无法创建ViewModel类的实例

Question

package com.example.photoalbum;

import android.app.Application;
import android.content.Context;

import androidx.annotation.NonNull;
import androidx.lifecycle.AndroidViewModel;
import androidx.lifecycle.LiveData;
import androidx.lifecycle.ViewModelProvider;
import androidx.lifecycle.ViewModelProvider;
import androidx.fragment.app.Fragment;
import androidx.lifecycle.Observer;

import java.util.List;

public class MyImagesVIewModel extends AndroidViewModel {

     private MyImagesRepository repository;
     private LiveData<List<MyImages>> imageList;
    public MyImagesVIewModel(@NonNull Application application) {
        super(application);
        repository = new MyImagesRepository(application);
        imageList = repository.getAllImages();
    }

    public void insert(MyImages myImages){
        repository.insert(myImages);
    }

    public void delete(MyImages myImages){
        repository.delete(myImages);
    }

    public void update(MyImages myImages){
        repository.update(myImages);
    }

    public LiveData<List<MyImages>> getAllImages(){
        return imageList;
    }
}

here is my repository classs

package com.example.photoalbum;

import android.app.Application;
import android.os.AsyncTask;

import androidx.lifecycle.LiveData;

import java.util.ArrayList;
import java.util.List;

public class MyImagesRepository {

    private MyImagesDAO myImagesDAO;
    private LiveData<List<MyImages>> imagesList;

    public MyImagesRepository(Application application){

        MyImagesDatabase database = MyImagesDatabase.getInstance(application);
        myImagesDAO = database.myImagesDAO();
        imagesList = myImagesDAO.getAllImages();
    }

    public void insert(MyImages myImages){

        new InsertImageAsyncTask(myImagesDAO).execute(myImages);
    }

    public void delete(MyImages myImages){

        new DeleteImageAsyncTask(myImagesDAO).execute(myImages);
    }

    public void update(MyImages myImages){

        new UpdateImageAsyncTask(myImagesDAO).execute(myImages);
    }

    public LiveData<List<MyImages>> getAllImages(){

        return imagesList;
    }

    public static class InsertImageAsyncTask extends AsyncTask<MyImages,Void,Void>{

        MyImagesDAO myImagesDAO;

        public InsertImageAsyncTask(MyImagesDAO myImagesDAO) {
            this.myImagesDAO = myImagesDAO;
        }

        @Override
        protected Void doInBackground(MyImages... myImages) {
            myImagesDAO.insert(myImages[0]);
            return null;
        }
    }

    public static class DeleteImageAsyncTask extends AsyncTask<MyImages,Void,Void>{

        MyImagesDAO myImagesDAO;

        public DeleteImageAsyncTask(MyImagesDAO myImagesDAO) {
            this.myImagesDAO = myImagesDAO;
        }

        @Override
        protected Void doInBackground(MyImages... myImages) {
            myImagesDAO.delete(myImages[0]);
            return null;
        }
    }

    public static class UpdateImageAsyncTask extends AsyncTask<MyImages,Void,Void>{

        MyImagesDAO myImagesDAO;

        public UpdateImageAsyncTask(MyImagesDAO myImagesDAO) {
            this.myImagesDAO = myImagesDAO;
        }

        @Override
        protected Void doInBackground(MyImages... myImages) {
            myImagesDAO.update(myImages[0]);
            return null;
        }
    }
}

package com.example.photoalbum;

import androidx.appcompat.app.AppCompatActivity;
import androidx.lifecycle.Observer;
import androidx.lifecycle.ViewModelProvider;
import androidx.recyclerview.widget.LinearLayoutManager;
import androidx.recyclerview.widget.RecyclerView;

import android.content.Intent;
import android.os.Bundle;
import android.view.View;

import com.google.android.material.floatingactionbutton.FloatingActionButton;

import java.util.List;

public class MainActivity extends AppCompatActivity {

    private MyImagesVIewModel myImagesVIewModel;
    private RecyclerView rv;
    private FloatingActionButton fab;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        rv = findViewById(R.id.rv);
        fab = findViewById(R.id.fab);
        rv.setLayoutManager(new LinearLayoutManager(this));
        MyImagesAdapter adapter = new MyImagesAdapter();
        rv.setAdapter(adapter);
        myImagesVIewModel = new ViewModelProvider.AndroidViewModelFactory(getApplication())
                    .create(MyImagesVIewModel.class);
        //ViewModelProvider.Factory
        // which may create AndroidViewModel and ViewModel, which have an empty constructor.
        myImagesVIewModel.getAllImages().observe(MainActivity.this, new Observer<List<MyImages>>() {
            @Override
            public void onChanged(List<MyImages> myImages) {//called when data is changed

                adapter.setImagesList(myImages);
            }
            //observer updates the UI
        });

        fab.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {

                Intent intent = new Intent(MainActivity.this, AddImageActivity.class);
                startActivityForResult(intent, 3);
            }
        });

    }
}

package com.example.photoalbum;

import android.app.Application;
import android.os.AsyncTask;

import androidx.lifecycle.LiveData;

import java.util.ArrayList;
import java.util.List;

public class MyImagesRepository {

    private MyImagesDAO myImagesDAO;
    private LiveData<List<MyImages>> imagesList;

    public MyImagesRepository(Application application){

        MyImagesDatabase database = MyImagesDatabase.getInstance(application);
        myImagesDAO = database.myImagesDAO();
        imagesList = myImagesDAO.getAllImages();
    }

    public void insert(MyImages myImages){

        new InsertImageAsyncTask(myImagesDAO).execute(myImages);
    }

    public void delete(MyImages myImages){

        new DeleteImageAsyncTask(myImagesDAO).execute(myImages);
    }

    public void update(MyImages myImages){

        new UpdateImageAsyncTask(myImagesDAO).execute(myImages);
    }

    public LiveData<List<MyImages>> getAllImages(){

        return imagesList;
    }

    public static class InsertImageAsyncTask extends AsyncTask<MyImages,Void,Void>{

        MyImagesDAO myImagesDAO;

        public InsertImageAsyncTask(MyImagesDAO myImagesDAO) {
            this.myImagesDAO = myImagesDAO;
        }

        @Override
        protected Void doInBackground(MyImages... myImages) {
            myImagesDAO.insert(myImages[0]);
            return null;
        }
    }

    public static class DeleteImageAsyncTask extends AsyncTask<MyImages,Void,Void>{

        MyImagesDAO myImagesDAO;

        public DeleteImageAsyncTask(MyImagesDAO myImagesDAO) {
            this.myImagesDAO = myImagesDAO;
        }

        @Override
        protected Void doInBackground(MyImages... myImages) {
            myImagesDAO.delete(myImages[0]);
            return null;
        }
    }

    public static class UpdateImageAsyncTask extends AsyncTask<MyImages,Void,Void>{

        MyImagesDAO myImagesDAO;

        public UpdateImageAsyncTask(MyImagesDAO myImagesDAO) {
            this.myImagesDAO = myImagesDAO;
        }

        @Override
        protected Void doInBackground(MyImages... myImages) {
            myImagesDAO.update(myImages[0]);
            return null;
        }
    }
}

Answer 1

发生这种情况是因为viewmodel构造函数中的NullPointerException。

你在日志里看到了。

java.lang.NullPointerException: Attempt to invoke virtual method 'com.example.photoalbum.MyImagesDAO com.example.photoalbum.MyImagesDatabase.myImagesDAO()' on a null object reference at com.example.photoalbum.MyImagesRepository.

在存储库类中

public MyImagesRepository(Application application){
        MyImagesDatabase database = MyImagesDatabase.getInstance(application);
        myImagesDAO = database.myImagesDAO(); //here database seems to be null.
        imagesList = myImagesDAO.getAllImages();
    }