FirebaseDatabase：Model缺少没有参数的构造函数

Question

所以我添加了这样的Firebase数据：

我调用了ListView中的数据：

private void displayStudentChatMessages() {
    listofMessages = (ListView) findViewById(R.id.list_msg);
    DatabaseReference ref = FirebaseDatabase.getInstance().getReference(String.valueOf(generateChatID));

    adapter = new FirebaseListAdapter<StudentMessageModel>(this, StudentMessageModel.class, R.layout.item_chat_right, ref) {
        @Override
        protected void populateView(View v, StudentMessageModel model, int position) {
            TextView messageUsername = (TextView) findViewById(R.id.message_user_sender);
            TextView messageText = (TextView) findViewById(R.id.message_user);
            TextView messageTime = (TextView) findViewById(R.id.message_usertime);

            messageUsername.setText(model.getStudentName());
            messageText.setText(model.getStudentMessage());
            messageTime.setText(DateFormat.format("yyyy-MM-dd (HH:mm:ss)", model.getStudentTime()));
        }
    };

    listofMessages.setAdapter(adapter);
    listofMessages.setTranscriptMode(ListView.TRANSCRIPT_MODE_ALWAYS_SCROLL);
}

但是我得到了错误：

  

com.google.firebase.database.DatabaseException：类com.androidports.autosiskola.StudentMessageModel缺少没有参数的构造函数

我需要那些根目录的孩子是生成名称的数据（ test-testteacher-student-teacher ）。

以下是我添加构造函数的方法：

public class StudentMessageModel {

private String studentUsername;
private String studentName;
private String studentMessage;
private String generatedChatID;
private long studentTime;

public StudentMessageModel(String studentUsername, String studentName, String studentMessage, String generatedChatID) {
    this.studentUsername = studentUsername;
    this.studentName = studentName;
    this.studentMessage = studentMessage;
    this.generatedChatID = generatedChatID;

    studentTime = new Date().getTime();
}

和getter and setters。

Answer 1

我认为错误是不言自明的。您必须具有不带参数的构造函数。像这样：

public class StudentMessageModel {

private String studentUsername;
private String studentName;
private String studentMessage;
private String generatedChatID;
private long studentTime;

public StudentMessageModel(String studentUsername, String studentName, String studentMessage, String generatedChatID) {
    this.studentUsername = studentUsername;
    this.studentName = studentName;
    this.studentMessage = studentMessage;
    this.generatedChatID = generatedChatID;

    studentTime = new Date().getTime();
}

public StudentMessageModel(){}

Answer 2

正如错误消息所示，您必须拥有不带参数的公共构造函数。从数据库重新创建对象时使用此方法。

此外，您必须将字段标记为公共字段，或者为它们提供getter和setter。这样Firebase就可以知道从数据库中的JSON属性填充哪些字段。

所以：

public class StudentMessageModel {

    public String studentUsername;
    public String studentName;
    public String studentMessage;
    public String generatedChatID;
    public long studentTime;

    public StudentMessageModel(String studentUsername, String studentName, String studentMessage, String generatedChatID) {
        this.studentUsername = studentUsername;
        this.studentName = studentName;
        this.studentMessage = studentMessage;
        this.generatedChatID = generatedChatID;

        studentTime = new Date().getTime();
    }
    public StudentMessageModel() {
        // public default constructor, which Firebase uses
    }

Answer 3

将您的字段公开添加，也许获取器和设置器会有所帮助！