在我的聊天应用程序中，我有以下错误java.lang.NullPointerException

我正在制作聊天应用程序。。当我点击任何用户开始与他聊天时，我就遇到了这个问题。但当我第一次进入聊天室并发送第一条消息时，它已被发送“在我的firebase中，我看到了”，但当我试图再次获取聊天室时，会发生此错误

    java.lang.NullPointerException: Attempt to invoke virtual method 'boolean java.lang.String.equals(java.lang.Object)' on a null object reference
    at com.example.learno.SocialApp.ChatActivity$4.onDataChange(ChatActivity.java:157)

这是我的代码

public class ChatActivity extends AppCompatActivity {
Toolbar toolbar;
RecyclerView recyclerView;
ImageView profileTv;
TextView nameTv, userStatusTv;
EditText messageEt;
ImageButton sendBtn;
FirebaseAuth firebaseAuth;
FirebaseDatabase firebaseDatabase;
DatabaseReference usersDbRef;

ValueEventListener seenListener;
DatabaseReference userRefForSeen;

List<Modelchat> chatList;
AdapterChat adapterChat;

String hisUid;
String myUid;
String hisImage;

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_chat);
    toolbar = findViewById(R.id.toolbar);
    setSupportActionBar(toolbar);
    toolbar.setTitle("");
    recyclerView = findViewById(R.id.chat_RecylerView);
    profileTv = findViewById(R.id.profileTv);
    nameTv = findViewById(R.id.nameTv);
    userStatusTv = findViewById(R.id.userStatusTv);
    messageEt = findViewById(R.id.massageEt);
    sendBtn = findViewById(R.id.sendBtn);

    LinearLayoutManager linearLayoutManager = new LinearLayoutManager(this);
    linearLayoutManager.setStackFromEnd(true);
    recyclerView.setHasFixedSize(true);
    recyclerView.setLayoutManager(linearLayoutManager);

    Intent intent = getIntent();
    hisUid = intent.getStringExtra("hisUid");

    firebaseAuth = FirebaseAuth.getInstance();

    firebaseDatabase = FirebaseDatabase.getInstance();

    usersDbRef = firebaseDatabase.getReference("Users");

    Query userQuery = usersDbRef.orderByChild("uid").equalTo(hisUid);

    userQuery.addValueEventListener(new ValueEventListener() {
        @Override
        public void onDataChange(@NonNull DataSnapshot dataSnapshot) {
            for (DataSnapshot ds : dataSnapshot.getChildren()) {
                String name = "" + ds.child("name").getValue();
                hisImage = "" + ds.child("image").getValue();

                nameTv.setText(name);
                try {
                    Picasso.get().load(hisImage).placeholder(R.drawable.ic_defult_img_face).into(profileTv);
                } catch (Exception e) {
                    Picasso.get().load(R.drawable.ic_defult_img_face).into(profileTv);
                }
            }
        }

        @Override
        public void onCancelled(@NonNull DatabaseError databaseError) {

        }
    });

    sendBtn.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View v) {
            String message = messageEt.getText().toString().trim();
            if (TextUtils.isEmpty(message)) {
                Toast.makeText(ChatActivity.this, "Cannot send empty message...", Toast.LENGTH_SHORT).show();
            } else {
                sendMessage(message);
            }
        }
    });
    readMessages();
    seenMessages();
}

private void seenMessages() {
    userRefForSeen = FirebaseDatabase.getInstance().getReference("Chats");
    seenListener = userRefForSeen.addValueEventListener(new ValueEventListener() {
        @Override
        public void onDataChange(@NonNull DataSnapshot dataSnapshot) {
            for (DataSnapshot ds: dataSnapshot.getChildren()) {
                Modelchat chat = ds.getValue(Modelchat.class);
                if (chat.getReceiver().equals(myUid) && chat.getSender().equals(hisUid)) {
                    HashMap<String, Object> hasSeenHashMap = new HashMap<>();
                    hasSeenHashMap.put("isSeen", true);
                    ds.getRef().updateChildren(hasSeenHashMap);
                }
            }
        }

        @Override
        public void onCancelled(@NonNull DatabaseError databaseError) {

        }
    });
}

private void readMessages() {
    chatList = new ArrayList<>();
    DatabaseReference dbRef = FirebaseDatabase.getInstance().getReference("Chats");
    dbRef.addValueEventListener(new ValueEventListener() {
        @Override
        public void onDataChange(@NonNull DataSnapshot dataSnapshot) {
            chatList.clear();
            for (DataSnapshot ds : dataSnapshot.getChildren()) {
                Modelchat chat = ds.getValue(Modelchat.class);
                if (chat.getReceiver().equals(myUid) && chat.getSender().equals(hisUid) ||
                        chat.getReceiver().equals(hisUid) && chat.getSender().equals(myUid)) {
                    chatList.add(chat);
                }
                adapterChat = new

                        AdapterChat(ChatActivity.this, chatList, hisImage);
                adapterChat.notifyDataSetChanged();
                recyclerView.setAdapter(adapterChat);
            }
        }


        @Override
        public void onCancelled(@NonNull DatabaseError databaseError) {

        }
    });
}


private void sendMessage(String message) {


    DatabaseReference databaseReference = FirebaseDatabase.getInstance().getReference();

    String timeStamp = String.valueOf(System.currentTimeMillis());

    HashMap<String, Object> hashMap = new HashMap<>();
    hashMap.put("Sender", myUid);
    hashMap.put("receiver", hisUid);
    hashMap.put("message", message);
    hashMap.put("timeStamp", timeStamp);
    hashMap.put("isSeen", false);

    databaseReference.child("Chats").push().setValue(hashMap);

    messageEt.setText("");
}

private void checkuserstatus() {
    FirebaseUser user = firebaseAuth.getCurrentUser();
    if (user != null) {
        myUid = user.getUid();
    } else {
        startActivity(new Intent(this, Register.class));
        finish();
    }
}

@Override
protected void onStart() {
    checkuserstatus();
    super.onStart();
}

@Override
protected void onPause() {
    super.onPause();
    userRefForSeen.removeEventListener(seenListener);
}

@Override
public boolean onCreateOptionsMenu(Menu menu) {
    getMenuInflater().inflate(R.menu.menu_logout, menu);
    menu.findItem(R.id.actiob_search).setVisible(false);
    return super.onCreateOptionsMenu(menu);
}

@Override
public boolean onOptionsItemSelected(@NonNull MenuItem item) {
    int id = item.getItemId();
    if (id == R.id.logout) ;
    {
        firebaseAuth.signOut();
        checkuserstatus();
    }

    return super.onOptionsItemSelected(item);
}

因此，当我进入我的用户片段并点击任何用户开始与他聊天时，就会发生此错误。。

---

有人能帮我吗？

在你的代码

聊天中。getReceiver（）

和

聊天。getSender（）

返回

null

值，你有很多调用，比如

chat.getReceiver（）.equals（myUid）

和

聊天。getSender（）.equals（hisUid）

，所有这些调用都可以生成

NullPointerException

因此，请更正

getReceiver（）

和

getSender（）

代码，并确保它们在调用

equals

之前不会返回

null

值或检查

null

或者将

chat.getSender（）.equals（hisUid）

更改为

hisUid.equals（chat.getSender（））

---

和

chat.getReceiver（）.equals（myUid）

到

myUid.equals（chat.getReceiver（））

如果

hisUid

和

myUid

始终不为

null

请输入较少的代码，显示引发错误的行。

public class Modelchat {
String message,receiver,sender,timestamp;
boolean isSeen;

public Modelchat(){}

public Modelchat(String message, String receiver, String sender, String timestamp, boolean isSeen) {
    this.message = message;
    this.receiver = receiver;
    this.sender = sender;
    this.timestamp = timestamp;
    this.isSeen = isSeen;
}

public String getMessage() {
    return message;
}

public void setMessage(String message) {
    this.message = message;
}

public String getReceiver() {
    return receiver;
}

public void setReceiver(String receiver) {
    this.receiver = receiver;
}

public String getSender() {
    return sender;
}

public void setSender(String sender) {
    this.sender = sender;
}

public String getTimestamp() {
    return timestamp;
}

public void setTimestamp(String timestamp) {
    this.timestamp = timestamp;
}

public boolean isSeen() {
    return isSeen;
}

public void setSeen(boolean seen) {
    isSeen = seen;
}