我在点击按钮时遇到Null Pointer Exception。以下是错误日志和代码。请帮助。提前致谢。
LOGCAT：
03-15 11:56:06.611: E/AndroidRuntime(856): FATAL EXCEPTION: main
03-15 11:56:06.611: E/AndroidRuntime(856): java.lang.NullPointerException
03-15 11:56:06.611: E/AndroidRuntime(856): at com.android.database.Database_Manipulation.insertTitle(Database_Manipulation.java:58)
03-15 11:56:06.611: E/AndroidRuntime(856): at com.android.database.Task3Activity$1.onClick(Task3Activity.java:49)
03-15 11:56:06.611: E/AndroidRuntime(856): at android.view.View.performClick(View.java:3511)
03-15 11:56:06.611: E/AndroidRuntime(856): at android.view.View$PerformClick.run(View.java:14105)
03-15 11:56:06.611: E/AndroidRuntime(856): at android.os.Handler.handleCallback(Handler.java:605)
03-15 11:56:06.611: E/AndroidRuntime(856): at android.os.Handler.dispatchMessage(Handler.java:92)
03-15 11:56:06.611: E/AndroidRuntime(856): at android.os.Looper.loop(Looper.java:137)
03-15 11:56:06.611: E/AndroidRuntime(856): at android.app.ActivityThread.main(ActivityThread.java:4424)
03-15 11:56:06.611: E/AndroidRuntime(856): at java.lang.reflect.Method.invokeNative(Native Method)
03-15 11:56:06.611: E/AndroidRuntime(856): at java.lang.reflect.Method.invoke(Method.java:511)
03-15 11:56:06.611: E/AndroidRuntime(856): at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:784)
03-15 11:56:06.611: E/AndroidRuntime(856): at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:551)
03-15 11:56:06.611: E/AndroidRuntime(856): at dalvik.system.NativeStart.main(Native Method)
03-15 11:56:08.102: I/Process(856): Sending signal. PID: 856 SIG: 9
CODE：
public class Database_Manipulation extends Activity
{
public static final String KEY_ROWID="id";
public static final String KEY_ISBN="isbn";
public static final String KEY_TITLE="title";
public static final String KEY_PUBLISHER="publisher";
private static final String TAG="DBAdapter";
private static final String DATABASE_NAME="books";
private static final String DATABASE_TABLE="titles";
private static final int DATABASE_VERSION=1;
private static final String DATABASE_CREATE="create table titles(id integer primary key autoincrement,"+"isbn text not null,title text not null,"+"publisher text not null);";
private final Context context;
private DatabaseHelper DBhelper;
private SQLiteDatabase db;
public Database_Manipulation(Context ctx)
{
this.context=ctx;
DBhelper=new DatabaseHelper(context);
}
public Database_Manipulation open() throws SQLException
{
db=DBhelper.getWritableDatabase();
return this;
}
public void close()
{
DBhelper.close();
}
public long insertTitle(String isbn,String title,String publisher)
{
ContentValues initialvalues=new ContentValues();
initialvalues.put(KEY_ISBN,isbn);
initialvalues.put(KEY_TITLE, title);
initialvalues.put(KEY_PUBLISHER, publisher);
return db.insert(DATABASE_TABLE, null, initialvalues);
}
private static class DatabaseHelper extends SQLiteOpenHelper
{
public DatabaseHelper(Context context)
{
// TODO Auto-generated constructor stub
super(context, DATABASE_NAME, null, DATABASE_VERSION);
}
@Override
public void onCreate(SQLiteDatabase db) {
// TODO Auto-generated method stub
db.execSQL(DATABASE_CREATE);
}
@Override
public void onUpgrade(SQLiteDatabase db, int oldVersion, int newVersion) {
// TODO Auto-generated method stub
Log.w(TAG,"UPGRADING"+oldVersion+"to"+newVersion+"which will destroy every old data");
db.execSQL("DROP TABLE IF EXISTS titles");
onCreate(db);
}
}
}
//calling the insert title method from the class below at the click event...Everything runs fine but when i click on save button
it shows the above given log stack....HERE iS THE CLASS
public class Task3Activity extends Activity {
/** Called when the activity is first created. */
Context ctx;
private String name_get=null,address_get=null,contact_no_get=null;
public void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.main);
//getting the name
final EditText name=(EditText)findViewById(R.id.editText1);
//getting the address
final EditText address=(EditText)findViewById(R.id.editText2);
//getting the contact no
final EditText contactno=(EditText)findViewById(R.id.editText3);
name_get=name.getText().toString();
address_get=address.getText().toString();
contact_no_get=contactno.getText().toString();
Toast.makeText(getApplicationContext(),name_get+address_get+contact_no_get,Toast.LENGTH_LONG).show();
//buttons
final Button save=(Button)findViewById(R.id.button1);
final Button update=(Button)findViewById(R.id.button2);
final Button delete=(Button)findViewById(R.id.button3);
//SAVE BUTTON
save.setOnClickListener(new View.OnClickListener() {
@Override
public void onClick(View v) {
long ret=0;
Database_Manipulation dm=new Database_Manipulation(ctx);
if(dm!=null)
{ret=dm.insertTitle(name_get,address_get,contact_no_get);
Toast.makeText(getApplicationContext(),"recieved somthing",Toast.LENGTH_SHORT).show();
}
else
Toast.makeText(getApplicationContext(), "empty context",Toast.LENGTH_SHORT).show();
}
});
//UPDATE BUTTON
update.setOnClickListener(new View.OnClickListener() {
@Override
public void onClick(View v) {
Toast.makeText(getApplicationContext(), address_get,Toast.LENGTH_SHORT).show();
}
});
//DELETE BUTTON
delete.setOnClickListener(new View.OnClickListener() {
@Override
public void onClick(View v) {
Toast.makeText(getApplicationContext(), contact_no_get,Toast.LENGTH_SHORT).show();
}
});
}}
答案 0 :(得分：1)
我认为在这种方法中（如果我没有错），
public long insertTitle(String isbn,String title,String publisher)
{
ContentValues initialvalues=new ContentValues();
initialvalues.put(KEY_ISBN,isbn);
initialvalues.put(KEY_TITLE, title);
initialvalues.put(KEY_PUBLISHER, publisher);
return db.insert(DATABASE_TABLE, null, initialvalues);
}
db对象为空 ..
只需使用可写数据库初始化它。
<强>更新
把这一行，
if (db == null || !db.isOpen())
{
db = DBhelper.getWritableDatabase();
}
insertTitle方法中，在数据库中插入值之前，
答案 1 :(得分：1)
我认为您应该检查正在使用的未初始化对象 检查必要的permisions