....没有定义无参数构造函数？ [复制]

Posted 2023-02-22

【中文标题】....没有定义无参数构造函数？ [复制]

【英文标题】：....does not define a no-argument constructor? [duplicate]

【发布时间】：2018-01-04 23:22:02

【问题描述】：

我收到以下错误，强制关闭一个活动（不是整个应用程序）：

com.google.firebase.database.DatabaseException: Class com.example.android.frapp.Notes does not define a no-argument constructor. If you are using ProGuard, make sure these constructors are not stripped.

它似乎表明问题出在这里：

at com.example.android.frapp.NoteTakerActivity$3.onDataChange(NoteTakerActivity.java:93)

下面是 Notes 类的代码：

public class Notes 

String notesId;
String notesTitle;
String notesType;

public Notes(String id, String mainNotes) 



public Notes(String notesId, String notesTitle, String notesType) 
    this.notesId = notesId;
    this.notesTitle = notesTitle;
    this.notesType = notesType;


public String getNotesId() 
    return notesId;


public String getNotesTitle() 

    return notesTitle;


public String getNotesType() 

    return notesType;

下面是 NoteTakerActivity 类的代码：

public class NoteTakerActivity extends AppCompatActivity 

public static final String NOTES_TITLE = "notestitle";
public static final String NOTES_ID = "notesid";

EditText editNoteTitle;
Button addButton;
Spinner spinnerType;

DatabaseReference databaseNotes;

ListView listViewNotes;

List<Notes> notesList;

@Override
protected void onCreate(Bundle savedInstanceState) 
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_note_taker);
    Toolbar toolbar = (Toolbar) findViewById(R.id.toolbar);
    setSupportActionBar(toolbar);

    databaseNotes = FirebaseDatabase.getInstance().getReference("notes");

    editNoteTitle = (EditText) findViewById(R.id.noteTitleTxt);
    addButton = (Button) findViewById(R.id.addNoteBtn);
    spinnerType = (Spinner) findViewById(R.id.spinnerNoteType);

    listViewNotes = (ListView) findViewById(R.id.listViewNotes);

    notesList = new ArrayList<>();

    addButton.setOnClickListener(new View.OnClickListener() 
        @Override
        public void onClick(View view) 
            addNote();
        
    );

    // for when a note title is clicked to open the next activity to write extensive notes
    listViewNotes.setOnItemClickListener(new AdapterView.OnItemClickListener() 
        @Override
        public void onItemClick(AdapterView<?> adapterView, View view, int i, long l) 
            Notes notes = notesList.get(i);
            Intent intent = new Intent(getApplicationContext(), AddMainNotesActivity.class);

            intent.putExtra(NOTES_ID, notes.getNotesId());
            intent.putExtra(NOTES_TITLE, notes.getNotesTitle());

            startActivity(intent);
        
    );



@Override
protected void onStart() 
    super.onStart();

    // every time this method is executed it will fecth all the notes from the database and populate the list view
    databaseNotes.addValueEventListener(new ValueEventListener() 
        @Override
        // executed any time something is changed in the database
        public void onDataChange(DataSnapshot dataSnapshot) 

            notesList.clear();

            for (DataSnapshot notesSnapshot: dataSnapshot.getChildren()) 
                Notes notes = notesSnapshot.getValue(Notes.class); // this is line 93

                notesList.add(notes);
            

            NotesList adapter = new NotesList(NoteTakerActivity.this, notesList);
            listViewNotes.setAdapter(adapter);
        

        @Override
        // executed if there is some kind of error
        public void onCancelled(DatabaseError databaseError) 

        
    );


private void addNote() 
    String title = editNoteTitle.getText().toString().trim();
    String type = spinnerType.getSelectedItem().toString();

    if (!TextUtils.isEmpty(title)) 

        String id = databaseNotes.push().getKey(); // id being created is unique every time

        Notes notes = new Notes(id, title, type);

        databaseNotes.child(id).setValue(notes); // to send data to database

        Toast.makeText(this, "Title added", Toast.LENGTH_SHORT).show();

        startActivity(getIntent());

     else 
        Toast.makeText(this, "You must give the note a title", Toast.LENGTH_SHORT).show();
    

我很难看到错误。我觉得我已经正确构建了所有内容，无法正确理解或解释它所指向的错误。 谁能帮帮我？

【问题讨论】：

在您的 Notes.java 文件中，我认为您需要添加一个无构造函数部分 public Notes()

你为什么认为你做得对？

【参考方案1】：

除了@omodemilade-bamgbose 的回答，你还需要了解 Java 构造函数的基本原理。

Java 基本上默认提供默认构造函数（不带参数），即使你没有定义它。

但是，在您的情况下，您已经定义了另一个带有一些参数的构造函数。这会导致您让 JVM 了解您没有使用默认构造函数。

【参考方案2】：

错误信息意味着您需要为Notes class 定义一个不带参数的构造函数，如下所示：

public Notes() 

所以你会：

public class Notes 

 String notesId;
 String notesTitle;
 String notesType;

 public Notes(String id, String mainNotes) 

 

 //a blank constructor//
 public Notes() 

 

 public Notes(String notesId, String notesTitle, String notesType) 
   this.notesId = notesId;
   this.notesTitle = notesTitle;
   this.notesType = notesType;
 

 public String getNotesId() 
   return notesId;
 

 public String getNotesTitle() 

  return notesTitle;
 

 public String getNotesType() 

  return notesType;