android - 当我尝试打开使用 SQLite 的 Activity 时，联系人管理器应用程序崩溃

Question

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。现在我开始为一个更大的项目开发一个模块，这个项目是我和我的编程爱好者伙伴正在进行的，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 Activity ，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 之前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，现在我附加了 Fragment 类、数据库适配器类和数据库助手类，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)

我意识到这可能是非常愚蠢和明显的事情，但请耐心等待并帮助 friend 。和平!

Answer 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 将有助于通过 getActivity() 获取 Activity

EDIT 1:

代替:

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

这样写:

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