当我尝试打开使用 SQLite 的活动时，Contact Manager App 崩溃答案

【问题标题】：Contact Manager App crashing when I try to open an activity that uses SQLite当我尝试打开使用 SQLite 的活动时，Contact Manager App 崩溃
【发布时间】：2016-06-15 06:19:09
【问题描述】：

所以基本上我是一个真正的业余爱好者，对编程非常感兴趣。我精通核心 Java 甚至 Swing。现在我开始为我和我的编程爱好者朋友正在研究的一个更大的项目开发一个模块，我需要一些帮助。我正在开发一个非常简单的联系人管理器活动，它使用 ViewPager 显示三个屏幕，即只有两个标签的介绍屏幕、添加联系人屏幕和列表联系人屏幕。现在，打开寻呼机 FragmentActivity 的一切都进行得很顺利，但是一旦我将寻呼机设置为打开添加联系人 Fragment，应用程序就会崩溃。我正在使用简单的 SQLite 方式，现在正在附加 Fragment 类、数据库适配器类和数据库帮助器类，以及来自 logcat 的错误消息。请帮忙。

ContactEdit.class

package com.example.nikhil.test1;

import android.os.Bundle;
import android.support.v4.app.Fragment;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;
import android.widget.Button;
import android.widget.EditText;

public class ContactEdit extends Fragment {
    ContactsAdapter databaseAdapter = new ContactsAdapter(this.getActivity());
    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container,
                             Bundle savedInstanceState) {
        ViewGroup rootView = (ViewGroup) inflater.inflate(
                R.layout.fragment_contact_edit, container, false);
databaseAdapter.open();

        Button SaveBTN = (Button) getActivity().findViewById(R.id.BtnSave);
        SaveBTN.setOnClickListener(new View.OnClickListener() {
            public void onClick(View v) {
                EditText _Name = (EditText) getView().findViewById(R.id.textName);
                EditText _Mobile = (EditText) getView().findViewById(R.id.textMobile);
                EditText _Home = (EditText) getView().findViewById(R.id.textHome);
                EditText _Address = (EditText) getView().findViewById(R.id.textAddress);

                String name = _Name.getText().toString();
                String mobile = _Mobile.getText().toString();
                String home = _Home.getText().toString();
                String address = _Address.getText().toString();

             databaseAdapter.addFriend(name, address, mobile, home);

            }
        });
databaseAdapter.close();
        return rootView;

    }

}

ContactsAdapter.class

package com.example.nikhil.test1;
import android.content.ContentValues;
import android.content.Context;
import android.database.*;
import android.database.sqlite.*;
public class ContactsAdapter {
    private static final String DATABASE_NAME ="myDatabase.db";
    private static final String DATABASE_TABLE ="mainTable";
    private static final int DATABASE_VERSION = 1;


     private SQLiteDatabase db;

    private final Context context;

    private MyDatabaseHelper dbHelper;
    public ContactsAdapter(Context _context){
        context = _context;
        dbHelper = new MyDatabaseHelper(context, DATABASE_NAME, null,DATABASE_VERSION);
    }
    public ContactsAdapter open() throws SQLException {
        try {
            db = dbHelper.getWritableDatabase();
        }
        catch(SQLException ex){
            db=dbHelper.getReadableDatabase();
        }
        return this;
    }
    public void close() {
        db.close();
    }
    public void addFriend(String name, String address, String mobile, String home)

    {

        ContentValues values=new ContentValues(4);

        values.put("name", name);
        values.put("address", address);
        values.put("mobile", mobile);
        values.put("home", home);

        db.insert(DATABASE_TABLE, null, values);

    }
    public Cursor getFriend(int id)
    {
        Cursor res = db.rawQuery( "SELECT * FROM " + DATABASE_TABLE + " WHERE _id=", new String[] { Integer.toString(id) } );
        return res;
    }
    public Cursor getAllFriends() {
        Cursor res = db.rawQuery( "SELECT * FROM friends", null );
        return res;
    }
}

MyDatabaseHelper.class

package com.example.nikhil.test1;

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


public class MyDatabaseHelper extends SQLiteOpenHelper {
    private static final String DATABASE_TABLE ="mainTable";
    public static final String KEY_ID="_id";
    public static final String KEY_NAME="name";
    public static final int NAME_COLUMN = 1;


    public static final String KEY_Phone="phone";
    public static final int Phone_COLUMN = 2;


    public static final String KEY_Home="home";
    public static final int Home_COLUMN = 3;


    public static final String KEY_Address="address";
    public static final int Address_COLUMN = 4;

    private static final String DATABASE_CREATE = "create table " +
            DATABASE_TABLE + " (" + KEY_ID +" integer primary key autoincrement, " +KEY_NAME + " text not null,"+KEY_Phone+" text not null,"+KEY_Home+" text,"+KEY_Address+" text);";

    private SQLiteDatabase db;
    public MyDatabaseHelper(Context context, String name,
                            SQLiteDatabase.CursorFactory factory, int version) {

        super(context, name, factory, version);

    }



    @Override
    public void onCreate(SQLiteDatabase database) {

        database.execSQL(DATABASE_CREATE);
    }

    @Override
    public void onUpgrade(SQLiteDatabase db, int oldVersion, int newVersion) {
        db.execSQL("DROP TABLE IF EXISTS "+DATABASE_TABLE+";");

        onCreate(db);
    }



}

这里是logcat错误日志：

06-14 22:31:59.660  30738-30738/com.example.nikhil.test1 E/AndroidRuntime﹕ FATAL EXCEPTION: main
    Process: com.example.nikhil.test1, PID: 30738
    java.lang.NullPointerException: Attempt to invoke virtual method 'android.database.sqlite.SQLiteDatabase android.content.Context.openOrCreateDatabase(java.lang.String, int, android.database.sqlite.SQLiteDatabase$CursorFactory, android.database.DatabaseErrorHandler)' on a null object reference
            at android.database.sqlite.SQLiteOpenHelper.getDatabaseLocked(SQLiteOpenHelper.java:223)
            at android.database.sqlite.SQLiteOpenHelper.getWritableDatabase(SQLiteOpenHelper.java:163)
            at com.example.nikhil.test1.ContactsAdapter.open(ContactsAdapter.java:24)
            at com.example.nikhil.test1.ContactEdit.onCreateView(ContactEdit.java:21)
            at android.support.v4.app.Fragment.performCreateView(Fragment.java:1789)
            at android.support.v4.app.FragmentManagerImpl.moveToState(FragmentManager.java:955)
            at android.support.v4.app.FragmentManagerImpl.moveToState(FragmentManager.java:1138)
            at android.support.v4.app.BackStackRecord.run(BackStackRecord.java:740)
            at android.support.v4.app.FragmentManagerImpl.execPendingActions(FragmentManager.java:1501)
            at android.support.v4.app.FragmentManagerImpl.executePendingTransactions(FragmentManager.java:490)
            at android.support.v4.app.FragmentStatePagerAdapter.finishUpdate(FragmentStatePagerAdapter.java:163)
            at android.support.v4.view.ViewPager.populate(ViewPager.java:1072)
            at android.support.v4.view.ViewPager.populate(ViewPager.java:918)
            at android.support.v4.view.ViewPager.onMeasure(ViewPager.java:1440)
            at android.view.View.measure(View.java:17565)
            at android.widget.RelativeLayout.measureChildHorizontal(RelativeLayout.java:728)
            at android.widget.RelativeLayout.onMeasure(RelativeLayout.java:464)
            at android.view.View.measure(View.java:17565)
            at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:5545)
            at android.widget.FrameLayout.onMeasure(FrameLayout.java:436)
            at android.view.View.measure(View.java:17565)
            at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:5545)
            at android.widget.LinearLayout.measureChildBeforeLayout(LinearLayout.java:1436)
            at android.widget.LinearLayout.measureVertical(LinearLayout.java:722)
            at android.widget.LinearLayout.onMeasure(LinearLayout.java:613)
            at android.view.View.measure(View.java:17565)
            at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:5545)
            at android.widget.FrameLayout.onMeasure(FrameLayout.java:436)
            at com.android.internal.policy.impl.PhoneWindow$DecorView.onMeasure(PhoneWindow.java:2652)
            at android.view.View.measure(View.java:17565)
            at android.view.ViewRootImpl.performMeasure(ViewRootImpl.java:2027)
            at android.view.ViewRootImpl.measureHierarchy(ViewRootImpl.java:1185)
            at android.view.ViewRootImpl.performTraversals(ViewRootImpl.java:1391)
            at android.view.ViewRootImpl.doTraversal(ViewRootImpl.java:1073)
            at android.view.ViewRootImpl$TraversalRunnable.run(ViewRootImpl.java:5903)
            at android.view.Choreographer$CallbackRecord.run(Choreographer.java:828)
            at android.view.Choreographer.doCallbacks(Choreographer.java:588)
            at android.view.Choreographer.doFrame(Choreographer.java:558)
            at android.view.Choreographer$FrameDisplayEventReceiver.run(Choreographer.java:814)
            at android.os.Handler.handleCallback(Handler.java:739)
            at android.os.Handler.dispatchMessage(Handler.java:95)
            at android.os.Looper.loop(Looper.java:135)
            at android.app.ActivityThread.main(ActivityThread.java:5268)
            at java.lang.reflect.Method.invoke(Native Method)
            at java.lang.reflect.Method.invoke(Method.java:372)
            at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:902)
            at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:697)

我知道这可能是非常愚蠢和明显的事情，但请耐心等待并帮助朋友。安息吧！

【问题讨论】：

使用带断点的调试器并开始调试
试试这个stackoverflow.com/questions/34834620/…
What is a NullPointerException, and how do I fix it?的可能重复

标签： android sqlite android-fragments android-viewpager

【解决方案1】：

这样写：

public class ContactEdit extends Fragment {

@Override
public View onCreateView(LayoutInflater inflater, ViewGroup container,
                         Bundle savedInstanceState) {
    ViewGroup rootView = (ViewGroup) inflater.inflate(
            R.layout.fragment_contact_edit, container, false);
    ContactsAdapter databaseAdapter = new ContactsAdapter(this.getActivity());
    databaseAdapter.open();

    // Rest of your code..!!
}

在onCreateView() 中初始化ContactsAdapter 将有助于getActivity() 获取活动

编辑 1：

而不是：

 Button SaveBTN = (Button) getActivity().findViewById(R.id.BtnSave);

写这个：

 Button SaveBTN = (Button) rootView.findViewById(R.id.BtnSave);

【讨论】：

【解决方案2】：

问题出在这一行：

ContactsAdapter databaseAdapter = new ContactsAdapter(this.getActivity());

这在构造时运行，此时您的 Fragment 未附加到任何 Activity 并且getActivity() 返回 null。您需要在生命周期回调方法之一中实例化databaseAdapter，例如onActivityCreated()。

另外，最好让您的 SQLiteOpenHelper 实现成为一个单例，它使用单个 SQLiteDatabase 进行所有操作。

【讨论】：