java - 尝试在空对象上调用虚拟方法-6ren

java - 尝试在空对象上调用虚拟方法

转载作者：行者123 更新时间：2023-12-02 12:25:26

我正在尝试在加载器和内容提供程序上进行练习我有一个学生数据库，其中在activity_main.xml中包含(姓名，学位)我有一个recyclerview和2个editText，一个用于名称，另一个用于学位，我的目标是当用户单击按钮，名称将出现在回收 View 中这是我的日志

FATAL EXCEPTION: main Process: com.example.abdelmagied.myapplication, PID: 16160 java.lang.NullPointerException: Attempt to invoke virtual method 'void com.example.abdelmagied.myapplication.recyclerview.swapcursor(android.database.Cursor)' on a null object reference at com.example.abdelmagied.myapplication.MainActivity.onLoadFinished(MainActivity.java:97) at com.example.abdelmagied.myapplication.MainActivity.onLoadFinished(MainActivity.java:17) at android.support.v4.app.LoaderManagerImpl$LoaderInfo.callOnLoadFinished(LoaderManager.java:476) at android.support.v4.app.LoaderManagerImpl$LoaderInfo.onLoadComplete(LoaderManager.java:444) at android.support.v4.content.Loader.deliverResult(Loader.java:126) at android.support.v4.content.CursorLoader.deliverResult(CursorLoader.java:105) at android.support.v4.content.CursorLoader.deliverResult(CursorLoader.java:37) at android.support.v4.content.AsyncTaskLoader.dispatchOnLoadComplete(AsyncTaskLoader.java:255) at android.support.v4.content.AsyncTaskLoader$LoadTask.onPostExecute(AsyncTaskLoader.java:80) at android.support.v4.content.ModernAsyncTask.finish(ModernAsyncTask.java:485) at android.support.v4.content.ModernAsyncTask$InternalHandler.handleMessage(ModernAsyncTask.java:502) at android.os.Handler.dispatchMessage(Handler.java:102) at android.os.Looper.loop(Looper.java:148) at android.app.ActivityThread.main(ActivityThread.java:5417) 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)

这是我的主要 Activity 类(class)

package com.example.abdelmagied.myapplication;

import android.content.ContentValues;
import android.database.Cursor;
import android.net.Uri;
import android.support.v4.app.LoaderManager;
import android.support.v4.content.CursorLoader;
import android.support.v4.content.Loader;
import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.support.v7.widget.LinearLayoutManager;
import android.support.v7.widget.RecyclerView;
import android.util.Log;
import android.view.View;
import android.widget.EditText;

public class MainActivity extends AppCompatActivity implements LoaderManager.LoaderCallbacks<Cursor>{

    public static final String[] MAIN_STUDENT_PROJECTION = {

            studentContract.StudentEntry._ID ,

            studentContract.StudentEntry.COLUMN_NAME ,

            studentContract.StudentEntry.COLUMN_DEGREE ,

    };

    public static final int INDEX_COLUMN_ID      = 0;
    public static final int INDEX_COLUMN_NAME    = 1;
    public static final int INDEX_COLUMN_DEGREE  = 2;


    private recyclerview recyclerAdapter;
    private int mposition = RecyclerView.NO_POSITION;

    public static final int ID_STUDNET_LOADER = 99;

    // details of the recyclerview

     RecyclerView recycler;
     RecyclerView.Adapter myadapter;
     RecyclerView.LayoutManager mymanager;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);

        setContentView(R.layout.activity_main);

        // adjust the recyclerview
        recycler  = (RecyclerView) findViewById(R.id.recyclerid);
        myadapter = new recyclerview(this);
        mymanager = new LinearLayoutManager(this);

        recycler.setAdapter(myadapter);
        recycler.setLayoutManager(mymanager);

        getSupportLoaderManager().initLoader(ID_STUDNET_LOADER , null , this);
    }

    @Override
    public Loader<Cursor> onCreateLoader(int id, Bundle args) {
        switch(id)
        {
            case ID_STUDNET_LOADER :

                Uri  Query    = studentContract.StudentEntry.CONTENT_URI;


                return new CursorLoader(this  ,
                              Query ,
                              MAIN_STUDENT_PROJECTION ,
                              null ,
                              null  ,
                              null
                           );

             default:
                  throw new RuntimeException("Loader is not implemented : " + id);
        }
    }

    @Override
    public void onLoadFinished(Loader<Cursor> loader, Cursor data) {

        recyclerAdapter.swapcursor(data);
        if(mposition == RecyclerView.NO_POSITION) mposition = 0;
        recycler.smoothScrollToPosition(mposition);
    }

    @Override
    public void onLoaderReset(Loader<Cursor> loader) {
           recyclerAdapter.swapcursor(null);
    }

    public void addtodatabase(View view) {

        EditText name = (EditText) findViewById(R.id.name);
        EditText degree=(EditText) findViewById(R.id.degree);

        // make a contentvalues and put name  , degree , salary on  it...
        ContentValues values = new ContentValues();
        values.put(studentContract.StudentEntry.COLUMN_NAME   , name.getText().toString());
        values.put(studentContract.StudentEntry.COLUMN_DEGREE , degree.getText().toString());
        getContentResolver().insert(studentContract.StudentEntry.CONTENT_URI , values);
    }
}

这是 recyclerview 适配器

package com.example.abdelmagied.myapplication;

import android.content.Context;
import android.database.Cursor;
import android.support.v7.widget.RecyclerView;
import android.util.Log;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;
import android.widget.TextView;

/**
 * Created by AbdELMagied on 8/4/2017.
 */

public class recyclerview extends RecyclerView.Adapter<RecyclerView.ViewHolder> {


    Context context;

    private Cursor mycursor;
    public recyclerview(Context context) {
        this.context = context;
    }

    @Override
    public RecyclerView.ViewHolder onCreateViewHolder(ViewGroup parent, int viewType) {

        View view =   LayoutInflater.from(context).inflate(R.layout.recyclerrow , parent , false);
        return new viewholder(view);
    }

    @Override
    public void onBindViewHolder(RecyclerView.ViewHolder holder, int position) {

         mycursor.move(position);
         viewholder myholder = (viewholder) holder;
        myholder.mytxt.setText(mycursor.getString(MainActivity.INDEX_COLUMN_NAME));

    }

    void swapcursor(Cursor mycurso)
    {
        this.mycursor = mycurso;
        notifyDataSetChanged();
    }

    @Override
    public int getItemCount() {
        if(mycursor == null) return 0;
        return mycursor.getCount();
    }

    public class viewholder extends RecyclerView.ViewHolder
    {
        public  TextView mytxt;
        public viewholder(View itemView) {
            super(itemView);
            mytxt = (TextView) itemView.findViewById(R.id.textView);
        }
    }
}

这是我的内容提供商

package com.example.abdelmagied.myapplication;

import android.content.ContentProvider;
import android.content.ContentUris;
import android.content.ContentValues;
import android.content.UriMatcher;
import android.database.Cursor;
import android.database.SQLException;
import android.database.sqlite.SQLiteDatabase;
import android.net.Uri;
import android.support.annotation.Nullable;
import android.text.TextUtils;

/**
 * Created by AbdELMagied on 8/4/2017.
 */

public class studentprovider extends ContentProvider {

    databaseopenhelper mydatabase;

    static final int STUDENT = 100;

    static final int STUDENT_WITH_ID = 101;

    private UriMatcher urimatcher = matcher();

    private UriMatcher matcher() {

        UriMatcher mymatcher = new UriMatcher(UriMatcher.NO_MATCH);
        mymatcher.addURI(studentContract.authority, "student", STUDENT);
        mymatcher.addURI(studentContract.authority, "student/#", STUDENT_WITH_ID);
        return mymatcher;

    }

    @Override
    public boolean onCreate() {
        mydatabase = new databaseopenhelper(getContext());
        return true;
    }


    @Nullable
    @Override
    public Cursor query(Uri uri, String[] projection, String selection, String[] selectionArgs, String sortOrder) {
        Cursor returncursor;
        switch (urimatcher.match(uri)) {

            case STUDENT:

                returncursor = mydatabase.getReadableDatabase().query(
                        studentContract.StudentEntry.TABLE_NAME,
                        projection,
                        selection,
                        selectionArgs,
                        null,
                        null,
                        sortOrder
                );
                break;

            case STUDENT_WITH_ID:

                String where = "_id = " + uri.getLastPathSegment() + (!TextUtils.isEmpty(selection)? " AND " +  selection : "");

                returncursor = mydatabase.getReadableDatabase().query(

                        studentContract.StudentEntry.TABLE_NAME,
                        projection,
                        where,
                        selectionArgs,
                        null,
                        null,
                        sortOrder

                );
                break;

            default:
                throw new UnsupportedOperationException("not found  uri " + uri);
        }

        returncursor.setNotificationUri(getContext().getContentResolver() , uri);
        return returncursor;
    }


    @Nullable
    @Override
    public String getType(Uri uri) {
        return null;
    }

    @Nullable
    @Override
    public Uri insert(Uri uri, ContentValues values) {
         long rowid =  mydatabase.getWritableDatabase().insert(studentContract.StudentEntry.TABLE_NAME, null , values);
          if(rowid > 0)
          {
              Uri _uri = ContentUris.withAppendedId(studentContract.StudentEntry.CONTENT_URI, rowid);
              getContext().getContentResolver().notifyChange(_uri, null);
              return _uri;
          }

        throw new SQLException("Faild to add a record " + uri);
    }

    @Override
    public int delete(Uri uri, String selection, String[] selectionArgs) {
        if(selection == "") selection= "1";
        int numberofdeleted = 0;
        switch (urimatcher.match(uri))
        {
            case STUDENT:
                 String where = "_id = " + uri.getLastPathSegment()  + (!TextUtils.isEmpty(selection)? "AND " + selection : "");
                 numberofdeleted = mydatabase.getWritableDatabase().delete("students" , where , selectionArgs);
                break;
            default:
                throw new UnsupportedOperationException("uri not found  " + uri);
        }
           if(numberofdeleted != 0)
           {
               getContext().getContentResolver().notifyChange(uri , null);
           }
        return numberofdeleted;
    }


    @Override
    public int update(Uri uri, ContentValues values, String selection, String[] selectionArgs) {
        return 0;
    }
}

这是数据库类

package com.example.abdelmagied.myapplication;

import android.content.Context;
import android.database.sqlite.SQLiteDatabase;
import android.database.sqlite.SQLiteOpenHelper;

/**
 * Created by AbdELMagied on 8/4/2017.
 */

public class databaseopenhelper extends SQLiteOpenHelper {


    public databaseopenhelper(Context context) {

        super(context, "student.db", null, 1);
    }

    @Override
    public void onCreate(SQLiteDatabase db) {


          final String SQL_CREATE_STUDENT_TABLE =

                          "CREATE TABLE " + studentContract.StudentEntry.TABLE_NAME + " ("  +

                           studentContract.StudentEntry._ID + " INTEGER  PRIMARY KEY , "  +

                           studentContract.StudentEntry.COLUMN_NAME + " TEXT , " +

                           studentContract.StudentEntry.COLUMN_DEGREE + " TEXT ); " ;


          db.execSQL(SQL_CREATE_STUDENT_TABLE);

    }

    @Override
    public void onUpgrade(SQLiteDatabase db, int oldVersion, int newVersion) {

         db.execSQL("drop database if exists student");

         onCreate(db);
    }
}

这里是学生契约(Contract)类

package com.example.abdelmagied.myapplication;

import android.content.ContentProvider;
import android.content.ContentUris;
import android.content.ContentValues;
import android.content.UriMatcher;
import android.database.Cursor;
import android.database.SQLException;
import android.database.sqlite.SQLiteDatabase;
import android.net.Uri;
import android.support.annotation.Nullable;
import android.text.TextUtils;

/**
 * Created by AbdELMagied on 8/4/2017.
 */

public class studentprovider extends ContentProvider {

    databaseopenhelper mydatabase;

    static final int STUDENT = 100;

    static final int STUDENT_WITH_ID = 101;

    private UriMatcher urimatcher = matcher();

    private UriMatcher matcher() {

        UriMatcher mymatcher = new UriMatcher(UriMatcher.NO_MATCH);
        mymatcher.addURI(studentContract.authority, "student", STUDENT);
        mymatcher.addURI(studentContract.authority, "student/#", STUDENT_WITH_ID);
        return mymatcher;

    }

    @Override
    public boolean onCreate() {
        mydatabase = new databaseopenhelper(getContext());
        return true;
    }


    @Nullable
    @Override
    public Cursor query(Uri uri, String[] projection, String selection, String[] selectionArgs, String sortOrder) {
        Cursor returncursor;
        switch (urimatcher.match(uri)) {

            case STUDENT:

                returncursor = mydatabase.getReadableDatabase().query(
                        studentContract.StudentEntry.TABLE_NAME,
                        projection,
                        selection,
                        selectionArgs,
                        null,
                        null,
                        sortOrder
                );
                break;

            case STUDENT_WITH_ID:

                String where = "_id = " + uri.getLastPathSegment() + (!TextUtils.isEmpty(selection)? " AND " +  selection : "");

                returncursor = mydatabase.getReadableDatabase().query(

                        studentContract.StudentEntry.TABLE_NAME,
                        projection,
                        where,
                        selectionArgs,
                        null,
                        null,
                        sortOrder

                );
                break;

            default:
                throw new UnsupportedOperationException("not found  uri " + uri);
        }

        returncursor.setNotificationUri(getContext().getContentResolver() , uri);
        return returncursor;
    }


    @Nullable
    @Override
    public String getType(Uri uri) {
        return null;
    }

    @Nullable
    @Override
    public Uri insert(Uri uri, ContentValues values) {
         long rowid =  mydatabase.getWritableDatabase().insert(studentContract.StudentEntry.TABLE_NAME, null , values);
          if(rowid > 0)
          {
              Uri _uri = ContentUris.withAppendedId(studentContract.StudentEntry.CONTENT_URI, rowid);
              getContext().getContentResolver().notifyChange(_uri, null);
              return _uri;
          }

        throw new SQLException("Faild to add a record " + uri);
    }

    @Override
    public int delete(Uri uri, String selection, String[] selectionArgs) {
        if(selection == "") selection= "1";
        int numberofdeleted = 0;
        switch (urimatcher.match(uri))
        {
            case STUDENT:
                 String where = "_id = " + uri.getLastPathSegment()  + (!TextUtils.isEmpty(selection)? "AND " + selection : "");
                 numberofdeleted = mydatabase.getWritableDatabase().delete("students" , where , selectionArgs);
                break;
            default:
                throw new UnsupportedOperationException("uri not found  " + uri);
        }
           if(numberofdeleted != 0)
           {
               getContext().getContentResolver().notifyChange(uri , null);
           }
        return numberofdeleted;
    }


    @Override
    public int update(Uri uri, ContentValues values, String selection, String[] selectionArgs) {
        return 0;
    }
}

最佳答案

recyclerAdapter 永远不会被实例化。 onCreate 将是实例化此类型 recyclerview 对象的理想位置。

关于java - 尝试在空对象上调用虚拟方法，我们在Stack Overflow上找到一个类似的问题： https://stackoverflow.com/questions/45514119/

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java - 尝试在空对象上调用虚拟方法