这个问题可能已经在某个地方解决了，但我无法正确解决，所以请帮助我刚开始使用数据库，所以我对此知之甚少。
我正在尝试从创建的数据库中检索数据，并且调用了一个空指针异常。
这是显示列表 ID 的类。
ReviewList.java
public class ReviewList extends ListActivity{
private ReviewsDataSource datasource;
// Place Details
PlaceDetails placeDetails;
@Override
public void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.review);
Button b = (Button) findViewById(R.id.reviewbutton);
datasource = new ReviewsDataSource(this);
datasource.open();
List<Review> values = datasource.getAllReviews(placeDetails.result.reference);
// Use the SimpleCursorAdapter to show the
// elements in a ListView
ArrayAdapter<Review> adapter = new ArrayAdapter<Review>(this,
android.R.layout.simple_list_item_1, values);
setListAdapter(adapter);
b.setOnClickListener(new OnClickListener() {
@Override
public void onClick(View arg0) {
// TODO Auto-generated method stub
@SuppressWarnings("unchecked")
ArrayAdapter<Review> adapter = (ArrayAdapter<Review>) getListAdapter();
Review review = null;
EditText et = (EditText) findViewById(R.id.reviewtext);
String text= et.getText().toString();
review=datasource.createReview(text, placeDetails.result.reference);
adapter.add(review);
adapter.notifyDataSetChanged();
}
});
}
@Override
protected void onResume() {
datasource.open();
super.onResume();
}
@Override
protected void onPause() {
datasource.close();
super.onPause();
}
}
这是复习课
public class Review {
private String id;
private String review;
public String getId() {
return id;
}
public void setId(String id) {
this.id = id;
}
public String getReview() {
return review;
}
public void setReview(String review) {
this.review = review;
}
// Will be used by the ArrayAdapter in the ListView
@Override
public String toString() {
return review;
}
}
这是进行所有数据库操作的ReviewsDataSource.java。
public class ReviewsDataSource {
private SQLiteDatabase database;
private Database dbHelper;
private String[] allColumns = { Database.COLUMN_ID,
Database.COLUMN_REVIEW };
public ReviewsDataSource(Context context) {
dbHelper = new Database(context);
}
public void open() throws SQLException {
database = dbHelper.getWritableDatabase();
}
public void close() {
dbHelper.close();
}
public Review createReview(String review, String reference) {
ContentValues values = new ContentValues();
values.put(Database.COLUMN_REVIEW, review);
values.put(Database.COLUMN_ID, reference);
database.insert(Database.TABLE_REVIEWS, null, values);
Cursor cursor = database.query(Database.TABLE_REVIEWS,
allColumns, Database.COLUMN_ID + " = " + reference, null,
null, null, null);
cursor.moveToFirst();
Review newReview = cursorToReview(cursor);
cursor.close();
return newReview;
}
public void deleteReview(Review comment) {
String id = comment.getId();
System.out.println("Comment deleted with id: " + id);
database.delete(Database.TABLE_REVIEWS, Database.COLUMN_ID
+ " = " + id, null);
}
public List<Review> getAllReviews(String reference) {
List<Review> comments = new ArrayList<Review>();
Cursor cursor = database.query(Database.TABLE_REVIEWS,
allColumns, null, null, null, null, null);
cursor.moveToFirst();
while (!cursor.isAfterLast()) {
Review review = cursorToReview(cursor);
if(cursor.getString(0)==reference)
{
comments.add(review);
}
cursor.moveToNext();
}
// Make sure to close the cursor
cursor.close();
return comments;
}
private Review cursorToReview(Cursor cursor) {
// TODO Auto-generated method stub
Review review = new Review();
review.setId(cursor.getString(0));
review.setReview(cursor.getString(1));
return review;
}
}
最后是 database.java 类
public class Database extends SQLiteOpenHelper {
public static final String TABLE_REVIEWS = "reviews";
public static final String COLUMN_ID = "_id";
public static final String COLUMN_REVIEW = "REVIEW";
private static final String DATABASE_NAME = "reviews.db";
private static final int DATABASE_VERSION = 1;
private static final String DATABASE_CREATE = "create table "
+ TABLE_REVIEWS + "(" + COLUMN_ID
+ " text not null, " + COLUMN_REVIEW
+ " text not null);";
public Database(Context context) {
super(context, DATABASE_NAME, null, DATABASE_VERSION);
}
@Override
public void onCreate(SQLiteDatabase database) {
// TODO Auto-generated method stub
database.execSQL(DATABASE_CREATE);
}
@Override
public void onUpgrade(SQLiteDatabase db, int oldVersion, int newVersion) {
// TODO Auto-generated method stub
Log.w(Database.class.getName(),
"Upgrading database from version " + oldVersion + " to "
+ newVersion + ", which will destroy all old data");
db.execSQL("DROP TABLE IF EXISTS " + TABLE_REVIEWS);
onCreate(db);
}
}
这是我的日志猫
02-21 10:49:08.957: E/AndroidRuntime(24586): FATAL EXCEPTION: main
02-21 10:49:08.957: E/AndroidRuntime(24586):
java.lang.RuntimeException: Unable to start activity
ComponentInfo{com.example.fueled_assignment_1/com.example.fueled_assignment_1.ReviewList}:
java.lang.NullPointerException 02-21 10:49:08.957:
E/AndroidRuntime(24586): at
android.app.ActivityThread.performLaunchActivity(ActivityThread.java:1872)
02-21 10:49:08.957: E/AndroidRuntime(24586): at
android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:1893)
02-21 10:49:08.957: E/AndroidRuntime(24586): at
android.app.ActivityThread.access$1500(ActivityThread.java:135) 02-21
10:49:08.957: E/AndroidRuntime(24586): at
android.app.ActivityThread$H.handleMessage(ActivityThread.java:1054)
02-21 10:49:08.957: E/AndroidRuntime(24586): at
android.os.Handler.dispatchMessage(Handler.java:99) 02-21
10:49:08.957: E/AndroidRuntime(24586): at
android.os.Looper.loop(Looper.java:150) 02-21 10:49:08.957:
E/AndroidRuntime(24586): at
android.app.ActivityThread.main(ActivityThread.java:4389) 02-21
10:49:08.957: E/AndroidRuntime(24586): at
java.lang.reflect.Method.invokeNative(Native Method) 02-21
10:49:08.957: E/AndroidRuntime(24586): at
java.lang.reflect.Method.invoke(Method.java:507) 02-21 10:49:08.957:
E/AndroidRuntime(24586): at
com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:849)
02-21 10:49:08.957: E/AndroidRuntime(24586): at
com.android.internal.os.ZygoteInit.main(ZygoteInit.java:607) 02-21
10:49:08.957: E/AndroidRuntime(24586): at
dalvik.system.NativeStart.main(Native Method) 02-21 10:49:08.957:
E/AndroidRuntime(24586): **Caused by: java.lang.NullPointerException
02-21 10:49:08.957: E/AndroidRuntime(24586): at
com.example.fueled_assignment_1.ReviewList.onCreate(ReviewList.java:26)**
02-21 10:49:08.957: E/AndroidRuntime(24586): at
android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1072)
02-21 10:49:08.957: E/AndroidRuntime(24586): at
android.app.ActivityThread.performLaunchActivity(ActivityThread.java:1836)
02-21 10:49:08.957: E/AndroidRuntime(24586): ... 11 more
请帮我解决我一团糟的问题