在 Room 数据库中输入数据时，出现错误

Question

我正在使用 Room 数据库。在视频指南上做了。 在视频中，活动布局包含：一个数据库对象列表，一个允许您添加和更改记录的添加按钮。 我需要将此代码分成几部分。 我已经可以做一个单独的列表，单独更改记录，但是我不能做数据。

第一个代码显示有效的原始代码，第二个显示我的。

第一个代码

            @Override
        public void onItemClick(Note note) {
            Intent intent = new Intent(MainActivity.this, AddEditNoteActivity.class);
            intent.putExtra(AddEditNoteActivity.EXTRA_ID, note.getId());
            intent.putExtra(AddEditNoteActivity.EXTRA_TITLE, note.getTitle());
            intent.putExtra(AddEditNoteActivity.EXTRA_DESCRIPTION, note.getDescription());
            intent.putExtra(AddEditNoteActivity.EXTRA_PRIORITY, note.getPriority());
            startActivityForResult(intent, EDIT_NOTE_REQUEST);
        }
    });
}
@Override
protected void onActivityResult(int requestCode, int resultCode, Intent data) {
    super.onActivityResult(requestCode, resultCode, data);
    if (requestCode == ADD_NOTE_REQUEST && resultCode == RESULT_OK) {
        String title = data.getStringExtra(AddEditNoteActivity.EXTRA_TITLE);
        String description = data.getStringExtra(AddEditNoteActivity.EXTRA_DESCRIPTION);
        int priority = data.getIntExtra(AddEditNoteActivity.EXTRA_PRIORITY, 1);
        Note note = new Note(title, description, priority);
        noteViewModel.insert(note);
        Toast.makeText(this, "Note saved", Toast.LENGTH_SHORT).show();
    } else if (requestCode == EDIT_NOTE_REQUEST && resultCode == RESULT_OK) {
        int id = data.getIntExtra(AddEditNoteActivity.EXTRA_ID, -1);
        if (id == -1) {
            Toast.makeText(this, "Note can't be updated", Toast.LENGTH_SHORT).show();
            return;
        }
        String title = data.getStringExtra(AddEditNoteActivity.EXTRA_TITLE);
        String description = data.getStringExtra(AddEditNoteActivity.EXTRA_DESCRIPTION);
        int priority = data.getIntExtra(AddEditNoteActivity.EXTRA_PRIORITY, 1);
        Note note = new Note(title, description, priority);
        note.setId(id);
        noteViewModel.update(note);
        Toast.makeText(this, "Note updated", Toast.LENGTH_SHORT).show();
    } else {
        Toast.makeText(this, "Note not saved", Toast.LENGTH_SHORT).show();
    }
}

第二个代码

import androidx.appcompat.app.AppCompatActivity;

import android.content.Intent;
import android.os.Bundle;
import android.view.View;
import android.widget.Button;
import android.widget.EditText;
import android.widget.NumberPicker;
import android.widget.Toast;

public class Add_item extends AppCompatActivity implements View.OnClickListener {
public static NoteViewModel noteViewModel1;
private EditText editTextTitle;
private EditText editTextDescription;
private NumberPicker numberPickerPriority;
Button save;
@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_add_item);
    save = findViewById(R.id.Save);
    save.setOnClickListener(this);
    editTextTitle = findViewById(R.id.edit_text_title);
    editTextDescription = findViewById(R.id.edit_text_description);
    numberPickerPriority = findViewById(R.id.number_picker_priority);
    numberPickerPriority.setMinValue(1);
    numberPickerPriority.setMaxValue(10);
}

private void saveNotes() {
    String title = editTextTitle.getText().toString();
    String description = editTextDescription.getText().toString();
    int priority = numberPickerPriority.getValue();
    if (title.trim().isEmpty() || description.trim().isEmpty()) {
        Toast.makeText(this, "Please insert a title and description", Toast.LENGTH_SHORT).show();
        return;
    }
    Note note = new Note(title, description, priority);
    noteViewModel1.insert(note);
    Toast.makeText(this, "Note saved", Toast.LENGTH_SHORT).show();

    finish();
}

@Override
public void onClick(View v) {
    switch (v.getId()){
        case R.id.Save:
            saveNotes();
    }

}
}

错误

05-19 21:01:30.440 26571-26571/com.example.recview E/AndroidRuntime: FATAL EXCEPTION: main
Process: com.example.recview, PID: 26571
java.lang.NullPointerException: Attempt to invoke virtual method 'void com.example.recview.NoteViewModel.insert(com.example.recview.Note)' on a null object reference
    at com.example.recview.Add_item.saveNotes(Add_item.java:41)
    at com.example.recview.Add_item.onClick(Add_item.java:51)
    at android.view.View.performClick(View.java:5204)
    at android.view.View$PerformClick.run(View.java:21153)
    at android.os.Handler.handleCallback(Handler.java:739)
    at android.os.Handler.dispatchMessage(Handler.java:95)
    at android.os.Looper.loop(Looper.java:148)
    at android.app.ActivityThread.main(ActivityThread.java:5480)
    at java.lang.reflect.Method.invoke(Native Method)
    at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:726)
    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:616)

Answer 1

在你的 onCreate 方法中初始化ViewModel 像这样：

 NoteViewModel model = new ViewModelProvider(this).get(NoteViewModel.class);