java - 无法启动 ComponentInfo java.lang.NullPointerException

Question

我收到 java.lang.NullPointerException 错误，但不确定原因。我认为这可能与我的数据库中没有数据有关，但我不确定。有人对如何让我的数据库接受没有数据有任何建议吗？我已经在多个网站/书籍上获取了代码并进行了研究，但这是我得到的最接近的结果，但仍然出现错误!非常感谢帮助!预先感谢您!

日志猫:

09-27 01:17:14.656: E/AndroidRuntime(3331): FATAL EXCEPTION: main
09-27 01:17:14.656: E/AndroidRuntime(3331): java.lang.RuntimeException: Unable to start activity ComponentInfo{com.pocketbotanist/com.pocketbotanist.HomeScreen}: java.lang.NullPointerException
09-27 01:17:14.656: E/AndroidRuntime(3331):     at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2247)
09-27 01:17:14.656: E/AndroidRuntime(3331):     at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2297)
09-27 01:17:14.656: E/AndroidRuntime(3331):     at android.app.ActivityThread.access$700(ActivityThread.java:152)
09-27 01:17:14.656: E/AndroidRuntime(3331):     at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1282)
09-27 01:17:14.656: E/AndroidRuntime(3331):     at android.os.Handler.dispatchMessage(Handler.java:99)
09-27 01:17:14.656: E/AndroidRuntime(3331):     at android.os.Looper.loop(Looper.java:137)
09-27 01:17:14.656: E/AndroidRuntime(3331):     at android.app.ActivityThread.main(ActivityThread.java:5328)
09-27 01:17:14.656: E/AndroidRuntime(3331):     at java.lang.reflect.Method.invokeNative(Native Method)
09-27 01:17:14.656: E/AndroidRuntime(3331):     at java.lang.reflect.Method.invoke(Method.java:511)
09-27 01:17:14.656: E/AndroidRuntime(3331):     at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1102)
09-27 01:17:14.656: E/AndroidRuntime(3331):     at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:869)
09-27 01:17:14.656: E/AndroidRuntime(3331):     at dalvik.system.NativeStart.main(Native Method)
09-27 01:17:14.656: E/AndroidRuntime(3331): Caused by: java.lang.NullPointerException
09-27 01:17:14.656: E/AndroidRuntime(3331):     at com.pocketbotanist.HomeScreen.onCreate(HomeScreen.java:69)
09-27 01:17:14.656: E/AndroidRuntime(3331):     at android.app.Activity.performCreate(Activity.java:5250)
09-27 01:17:14.656: E/AndroidRuntime(3331):     at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1097)
09-27 01:17:14.656: E/AndroidRuntime(3331):     at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2211)
09-27 01:17:14.656: E/AndroidRuntime(3331):     ... 11 more

代码:

package com.pocketbotanist;

import java.io.File;

import android.app.ListActivity;
import android.app.LoaderManager;
import android.content.CursorLoader;
import android.content.Intent;
import android.content.Loader;
import android.content.pm.ActivityInfo;
import android.content.res.Configuration;
import android.database.Cursor;
import android.net.Uri;
import android.os.Bundle;
import android.os.Environment;
import android.view.ContextMenu;
import android.view.ContextMenu.ContextMenuInfo;
import android.view.Menu;
import android.view.MenuItem;
import android.view.View;
import android.widget.AdapterView.AdapterContextMenuInfo;
import android.widget.ArrayAdapter;
import android.widget.ListView;
import android.widget.SimpleCursorAdapter;
import android.widget.Spinner;

import com.pocketbotanist.contentprovider.MyEntryContentProvider;
import com.pocketbotanist.database.EntryTable;



public class HomeScreen extends ListActivity implements LoaderManager.LoaderCallbacks<Cursor> {

    private static final int DELETE_ID = Menu.FIRST + 1;
    // private Cursor cursor;
    private SimpleCursorAdapter adapter;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_home_screen);
        this.getListView().setDividerHeight(2);
        setRequestedOrientation(ActivityInfo.SCREEN_ORIENTATION_PORTRAIT);

        //Initial folder creation code
        File appDirectory = new File(Environment.getExternalStorageDirectory().toString()+"/Pocket Botanist/");
        if (!appDirectory.exists()){
            appDirectory.mkdir();
        }

        //Drop down list code
        Spinner spinner = (Spinner) findViewById(R.id.action_spinner);
        ArrayAdapter<CharSequence> adapter = ArrayAdapter.createFromResource(this, R.array.action_list, android.R.layout.simple_spinner_item);
        adapter.setDropDownViewResource(android.R.layout.simple_spinner_dropdown_item);
        spinner.setAdapter(adapter);


        fillData();
        @SuppressWarnings("unused")
        View view = (View) getListView().getItemAtPosition(0);
        registerForContextMenu(getListView());
        if (adapter.isEmpty())
            System.out.println("IT'S EMPTY");
        String[] projection = { EntryTable.COLUMN_CUSTOMID };
        long info =  adapter.getItemId(0);
        Uri uri = Uri.parse(MyEntryContentProvider.CONTENT_URI + "/"
                + info);
        Cursor cursor = getContentResolver().query(uri, projection, null, null, null);
        cursor.moveToFirst();
        System.out.println((cursor.getString(cursor
                    .getColumnIndexOrThrow(EntryTable.COLUMN_CUSTOMID))));
    }

    @Override
    public void onConfigurationChanged(Configuration newConfig) {
        super.onConfigurationChanged(newConfig);
        setRequestedOrientation(ActivityInfo.SCREEN_ORIENTATION_PORTRAIT);
    }


    @Override
    public boolean onContextItemSelected(MenuItem item) {
        switch (item.getItemId()) {
        case DELETE_ID:
            String[] projection = { EntryTable.COLUMN_PHOTOS };
            AdapterContextMenuInfo info = (AdapterContextMenuInfo) item
            .getMenuInfo();
            Uri uri = Uri.parse(MyEntryContentProvider.CONTENT_URI + "/"
                    + info.id);
            Cursor cursor = getContentResolver().query(uri, projection, null, null, null);
            cursor.moveToFirst();
            File temp = new File(cursor.getString(cursor
                    .getColumnIndexOrThrow(EntryTable.COLUMN_PHOTOS)));
            if(temp.exists()){
                File[] files = temp.listFiles();
                for(int i = 0; i < files.length; i++){
                    files[i].delete();
                }
                temp.delete();
            }
            getContentResolver().delete(uri, null, null);
            fillData();
            return true;
        }

        return super.onContextItemSelected(item);

    }


    @Override
    public boolean onCreateOptionsMenu(Menu menu) {
        // Inflate the menu; this adds items to the action bar if it is present.
        getMenuInflater().inflate(R.menu.activity_home_screen, menu);
        //SearchView searchView = (SearchView) menu.findItem(R.id.menu_search).getActionView();
        return true;
    }

    @Override
    public boolean onOptionsItemSelected(MenuItem item) {
        // Handle item selection
        switch (item.getItemId()) {
        case R.id.menu_settings:
            settCall();
            return true;
        case R.id.menu_map:
            map1_3();
            return true;
        case R.id.menu_edit:
            entryScreen("New Entry");
            return true;
        default:
            return super.onOptionsItemSelected(item);
        }
    }

    public void entryScreen(String t){
        Intent intent = new Intent(this, EntryScreen.class);
        intent.putExtra("passer", t);
        startActivity(intent);
    }

    public void settCall(){
        Intent sett = new Intent(this,SettingsActivity.class);
        startActivity(sett);
    }

    public void map1_3(){

        Intent map13 = new Intent(this,Entrymap_1_3.class);
        startActivity(map13);
    }


    // Opens the second activity if an entry is clicked
    @Override
    protected void onListItemClick(ListView l, View v, int position, long id) {
        super.onListItemClick(l, v, position, id);
        Intent i = new Intent(this, EntryScreen.class);
        Uri itemUri = Uri.parse(MyEntryContentProvider.CONTENT_URI + "/" + id);
        i.putExtra(MyEntryContentProvider.CONTENT_ITEM_TYPE, itemUri);

        startActivity(i);
    }

    //@Override
    //When we create the loader we're going to get the projection for the ID, customID, species name, and time columns
    //(insures that these exist within the database)
    //then we create our cursor loader which will be responsible for loading data from the database
    //using the projection and our content provider
    public Loader<Cursor> onCreateLoader(int arg0, Bundle arg1) {
        String[] projection = { EntryTable.COLUMN_ID, EntryTable.COLUMN_CUSTOMID, EntryTable.COLUMN_SPECIES, EntryTable.COLUMN_TIME, EntryTable.COLUMN_PHOTOS, EntryTable.COLUMN_PHOTO };
        CursorLoader cursorLoader = new CursorLoader(this,
                MyEntryContentProvider.CONTENT_URI, projection, null, null, null);
        return cursorLoader;
    }

    private void fillData() {

        // Fields from the database (projection)
        String[] from = new String[] { EntryTable.COLUMN_CUSTOMID, EntryTable.COLUMN_SPECIES , EntryTable.COLUMN_TIME, EntryTable.COLUMN_PHOTO};
        // Fields on the UI to which we map
        int[] to = new int[] { R.id.customidlabel, R.id.namelabel, R.id.timelabel, R.id.imageView };

        getLoaderManager().initLoader(1, null, this);       //changed to 1
        adapter = new SimpleCursorAdapter(this, R.layout.list_row, null, from, to, 0);

        setListAdapter(adapter);
    }

    @Override
      public void onCreateContextMenu(ContextMenu menu, View v, ContextMenuInfo menuInfo) {
        super.onCreateContextMenu(menu, v, menuInfo);
        menu.add(0, DELETE_ID, 0, R.string.menu_delete);
      }


    @Override
    public void onLoadFinished(Loader<Cursor> arg0, Cursor data) {
        adapter.swapCursor(data);       
    }

    @Override
    public void onLoaderReset(Loader<Cursor> arg0) {
        adapter.swapCursor(null);
    }

}

Answer 1

这是返回null的查询方法，你的光标为null:

Cursor cursor = getContentResolver().query(uri, projection, null, null, null);// check why your query is returning null
        cursor.moveToFirst(); // cursor is null