java - NullPointerException Logcat 错误 Android

Question

LogCat NPE 错误

    02-03 16:19:48.172: D/AndroidRuntime(959): Shutting down VM
02-03 16:19:48.182: W/dalvikvm(959): threadid=1: thread exiting with uncaught exception (group=0x41465700)
02-03 16:19:48.322: E/AndroidRuntime(959): FATAL EXCEPTION: main
02-03 16:19:48.322: E/AndroidRuntime(959): java.lang.IllegalStateException: Could not execute method of the activity
02-03 16:19:48.322: E/AndroidRuntime(959):  at android.view.View$1.onClick(View.java:3633)
02-03 16:19:48.322: E/AndroidRuntime(959):  at android.view.View.performClick(View.java:4240)
02-03 16:19:48.322: E/AndroidRuntime(959):  at android.view.View$PerformClick.run(View.java:17721)
02-03 16:19:48.322: E/AndroidRuntime(959):  at android.os.Handler.handleCallback(Handler.java:730)
02-03 16:19:48.322: E/AndroidRuntime(959):  at android.os.Handler.dispatchMessage(Handler.java:92)
02-03 16:19:48.322: E/AndroidRuntime(959):  at android.os.Looper.loop(Looper.java:137)
02-03 16:19:48.322: E/AndroidRuntime(959):  at android.app.ActivityThread.main(ActivityThread.java:5103)
02-03 16:19:48.322: E/AndroidRuntime(959):  at java.lang.reflect.Method.invokeNative(Native Method)
02-03 16:19:48.322: E/AndroidRuntime(959):  at java.lang.reflect.Method.invoke(Method.java:525)
02-03 16:19:48.322: E/AndroidRuntime(959):  at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:737)
02-03 16:19:48.322: E/AndroidRuntime(959):  at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:553)
02-03 16:19:48.322: E/AndroidRuntime(959):  at dalvik.system.NativeStart.main(Native Method)
02-03 16:19:48.322: E/AndroidRuntime(959): Caused by: java.lang.reflect.InvocationTargetException
02-03 16:19:48.322: E/AndroidRuntime(959):  at java.lang.reflect.Method.invokeNative(Native Method)
02-03 16:19:48.322: E/AndroidRuntime(959):  at java.lang.reflect.Method.invoke(Method.java:525)
02-03 16:19:48.322: E/AndroidRuntime(959):  at android.view.View$1.onClick(View.java:3628)
02-03 16:19:48.322: E/AndroidRuntime(959):  ... 11 more
02-03 16:19:48.322: E/AndroidRuntime(959): Caused by: java.lang.NullPointerException
02-03 16:19:48.322: E/AndroidRuntime(959):  at com.example.universitybudgetub.MainMenu.populateListViewFromDB(MainMenu.java:184)
02-03 16:19:48.322: E/AndroidRuntime(959):  at com.example.universitybudgetub.MainMenu.addRecord(MainMenu.java:135)
02-03 16:19:48.322: E/AndroidRuntime(959):  ... 14 more

这是我收到的错误消息，我似乎无法在代码中找到 NPE 错误的位置。我将在下面提供 MainMenu.java 和 DBAdpater.java。

MainMenu.java

package com.example.universitybudgetub;

import java.io.File;

import android.database.Cursor;
import android.os.Bundle;
import android.support.v4.app.Fragment;
import android.support.v4.app.FragmentActivity;
import android.support.v4.app.FragmentManager;
import android.support.v4.app.FragmentPagerAdapter;
import android.support.v4.view.ViewPager;
import android.support.v4.widget.SimpleCursorAdapter;
import android.view.Menu;
import android.view.View;
import android.widget.EditText;
import android.widget.ListView;
import android.widget.Toast;


public class MainMenu extends FragmentActivity{

    DBAdapter db = new DBAdapter(this); //Initiate DB class methods
    SectionsPagerAdapter mSectionsPagerAdapter;
    ViewPager mViewPager;


    //OnCreate
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.main_menu);
        mSectionsPagerAdapter = new SectionsPagerAdapter(getSupportFragmentManager());
        mViewPager = (ViewPager) findViewById(R.id.pager);
        mViewPager.setAdapter(mSectionsPagerAdapter);
    }

    public void onClick(View v){
            String destPath = "/data/data/" + getPackageName() + "/databases/ExpensesDB";
            @SuppressWarnings("unused")
            File f = new File(destPath);
    }

    //OnCreateOptionsMenu 
    @Override
    public boolean onCreateOptionsMenu(Menu menu) {
        getMenuInflater().inflate(R.menu.main_menu, menu);
        return true;
    }

    //SectionsPagerAdapter Controls which Fragment is retrieved for each page
    public class SectionsPagerAdapter extends FragmentPagerAdapter {

        public SectionsPagerAdapter(FragmentManager fm) {
            super(fm);
        }

        @Override
        public Fragment getItem(int position) {
            Fragment fragment;
            switch (position) {
                case 0:
                    fragment = new Fragment1();
                    break;
                case 1:
                    fragment = new Fragment2();
                    break;
                case 2:
                    fragment = new Fragment3();
                    break;
                case 3:
                    fragment = new Fragment4();
                    break;
                case 4:
                    fragment = new Fragment5();
                    break;
                default:
                    fragment  = null;
                    break;
            }
            return fragment;
        }

        //Returns the amount of pages to display
        @Override
        public int getCount() {
            // Show 3 total pages.
            return 5;
        }

        //Page Title
        @Override
        public CharSequence getPageTitle(int position) {
            switch (position) {
            case 0:
                return getString(R.string.title_section1);
            case 1:
                return getString(R.string.title_section2);
            case 2:
                return getString(R.string.title_section3);
            case 3:
                return getString(R.string.title_section4);
            case 4:
                return getString(R.string.title_section5);
            }
            return null;
        }
    }

    //Add New Record
    public void addRecord (View v){

        //Get data from form
        EditText foodTxt = (EditText)findViewById(R.id.editText_food);
        EditText taxiTxt = (EditText)findViewById(R.id.editText_taxi);
        EditText clothesTxt = (EditText)findViewById(R.id.editText_clothes);
        EditText sportsTxt = (EditText)findViewById(R.id.editText_sports);
        EditText supermarketTxt = (EditText)findViewById(R.id.editText_supermarket);
        EditText clubs_barsTxt = (EditText)findViewById(R.id.editText_clubs);
        EditText owed_moneyTxt = (EditText)findViewById(R.id.editText_owed_money);
        EditText otherTxt = (EditText)findViewById(R.id.editText_other);
        EditText electric_gasTxt = (EditText)findViewById(R.id.editText_electric_gas);
        EditText waterbillTxt = (EditText)findViewById(R.id.editText_water);
        EditText houseTxt = (EditText)findViewById(R.id.editText_house);
        EditText rentTxt = (EditText)findViewById(R.id.editText_rent);
        EditText internetTxt = (EditText)findViewById(R.id.editText_internet);

        db.open();
        @SuppressWarnings("unused")
        long id = db.insertRecord(foodTxt.getText().toString(), clothesTxt.getText().toString(), 
                supermarketTxt.getText().toString(), taxiTxt.getText().toString(), sportsTxt.getText().toString(), 
                clubs_barsTxt.getText().toString(), owed_moneyTxt.getText().toString(), otherTxt.getText().toString(), 
                electric_gasTxt.getText().toString(), waterbillTxt.getText().toString(), rentTxt.getText().toString(),
                houseTxt.getText().toString(), internetTxt.getText().toString()); 

        populateListViewFromDB(); //Re-populate the list after new record is added

        db.close();

        //Set editText fields to empty
        foodTxt.setText("");
        taxiTxt.setText("");
        clothesTxt.setText("");
        sportsTxt.setText("");
        supermarketTxt.setText("");
        clubs_barsTxt.setText("");
        owed_moneyTxt.setText("");
        otherTxt.setText("");
        electric_gasTxt.setText("");
        waterbillTxt.setText("");
        houseTxt.setText("");
        rentTxt.setText("");
        internetTxt.setText("");
        Toast.makeText(MainMenu.this, "Expenses Added", Toast.LENGTH_LONG).show();
    }

    private void openDB(){
        db = new DBAdapter(this);
        db.open();
    }

    private void populateListViewFromDB() {
        Cursor cursor = db.getAllRows();

        //Allow activity to manage lifetime of the cursor
        //DEPRECATED!
        startManagingCursor(cursor);

        //Setup mapping from cursor to view fields
        String[] fromFieldNames = new String[] 
                {DBAdapter.KEY_ROWID, DBAdapter.KEY_FOOD, DBAdapter.KEY_CLOTHES, DBAdapter.KEY_SUPERMARKET, DBAdapter.KEY_TAXI, DBAdapter.KEY_SPORTS,
                    DBAdapter.KEY_CLUBS_BARS, DBAdapter.KEY_OWED_MONEY, DBAdapter.KEY_OTHER, DBAdapter.KEY_ELECTRIC_GAS, DBAdapter.KEY_WATERBILL,
                        DBAdapter.KEY_RENT, DBAdapter.KEY_HOUSE, DBAdapter.KEY_INTERNETBILL,};
        int[] toViewIDs = new int[]
                {R.id.textView_itemid, R.id.textView_itemfood, R.id.textView_itemclothes, R.id.textView_itemsupermarket, R.id.textView_itemtaxi, R.id.textView_itemsports,
                    R.id.textView_itemclubs, R.id.textView_itemowed, R.id.textView_itemother, R.id.textView_itemelectric, R.id.textView_itemwater, 
                        R.id.textView_itemrent, R.id.textView_itemhouse, R.id.textView_iteminternet,};

        //Create Adapter to map columns of the DB into elements in the UI
        SimpleCursorAdapter myCursorAdapter = new SimpleCursorAdapter(
                this, //Context
                R.layout.history_expenses_layout, //Row layout template
                cursor, //cursor (set of DB REcords to map)
                fromFieldNames, //DB Column names
                toViewIDs // View IDs to put information in
                );

        //Set the adapter for the list view
        ListView myList = (ListView) findViewById(R.id.listViewFromDB);
        myList.setAdapter(myCursorAdapter);
    }


}

DBAdapter.java

package com.example.universitybudgetub;

import android.content.ContentValues;
import android.content.Context;
import android.database.Cursor;
import android.database.SQLException;
import android.database.sqlite.SQLiteDatabase;
import android.database.sqlite.SQLiteOpenHelper;
import android.util.Log;


public class DBAdapter {
    public static final String KEY_ROWID = "_id";
    public static final String KEY_FOOD = "food";
    public static final String KEY_CLOTHES = "clothes";
    public static final String KEY_SUPERMARKET = "supermarket";
    public static final String KEY_TAXI= "taxi";
    public static final String KEY_SPORTS = "sports";
    public static final String KEY_CLUBS_BARS = "clubs_bars";
    public static final String KEY_OWED_MONEY = "owed_money";
    public static final String KEY_OTHER = "other";
    public static final String KEY_ELECTRIC_GAS = "electric_gas";
    public static final String KEY_WATERBILL = "waterbill";
    public static final String KEY_RENT = "rent";
    public static final String KEY_HOUSE = "house";
    public static final String KEY_INTERNETBILL = "internetbill";
    private static final String TAG = "DBAdapter";

    public static final String[] ALL_KEYS = new String[] {KEY_ROWID, KEY_FOOD, KEY_CLOTHES, KEY_SUPERMARKET, KEY_TAXI, KEY_SPORTS,
    KEY_CLUBS_BARS, KEY_OWED_MONEY, KEY_OTHER, KEY_ELECTRIC_GAS, KEY_WATERBILL, KEY_RENT, KEY_HOUSE, KEY_INTERNETBILL};

    private static String DATABASE_NAME = "ExpensesDB";
    private static final String DATABASE_TABLE = "expenses";
    private static final int DATABASE_VERSION = 4;

    private static final String DATABASE_CREATE = 
            "create table if not exists expenses (_id integer primary key autoincrement, " +
    "food VARCHAR, clothes VARCHAR, supermarket VARCHAR, taxi VARCHAR, sports VARCHAR, clubs_bars VARCHAR, owed_money VARCHAR, other VARCHAR, electric_gas VARCHAR, waterbill VARCHAR, rent VARCHAR, house VARCHAR, internetbill VARCHAR);";

private final Context context;

private DatabaseHelper DBHelper;
private SQLiteDatabase db;

public DBAdapter(Context ctx){
    this.context = ctx;
    DBHelper = new DatabaseHelper(context);
}

//Instantiate's all the database methods
public static class DatabaseHelper extends SQLiteOpenHelper {
    DatabaseHelper(Context context){
        super (context, DATABASE_NAME, null, DATABASE_VERSION);
    }

    @Override
    public void onCreate(SQLiteDatabase db){
        try{
            db.execSQL(DATABASE_CREATE);
        } catch (SQLException e) {
            e.printStackTrace();
        }
    }

    @Override 
    public void onUpgrade(SQLiteDatabase db, int oldVersion, int newVersion){
        Log.w(TAG, "Upgrading data from version " + oldVersion + " to " + newVersion + " , which wll destroy all old data");
        db.execSQL("DROP TABLE IF EXISTS contacts");
        onCreate(db);
    }
}

//Opens database
public DBAdapter open() throws SQLException{
    db = DBHelper.getWritableDatabase();
    return this;
}

//Close database
public void close(){
    DBHelper.close();
}

//Insert record to database
public long insertRecord(String food, String clothes, String supermarket, String taxi, String sports, String clubs_bars, 
        String owed_money, String other, String electric_gas, String waterbill, String rent, String house, String internetbill){
    ContentValues initialValues = new ContentValues();
    initialValues.put(KEY_FOOD, food);
    initialValues.put(KEY_CLOTHES, clothes);
    initialValues.put(KEY_SUPERMARKET, supermarket);
    initialValues.put(KEY_TAXI, taxi);
    initialValues.put(KEY_SPORTS, sports);
    initialValues.put(KEY_CLUBS_BARS, clubs_bars);
    initialValues.put(KEY_OWED_MONEY, owed_money);
    initialValues.put(KEY_OTHER, other);
    initialValues.put(KEY_ELECTRIC_GAS, electric_gas);
    initialValues.put(KEY_WATERBILL, waterbill);
    initialValues.put(KEY_RENT, rent);
    initialValues.put(KEY_HOUSE, house);
    initialValues.put(KEY_INTERNETBILL, internetbill);
    return db.insert(DATABASE_TABLE, null, initialValues);
}

//Delete particular record
public boolean deleteRecord(long rowId)
{
    return db.delete(DATABASE_TABLE, KEY_ROWID + "=" + rowId, null) > 0;
}

//Retrieve particular record
public Cursor getRecord(long rowId) throws SQLException
{
    Cursor mCursor = db.query(true, DATABASE_TABLE, new String[] {KEY_ROWID, KEY_FOOD, KEY_CLOTHES, KEY_TAXI, KEY_SPORTS, 
            KEY_CLUBS_BARS, KEY_OWED_MONEY, KEY_OTHER, KEY_ELECTRIC_GAS, KEY_WATERBILL, KEY_RENT, KEY_HOUSE, KEY_INTERNETBILL},
            KEY_ROWID + "=" + rowId, null, null, null, null, null);
    if (mCursor != null) {
            mCursor.moveToFirst();
    }
    return mCursor;
}

//Update particular record
public boolean updateRecord(long rowId, int food, int clothes, int taxi, int sports, int clubs_bars, int owed_money, int other,
        int electric_gas, int waterbill, int rent, int house, int internetbill){
    ContentValues args = new ContentValues();
    args.put(KEY_FOOD, food);
    args.put(KEY_CLOTHES, clothes);
    args.put(KEY_TAXI, taxi);
    args.put(KEY_SPORTS, sports);
    args.put(KEY_CLUBS_BARS, clubs_bars);
    args.put(KEY_OWED_MONEY, owed_money);
    args.put(KEY_OTHER, other);
    args.put(KEY_ELECTRIC_GAS, electric_gas);
    args.put(KEY_WATERBILL, waterbill);
    args.put(KEY_RENT, rent);
    args.put(KEY_HOUSE, house);
    args.put(KEY_INTERNETBILL, internetbill);
    return db.update(DATABASE_TABLE, args, KEY_ROWID + "=" + rowId, null) > 0;  
}

// Return all data in the database.
public Cursor getAllRows() {
    String where = null;
    Cursor c =  db.query(true, DATABASE_TABLE, ALL_KEYS, 
                        where, null, null, null, null, null);
    if (c != null) {
        c.moveToFirst();
    }
    return c;
}


}

我已将 ListView 放置在名为fragment5 的java 类中，并使用id listViewFromDB 对其进行命名。我想做的是在fragment5 上的列表中显示数据库信息。我不确定 fragment 是否会影响 MainMenu.java 找到 ListViewFromDB id 的方式，因为这可能会导致 NPE。

任何帮助都会很棒!

Answer 1

private void openDB(){
    db = new DBAdapter(this);
    db.open();
}

此方法不会被调用，因此当您尝试访问 db 实例时，它会抛出空指针异常。