無法將Firebase放入arraylist的自定義對象中

Question

運行代碼時，我收到NullPointerException ，因為我的Firestore文檔不會下載到我的對象中。 我不確定為什么，但是我環顧了對象getter和setter的格式，我認為我做對了，但是錯誤消息仍然顯示數據庫中的字段沒有getter或setter。

SubjectGroupPage.java

db.collection("messagesIT")
                /*.orderBy("TimeSent", Query.Direction.DESCENDING)*/
                .get()
                .addOnCompleteListener(new OnCompleteListener<QuerySnapshot>() {
                    @Override
                    public void onComplete(@NonNull Task<QuerySnapshot> task) {
                        if (task.isSuccessful()) {

                            ArrayList<MessageObj> messages = new ArrayList<>();
                            for (QueryDocumentSnapshot document : task.getResult()) {
                                MessageObj mess = document.toObject(MessageObj.class);
                                messages.add(mess);
                                Log.d(TAG, "" + mess);
                                showToast("b");
                            }

                            //reading and displaying messages
                            for (int i = 0; i < messages.size(); i++) {
                                showToast("d");
                                TableLayout tl = findViewById(R.id.TableLayoutSubjectGroupPage);
                                TableRow trMessage = new TableRow(SubjectGroupPage.this);
                                TableRow trSender = new TableRow(SubjectGroupPage.this);
                                TextView txtSender = new TextView(SubjectGroupPage.this);
                                TextView txtMessage = new TextView(SubjectGroupPage.this);
                                txtMessage.setText(messages.get(i).getMessage());
                                txtSender.setText(messages.get(i).getSender());
                                txtMessage.setTextSize(20);
                                txtMessage.setTextColor(0);
                                txtMessage.setWidth(0);
                                if (messages.get(i).getSender().equals(currentUser.getUid())) {
                                    txtMessage.setTextAlignment(View.TEXT_ALIGNMENT_TEXT_END);
                                    txtSender.setTextAlignment(View.TEXT_ALIGNMENT_TEXT_END);
                                } else {
                                    txtMessage.setTextAlignment(View.TEXT_ALIGNMENT_TEXT_START);
                                    txtSender.setTextAlignment(View.TEXT_ALIGNMENT_TEXT_START);
                                }
                                LinearLayout.LayoutParams param = new LinearLayout.LayoutParams(0, TableRow.LayoutParams.MATCH_PARENT, 1);
                                txtMessage.setLayoutParams(param);

                                //creating message display
                                trSender.addView(txtSender);
                                trMessage.addView(txtMessage);
                                tl.addView(txtSender);
                                tl.addView(trMessage);
                            }


                        } else {
                            Log.d(TAG, "Error getting documents: ", task.getException());
                        }
                    }
                }).addOnFailureListener(new OnFailureListener() {
            @Override
            public void onFailure(@NonNull Exception e) {

                showToast("There has been an error, please try again later.");
                Log.d(TAG, "Error: " + e);

            }
        });

LogChat

W/Firestore: (0.6.6-dev) [CustomClassMapper]: No setter/field for Sender found on class oxley.it.collectivelearning.MessageObj
W/Firestore: (0.6.6-dev) [CustomClassMapper]: No setter/field for Timesent found on class oxley.it.collectivelearning.MessageObj (fields/setters are case sensitive!)
(0.6.6-dev) [CustomClassMapper]: No setter/field for Message found on class oxley.it.collectivelearning.MessageObj
D/SubjectGroupPage.java: oxley.it.collectivelearning.MessageObj@e82129e
W/Firestore: (0.6.6-dev) [CustomClassMapper]: No setter/field for Sender found on class oxley.it.collectivelearning.MessageObj
W/Firestore: (0.6.6-dev) [CustomClassMapper]: No setter/field for TimeSent found on class oxley.it.collectivelearning.MessageObj (fields/setters are case sensitive!)
W/Firestore: (0.6.6-dev) [CustomClassMapper]: No setter/field for Timesent found on class oxley.it.collectivelearning.MessageObj (fields/setters are case sensitive!)
W/Firestore: (0.6.6-dev) [CustomClassMapper]: No setter/field for Message found on class oxley.it.collectivelearning.MessageObj
D/SubjectGroupPage.java: oxley.it.collectivelearning.MessageObj@743769b
D/AndroidRuntime: Shutting down VM
E/AndroidRuntime: FATAL EXCEPTION: main
Process: oxley.it.collectivelearning, PID: 23895
java.lang.NullPointerException: Attempt to invoke virtual method 'boolean 
java.lang.String.equals(java.lang.Object)' on a null object reference
    at 
oxley.it.collectivelearning.SubjectGroupPage$retrieveMessages$2
.onComplete(SubjectGroupPage.java:154)
    at com.google.android.gms.tasks.zzj.run(Unknown Source:4)
    at android.os.Handler.handleCallback(Handler.java:790)
    at android.os.Handler.dispatchMessage(Handler.java:99)
    at android.os.Looper.loop(Looper.java:187)
    at android.app.ActivityThread.main(ActivityThread.java:7025)
    at java.lang.reflect.Method.invoke(Native Method)
    at 
com.android.internal.os.RuntimeInit$MethodAndArgsCaller
.run(RuntimeInit.java:514)
    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:888)

MessageObj.java

public class MessageObj {

private String message;
private String sender;
private Timestamp timesent;

public MessageObj() {}

public void setMessage(String message, String sender, Timestamp timesent) {
    this.message = message;
}
@Exclude
public String getMessage() {
    return message;
}

public void setSender(String sender) {
    this.sender = sender;
}
@Exclude
public String getSender() {
    return sender;
}

public void setTimesent(Timestamp timesent) {
    this.timesent = timesent;
}

public Timestamp getTimesent() {
    return timesent;
}
}

正常運行時，我希望ArrayList能夠保存所有文檔，並能夠使用ArrayList上的對象獲取器和設置器。

我的數據庫

回答：

它只是通過將其解析為這樣的對象來工作的；

for (QueryDocumentSnapshot document : task.getResult()) {
    String message = document.getString("Message");
    String sender = document.getString("Sender");
    String timesent= document.getString("Timesent");

    MessageObj mess = new MessageObj (message, sender, timesent);
    messages.add(mess);
    Log.d(TAG, "" + mess);
    showToast("b");
}

Answer 1

之所以得到這些錯誤，是因為您在吸氣劑之前使用了@Exclude注釋。 除此之外， setMessage()方法應僅包含一個參數。 構造MessageObj類的正確方法是：

public class MessageObj {
    private String message;
    private String sender;
    private Timestamp timesent;

    public MessageObj() {}

    public MessageObj(String message, String sender, Timestamp timesent) {
        this.message = message;
        this.sender = sender;
        this.timesent = timesent;
    }

    public String getMessage() { return message; }

    public String getSender() { return sender; }

    public Timestamp getTimesent() { return timesent; }
}

使用公共字段甚至更簡單：

public class MessageObj {
    public String message;
    public String sender;
    public Timestamp timesent;

    public MessageObj() {}

    public MessageObj(String message, String sender, Timestamp timesent) {
        this.message = message;
        this.sender = sender;
        this.timesent = timesent;
    }
}