_RIGHT=1;
private Context mContext;
private List<Chat> mChat;
FirebaseUser fuser;
public MessageAdapter(Context mContext,List<Chat> mChat){
this.mChat=mChat;
this.mContext=mContext;
}
@NonNull
@Override
public ViewHolder onCreateViewHolder (@NonNull ViewGroup parent, int viewType) {
if(viewType==MSH_TYPE_RIGHT){
View view= LayoutInflater.from ( mContext ).inflate ( R.layout.chat_item_right ,parent,false);
return new MessageAdapter.ViewHolder ( view );
}else{
View view= LayoutInflater.from ( mContext ).inflate ( R.layout.chat_item_left ,parent,false);
return new MessageAdapter.ViewHolder ( view );
}
}
@Override
public void onBindViewHolder (@NonNull ViewHolder holder, int position) {
Chat chat=mChat.get ( position );
holder.show_message.setText ( chat.getMessage () );
}
@Override
public int getItemCount () {
return mChat.size ();
}
public class ViewHolder extends RecyclerView.ViewHolder{
public TextView show_message;
public ViewHolder(View itemView){
super(itemView);
show_message=itemView.findViewById ( R.id.show_message );
}
}
@Override
public int getItemViewType (int position) {
fuser= FirebaseAuth.getInstance ().getCurrentUser ();
if(mChat.get ( position ).getSender ().equals ( fuser.getUid () )){
return MSH_TYPE_RIGHT;
}else
{
return MSG_TYPE_LEFT;
}
}
}
聊天类:
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: 26180
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:134)
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)
我希望向我展示所有消息.
I expect to show me all the messages.
推荐答案
错误消息显示:
试图在空对象引用上调用虚拟方法
boolean java.lang.String.equals(java.lang.Object)
所以您在某处调用something.equals(...)，而something是null.堆栈跟踪告诉您问题所在:
So you're calling something.equals(...) somewhere, and something is null. The stack trace tells you that the problem is on:
MessageActivity $ 3.onDataChange(MessageActivity.java:134)
MessageActivity$3.onDataChange(MessageActivity.java:134)
因此在onDataChange方法内的MessageActivity.java的第134行.虽然我无法确定第134行是什么，但是您共享的代码中只有一个onDataChange方法，并且那里只有一个对equals的调用:
So on line 134 of MessageActivity.java, inside the onDataChange method. While I can't be certain what line 134 is, there is only one onDataChange method in the code you shared, and only one call to equals in there:
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 );
}
所以问题似乎出在这一行:
So the problem seems to come from this line:
if(!fuser.getUid().equals ( user.getFirstName () )){
具体来说:fuser.getUid()似乎返回null，这意味着您正在调用null.equals(...)，这就是错误消息所显示的内容.
Specifically: fuser.getUid() seems to return null, which means you're calling null.equals(...) and that's what the error message says.
FirebaseAuth.getInstance().getCurrentUser()返回用户对象，然后getUid()返回null的情况并不常见，但这确实是您正在发生的事情.
It's fairly uncommon for FirebaseAuth.getInstance().getCurrentUser() to return a user object, and then for getUid() to return null, but that does seem to be what's happening for you.
这篇关于字符串空对象引用的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！
