仮想メソッド 'java.lang.String android.content.Context.getPackageName（）'をNullobjectreferenceで呼び出そうとしています

Question

sqliteデータベースからいくつかのデータを取得しようとしています - > RecyclerViewに表示 - >私は新しい活動を始めたいと思う。

が、私は次のエラーがポップアップし、それらの一つをタップしたときに項目のリストを表示した後： のjava.lang.NullPointerException：仮想メソッドを呼び出そうと「はjava.lang.String型android.content.Context.getPackageName（ ）」ヌルオブジェクト参照に

マイmainActivityClass：

import android.content.Context; 
import android.content.Intent; 
import android.database.Cursor; 
import android.database.sqlite.SQLiteException; 
import android.support.annotation.NonNull; 
import android.support.design.widget.NavigationView; 
import android.support.v4.widget.DrawerLayout; 
import android.support.v7.app.ActionBarDrawerToggle; 
import android.support.v7.app.AppCompatActivity; 
import android.os.Bundle; 
import android.support.v7.widget.LinearLayoutManager; 
import android.support.v7.widget.RecyclerView; 
import android.support.v7.widget.Toolbar; 
import android.view.MenuItem; 
import android.widget.Toast; 
import com.example.blue2dew.trakin.Item.item; 
import com.example.blue2dew.trakin.MovieAdapter; 


import java.util.ArrayList; 




public class MainActivity extends AppCompatActivity { 
    private DrawerLayout mDrawerLayout; 
private ActionBarDrawerToggle mtoggle; 
private NavigationView navigationView; 
private Toolbar toolbar; 
private Movie_det movie_det; 
Context context; 

private RecyclerView recyclerView; 
private DatabaseHelper databaseHelper; 
ArrayList<item> arrayList=new ArrayList<item>(); 
private Cursor cursor; 
private MovieAdapter movieAdapter; 


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


    mDrawerLayout=(DrawerLayout) findViewById(R.id.activity_main); 
    navigationView=(NavigationView)findViewById(R.id.navigation_view); 
    toolbar=(Toolbar)findViewById(R.id.nav_action); 
    setSupportActionBar(toolbar); 
    mtoggle=new ActionBarDrawerToggle(this,mDrawerLayout,R.string.open,R.string.close); 

    recyclerView=(RecyclerView)findViewById(R.id.recycler_view); 
    mDrawerLayout.addDrawerListener(mtoggle); 
    mtoggle.syncState(); 

    getSupportActionBar().setDisplayHomeAsUpEnabled(true); 


    navigationView.setNavigationItemSelectedListener(new NavigationView.OnNavigationItemSelectedListener() { 
     @Override 
     public boolean onNavigationItemSelected(@NonNull MenuItem item) { 
      item.setChecked(true); 
      mDrawerLayout.closeDrawers(); 
      return false; 
     } 
    }); 

    loadDatabase(); 

} 


@Override 
public boolean onOptionsItemSelected(MenuItem item){ 
    if(mtoggle.onOptionsItemSelected(item)){ 
     return true; 
    } 

    return super.onOptionsItemSelected(item); 
} 

public void loadDatabase(){ 
    databaseHelper=new DatabaseHelper(MainActivity.this); 

    try{ 
     databaseHelper.checkAndCopyDatabase(); 
     databaseHelper.openDatabase(); 
    }catch(SQLiteException e){ 
     e.printStackTrace(); 
    } 


    try { 
     cursor = databaseHelper.QueryData("select * from movieDetails"); 
     if (cursor != null) { 
      if (cursor.moveToFirst()) { 
       do { 
        item itemv = new item(); 
        itemv.setMovieName(cursor.getString(2)); 
        itemv.setReleaseDate(cursor.getString(5)); 
        itemv.setMovieImage(cursor.getBlob(1)); 

        arrayList.add(itemv); 


       } while (cursor.moveToNext()); 
      } 
     } 
    }catch (SQLiteException e){ 
     e.printStackTrace(); 
    } 


    LinearLayoutManager linearLayoutManager=new LinearLayoutManager(getApplicationContext()); 
    movieAdapter=new MovieAdapter(MainActivity.this,context,arrayList); 



    recyclerView.setHasFixedSize(true); 
    recyclerView.setLayoutManager(linearLayoutManager); 
    recyclerView.setAdapter(movieAdapter); 

} 



} 

マイアダプタクラス：

package com.example.blue2dew.trakin; 

import android.app.Activity; 
import android.content.Context; 
import android.content.Intent; 
import android.support.v7.widget.RecyclerView; 
import android.view.LayoutInflater; 
import android.view.View; 
import android.view.ViewGroup; 
import android.widget.ImageView; 
import android.widget.TextView; 


import com.example.blue2dew.trakin.Item.item; 


import java.util.Collections; 
import java.util.List; 



public class MovieAdapter extends RecyclerView.Adapter<MovieAdapter.MovieViewHolder> { 
    Activity activity; 
    List<item> items= Collections.emptyList(); 
    Context context; 


public MovieAdapter(Activity activity,Context context, List<item> items){ 
    this.activity=activity; 
    this.context=context; 
    this.items=items; 
} 
@Override 
public MovieViewHolder onCreateViewHolder(ViewGroup parent, int viewType) { 
    View view= LayoutInflater.from(parent.getContext()).inflate(R.layout.custom_list_item,parent,false); 

    MovieViewHolder movieViewHolder=new MovieViewHolder(view,context,items); 
    return movieViewHolder; 
} 



public void onBindViewHolder(MovieViewHolder holder,final int position) { 
    holder.movie_name.setText(items.get(position).getMovieName()); 
    holder.release_date.setText(items.get(position).getReleaseDate()); 
    holder.imageView.setImageBitmap(items.get(position).getMovieImage()); 




} 

@Override 
public int getItemCount() { 
    return items.size(); 
} 




public class MovieViewHolder extends RecyclerView.ViewHolder implements View.OnClickListener{ 


    ImageView md_image; 
    TextView md_movie_name; 
    TextView md_length1; 
    TextView md_genre1; 
    TextView md_release_date1; 
    ImageView md_imdb_icon; 
    TextView md_rating1; 
    TextView md_director1; 
    TextView md_synopsis1; 

    TextView movie_name; 
    TextView release_date; 
    ImageView imageView; 

    List<item> items=Collections.emptyList(); 
    Context context; 

    public MovieViewHolder(View itemView,Context context,List<item> items) { 
     super(itemView); 
     this.context=context; 
     this.items=items; 
     itemView.setOnClickListener(this); 
     md_image = (ImageView) itemView.findViewById(R.id.md_image); 
     md_director1 = (TextView) itemView.findViewById(R.id.md_director1); 
     md_genre1 = (TextView) itemView.findViewById(R.id.md_genre1); 
     md_movie_name = (TextView) itemView.findViewById(R.id.md_movie_name); 
     md_length1 = (TextView) itemView.findViewById(R.id.md_length1); 
     md_release_date1 = (TextView) itemView.findViewById(R.id.md_release_date1); 
     md_rating1 = (TextView) itemView.findViewById(R.id.md_rating1); 
     md_synopsis1 = (TextView)itemView.findViewById(R.id.md_synopsis1); 
     md_imdb_icon = (ImageView) itemView.findViewById(R.id.md_imdb_icon); 

     movie_name=(TextView) itemView.findViewById(R.id.movie_name); 
     release_date=(TextView) itemView.findViewById(R.id.release_date); 
     imageView=(ImageView) itemView.findViewById(R.id.imageView); 


    } 


    @Override 
    public void onClick(View v) { 

     int position=getAdapterPosition(); 
     item item=this.items.get(position); 
     Intent intent=new Intent (this.context,Main2Activity.class); 
     this.context.startActivity(intent); 

    } 
} 


} 

マイデータベースヘルパークラス： パッケージcom.example.blue2dew.trakin;

import android.content.Context; 
import android.database.Cursor; 
import android.database.sqlite.SQLiteDatabase; 
import android.database.sqlite.SQLiteException; 
import android.database.sqlite.SQLiteOpenHelper; 
import android.os.Build; 
import android.os.Environment; 
import android.util.Log; 

import java.io.FileOutputStream; 
import java.io.IOException; 
import java.io.InputStream; 
import java.io.OutputStream; 



public class DatabaseHelper extends SQLiteOpenHelper { 
private static String DB_Name="movie.db"; 
private static String DB_Path; 
private SQLiteDatabase mydatabase; 
private final Context mycontext; 




public DatabaseHelper(Context context) { 
    super(context,DB_Name , null, 1); 
    if(Build.VERSION.SDK_INT>=15){ 
     DB_Path=context.getApplicationInfo().dataDir+"/databases/"; 
    } 
    else{ 
     DB_Path= Environment.getDataDirectory()+"/data/"+context.getPackageName()+"/databases/"; 
    } 
    this.mycontext=context; 
} 

@Override 
public void onCreate(SQLiteDatabase db) { 
    //using existing database so creation not needed 

} 

@Override 
public void onUpgrade(SQLiteDatabase db, int oldVersion, int newVersion) { 
    //the existing database need not be upgraded 

} 

public void checkAndCopyDatabase(){ 
    boolean dbExist=checkDatabase(); 
    if(dbExist){ 
     Log.d("TAG","database already exists"); 
    } 
    else { 
     this.getReadableDatabase(); 
    } 
    try { 
     copyDatabase(); 
    } 
    catch (IOException e){ 
     e.printStackTrace(); 
     Log.d("TAG","Error Copy Database"); 
    } 
} 

public boolean checkDatabase(){ 
    SQLiteDatabase checkDB=null; 
    try { 
     String mypath = DB_Path + DB_Name; 
     checkDB = SQLiteDatabase.openDatabase(mypath, null, SQLiteDatabase.OPEN_READWRITE); 
    }catch (SQLiteException e) { 
     e.printStackTrace(); 

    } 
    if(checkDB!=null){ 
     checkDB.close(); 
    } 
    return checkDB!=null?true:false; 

} 

public void copyDatabase() throws IOException{ 
    InputStream myInput=mycontext.getAssets().open(DB_Name); 
    String outfilename=DB_Path+DB_Name; 
    OutputStream myOutput=new FileOutputStream(outfilename); 
    byte[] buffer=new byte[1024]; 
    int length; 
    while ((length=myInput.read(buffer))>0){ 
     myOutput.write(buffer,0,length); 
    } 
    myOutput.flush(); 
    myOutput.close(); 
    myInput.close(); 
} 

public void openDatabase(){ 
    String myPath=DB_Path+DB_Name; 
    mydatabase=SQLiteDatabase.openDatabase(myPath,null, SQLiteDatabase.OPEN_READWRITE); 
} 

public synchronized void close(){ 
    if(mydatabase!=null){ 
     mydatabase.close(); 
    } 
    super.close(); 
} 

public Cursor QueryData(String query){ 
    return mydatabase.rawQuery(query,null); 
} 
} 

エラー：あなたはこれを行うことができますいずれか

E/AndroidRuntime: FATAL EXCEPTION: main Process: com.example.blue2dew.trakin, PID: 7430 java.lang.NullPointerException: Attempt to invoke virtual method 'java.lang.String android.content.Context.getPackageName()' on a null object reference 
at android.content.ComponentName.(ComponentName.java:128) 
at android.content.Intent.(Intent.java:4521) at com.example.blue2dew.trakin.MovieAdapter$MovieViewHolder.onClick(MovieAdapter.java:127) 
at android.view.View.performClick(View.java:5209) 
at android.view.View$PerformClick.run(View.java:21179) 
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:5437) 
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

、

@Override 
    public void onClick(View v) { 
    int position=getAdapterPosition(); 
    item item=this.items.get(position); 
    Intent intent=new Intent (v.getContext(),Main2Activity.class); 
    v.getContext().startActivity(intent); 
}