小贝子编程

Error java.lang.NullPointerException in SQLiteDatabase Andro



你能帮我解决错误"java.lang.NullPointerException"吗?我试图保存一些数据(姓名和电子邮件)在数据库中，然后加载它。当我试图启动应用程序时，它在模拟器中崩溃了。我寻找答案，撞到这个线程:在getReadableDatabase()方法上出现java.lang.NullPointerException错误，但没有解决问题。

MainActivity.java 

package com.example.test;
import android.support.v7.app.ActionBarActivity;
import android.support.v7.app.ActionBar;
import android.support.v4.app.Fragment;
import android.database.Cursor;
import android.os.Bundle;
import android.view.LayoutInflater;
import android.view.Menu;
import android.view.MenuItem;
import android.view.MotionEvent;
import android.view.View;
import android.view.View.OnTouchListener;
import android.view.ViewGroup;
import android.widget.Button;
import android.widget.EditText;
import android.widget.Toast;
import android.os.Build;
public class MainActivity extends ActionBarActivity {
Button save, load;
EditText name, email;
DataHandler handler;
@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);
    save = (Button)findViewById(R.id.save);
    load = (Button)findViewById(R.id.load);
    name = (EditText)findViewById(R.id.name);
    email = (EditText)findViewById(R.id.email);
    save.setOnTouchListener(new OnTouchListener(){
        public void onClick(View v){
        }
        @Override
        public boolean onTouch(View v, MotionEvent event) {
            // TODO Auto-generated method stub
            String getName = name.getText().toString();
            String getEmail = email.getText().toString();
            handler = new DataHandler(getBaseContext());
            handler.open();
            long id = handler.insertData(getName, getEmail);
            Toast.makeText(getBaseContext(), "Data inserted", Toast.LENGTH_LONG).show();
            handler.close();
            return false;
        }
    });
    load.setOnTouchListener(new OnTouchListener(){
        @Override
        public boolean onTouch(View v, MotionEvent event) {
            // TODO Auto-generated method stub
            String getName, getEmail;
            getName = "";
            getEmail = "";
            handler = new DataHandler(getBaseContext());
            handler.open();
            Cursor C = handler.returnData();
            if(C.moveToFirst())
            {
                do
                {
                    getName = C.getString(0);
                    getEmail = C.getString(1);
                }while(C.moveToNext());
            }
            handler.close();
            Toast.makeText(getBaseContext(), "Name : "+getName+" and email : "+getEmail, Toast.LENGTH_LONG).show();
            return false;
        }
    });
    if (savedInstanceState == null) {
        getSupportFragmentManager().beginTransaction()
                .add(R.id.container, new PlaceholderFragment())
                .commit();
    }
}

@Override
public boolean onCreateOptionsMenu(Menu menu) {
    // Inflate the menu; this adds items to the action bar if it is present.
    getMenuInflater().inflate(R.menu.main, menu);
    return true;
}
@Override
public boolean onOptionsItemSelected(MenuItem item) {
    // Handle action bar item clicks here. The action bar will
    // automatically handle clicks on the Home/Up button, so long
    // as you specify a parent activity in AndroidManifest.xml.
    int id = item.getItemId();
    if (id == R.id.action_settings) {
        return true;
    }
    return super.onOptionsItemSelected(item);
}
/**
 * A placeholder fragment containing a simple view.
 */
public static class PlaceholderFragment extends Fragment {
    public PlaceholderFragment() {
    }
    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container,
            Bundle savedInstanceState) {
        View rootView = inflater.inflate(R.layout.fragment_main, container, false);
        return rootView;
    }
}
}

DataHandler.java 

package com.example.test;
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.SQLiteDatabase.CursorFactory;
import android.database.sqlite.SQLiteOpenHelper;
public class DataHandler {
public static final String NAME = "name";
public static final String EMAIL = "email";
public static final String TABLE_NAME = "mytable";
public static final String DATA_BASE_NAME = "mydatabase";
public static final int DATABASE_VERSION = 1;
public static final String TABLE_CREATE = "create table  mytable (name text not null, email text not null);";
DataBaseHelper dbhelper;
private static Context ctx;
SQLiteDatabase db;
public DataHandler(Context ctx)
{
    this.ctx = ctx;
    dbhelper = new DataBaseHelper(ctx);
}
private static class DataBaseHelper extends SQLiteOpenHelper
{
    public DataBaseHelper(Context ctx)
    {
        super(ctx, DATA_BASE_NAME, null, DATABASE_VERSION);
    }
    @Override
    public void onCreate(SQLiteDatabase db)
    {
        try{
        db.execSQL("TABLE_CREATE");
        }
        catch(SQLException e)
        {
            e.printStackTrace();
        }
    }
    @Override
    public void onUpgrade(SQLiteDatabase db, int oldVersion, int newVersion)
    {
        db.execSQL("DROP TABLE IF EXISTS mytable");
        onCreate(db);
    }
}
public DataHandler open()
{
    db = dbhelper.getWritableDatabase();
    return this;
}
public void close()
{
    dbhelper.close();
}
public long insertData(String name, String email)
{
    ContentValues content =  new ContentValues();
    content.put(NAME, name);
    content.put(EMAIL, email);
    return db.insertOrThrow(TABLE_NAME, null, content);
}
public Cursor returnData()
{
    return db.query(TABLE_NAME, new String[] {NAME, EMAIL}, null, null, null, null, null);
}
}

activity_main.xml 

<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:tools="http://schemas.android.com/tools"
android:id="@+id/container"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:paddingBottom="@dimen/activity_vertical_margin"
android:paddingLeft="@dimen/activity_horizontal_margin"
android:paddingRight="@dimen/activity_horizontal_margin"
android:paddingTop="@dimen/activity_vertical_margin"
tools:context="com.example.test.MainActivity"   
tools:ignore="MergeRootFrame" > 
<EditText
    android:id="@+id/name"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:layout_below="@+id/textView1"
    android:layout_centerHorizontal="true"
    android:layout_marginTop="36dp"
    android:ems="10"
    android:hint="Enter name here" >
    <requestFocus />
</EditText>
<Button
    android:id="@+id/load"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:layout_alignParentBottom="true"
    android:layout_alignRight="@+id/save"
    android:layout_marginBottom="81dp"
    android:text="LOAD DATA" />
<EditText
    android:id="@+id/email"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:layout_alignLeft="@+id/name"
    android:layout_below="@+id/name"
    android:layout_marginTop="41dp"
    android:ems="10"
    android:hint="Enter email here" />
<Button
    android:id="@+id/save"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:layout_above="@+id/load"
    android:layout_centerHorizontal="true"
    android:layout_marginBottom="42dp"
    android:text="SAVE DATA" />
</RelativeLayout>

LogCat 

05-11 14:09:06.363: E/AndroidRuntime(2259): FATAL EXCEPTION: main
05-11 14:09:06.363: E/AndroidRuntime(2259): Process: com.example.test, PID: 2259
05-11 14:09:06.363: E/AndroidRuntime(2259): java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.test/com.example.test.MainActivity}: java.lang.NullPointerException
05-11 14:09:06.363: E/AndroidRuntime(2259):     at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2195)
05-11 14:09:06.363: E/AndroidRuntime(2259):     at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2245)
05-11 14:09:06.363: E/AndroidRuntime(2259):     at android.app.ActivityThread.access$800(ActivityThread.java:135)
05-11 14:09:06.363: E/AndroidRuntime(2259):     at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1196)
05-11 14:09:06.363: E/AndroidRuntime(2259):     at android.os.Handler.dispatchMessage(Handler.java:102)
05-11 14:09:06.363: E/AndroidRuntime(2259):     at android.os.Looper.loop(Looper.java:136)
05-11 14:09:06.363: E/AndroidRuntime(2259):     at android.app.ActivityThread.main(ActivityThread.java:5017)
05-11 14:09:06.363: E/AndroidRuntime(2259):     at java.lang.reflect.Method.invokeNative(Native Method)
05-11 14:09:06.363: E/AndroidRuntime(2259):     at java.lang.reflect.Method.invoke(Method.java:515)
05-11 14:09:06.363: E/AndroidRuntime(2259):     at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:779)
05-11 14:09:06.363: E/AndroidRuntime(2259):     at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:595)
05-11 14:09:06.363: E/AndroidRuntime(2259):     at dalvik.system.NativeStart.main(Native Method)
05-11 14:09:06.363: E/AndroidRuntime(2259): Caused by: java.lang.NullPointerException
05-11 14:09:06.363: E/AndroidRuntime(2259):     at com.example.test.MainActivity.onCreate(MainActivity.java:34)
05-11 14:09:06.363: E/AndroidRuntime(2259):     at android.app.Activity.performCreate(Activity.java:5231)
05-11 14:09:06.363: E/AndroidRuntime(2259):     at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1087)
05-11 14:09:06.363: E/AndroidRuntime(2259):     at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2159)
05-11 14:09:06.363: E/AndroidRuntime(2259):     ... 11 more

编辑后重新发布DataHandler.java 

package com.example.test;
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.SQLiteDatabase.CursorFactory;
import android.database.sqlite.SQLiteOpenHelper;
public class DataHandler {
public static final String NAME = "name";
public static final String EMAIL = "email";
public static final String TABLE_NAME = "mytable";
public static final String DATA_BASE_NAME = "mydatabase";
public static final int DATABASE_VERSION = 1;
public static final String TABLE_CREATE = "create table  mytable (name text not null, email text not null)";
DataBaseHelper dbhelper;
Context ctx;
SQLiteDatabase db;
public DataHandler(Context ctx)
{
    this.ctx = ctx;
    dbhelper = new DataBaseHelper(ctx);
}
private static class DataBaseHelper extends SQLiteOpenHelper
{
    public DataBaseHelper(Context ctx)
    {
        super(ctx, DATA_BASE_NAME, null, DATABASE_VERSION);
    }
    @Override
    public void onCreate(SQLiteDatabase db)
    {
        try{
        db.execSQL(TABLE_CREATE);
        }
        catch(SQLException e)
        {
            e.printStackTrace();
        }
    }
    @Override
    public void onUpgrade(SQLiteDatabase db, int oldVersion, int newVersion)
    {
        db.execSQL("DROP TABLE IF EXISTS mytable");
        onCreate(db);
    }
}
public DataHandler open()
{
    db = dbhelper.getWritableDatabase();
    return this;
}
public void close()
{
    dbhelper.close();
}
public long insertData(String name, String email)
{
    ContentValues content =  new ContentValues();
    content.put(NAME, name);
    content.put(EMAIL, email);
    return db.insertOrThrow(TABLE_NAME, null, content);
}
public Cursor returnData()
{
    return db.query(TABLE_NAME, new String[] {NAME, EMAIL}, null, null, null, null, null);
}
}

这似乎不是与SQLite相关的问题。

您发布的跟踪显示错误发生在第34行:

save.setOnTouchListener(new OnTouchListener(){

所以当你试图应用事件时，你的save按钮必须为null。

检查第29行是否返回了您和instance:

save = (Button)findViewById(R.id.save);

你做的所有事情都是正确的，但在你的onCreate()或DataHandler.java中遗漏了一件小事。

 @Override
        public void onCreate(SQLiteDatabase db)
        {
            try{
            //db.execSQL("TABLE_CREATE");
             // Here you passing `TABLE_CREATE` string instead of passing `TABLE_CREATE` variable's string. So kindly remove quotes surrounded by 'TABLE_CREATE' and try.
              db.execSQL(TABLE_CREATE);
            }
            catch(SQLException e)
            {
                e.printStackTrace();
            }
        }

然后改成

public static final String TABLE_CREATE = "create table mytable (name text not null, email text not null);";

public static final String TABLE_CREATE = "create table mytable (name text not null, email text not null)";

请使用这个…

save.setOnTouchListener(new View.OnTouchListener() {
    @Override
    public boolean onTouch(View v, MotionEvent event) {
        String getName = name.getText().toString();
        String getEmail = email.getText().toString();
        handler = new DataHandler(getBaseContext());
        handler.open();
        long id = handler.insertData(getName, getEmail);
        Toast.makeText(getBaseContext(), "Data inserted", Toast.LENGTH_LONG).show();
        handler.close();
        return false;
            }
        });
我认为你的ontouch()似乎有问题。请把我的代码，而不是你的代码，它可能是工作

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