通过SQLite Manager更新SQLite数据库列之后，为什么更改未反映在Android App中？

Question

I am writing an android app which uses an SQLite database to store information (id, title, author, and rating) about books. Eventually, the goal is to output that information to the user via CRUD implementation. Right now, I have a class to establish the various attributes of the Books, an SQLHelper class, a ListAdapter for the eventual listview, and of course the MainActivity class to run the app and implement everything.

I am hard-coding entries for the first 3 columns (id, title, and author) into the DB. After that, I'm using the Firefox extension SQLite Manager to update the "ratings" column. Right now however, I cannot get past an error that I have been getting; the app crashes when I attempt to run it and it does not appear that the changes I made in the SQLite Manager are reflected in the Logcat file (I export the DB file with , edit with the manager, then import back into the project).

I can and will work on the presentation of the app, UI, and other aesthetic factors at a later time, right now I would just like to figure out why my database changes aren't being reflected.

The full Logcat output is here , but I believe the pertinent information is this:

11-16 19:32:07.777 2433-2433/? E/AndroidRuntime: FATAL EXCEPTION: main
11-16 19:32:07.777 2433-2433/? E/AndroidRuntime: Process: com.bookreviews.hernandez.bookreviews, PID: 2433
11-16 19:32:07.777 2433-2433/? E/AndroidRuntime: java.lang.NullPointerException: Attempt to invoke virtual method 'java.lang.String java.lang.String.trim()' on a null object reference

From this I gather that the error lies somewhere in my MainActivity.java, but I have yet to determine the cause.

---

MainActivity.java

package com.bookreviews.hernandez.bookreviews;

import android.os.Bundle;
import android.support.design.widget.FloatingActionButton;
import android.support.design.widget.Snackbar;
import android.support.v7.app.AppCompatActivity;
import android.support.v7.widget.Toolbar;
import android.view.View;
import android.view.Menu;
import android.view.MenuItem;

import java.util.ArrayList;
import java.util.List;
import android.app.Activity;
import android.os.Bundle;
import android.util.Log;
import android.widget.ListView;

public class MainActivity extends Activity {

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);

    Log.d("Name", "C H");

    SqlHelper db = new SqlHelper(this);

/** CRUD Operations **/
// add Books
    /*
    db.addBook(new Book("Professional Android 4 Application Development",
    "Reto Meier"));
    db.addBook(new Book("Beginning Android 4 Application Development", "Wei-Meng Lee"));
    db.addBook(new Book("Programming Android", "Wallace Jackson"));
    db.addBook(new Book("Hello, Android", "Wallace Jackson"));
    */
// get all books
    List<Book> list = db.getAllBooks();
// update one book
    int j = db.updateBook(list.get(0), "Hello, Android", "Ben Wallace");
// delete one book
// db.deleteBook(list.get(0));
// get all books
    db.getAllBooks();
    ListView listContent = (ListView) findViewById(R.id.list);
    List<Book> books = new ArrayList<Book>();
    books=db.getAllBooks();
//get data from the table by the ListAdapter
    ListAdapter customAdapter = new ListAdapter(this, R.layout.itemlistrow, books );
    listContent.setAdapter(customAdapter);
// get # of records
    db.getIds(list.get(0));
}

@Override
public boolean onCreateOptionsMenu(Menu menu) {
    // Inflate the menu; this adds items to the action bar if it is present.
    getMenuInflater().inflate(R.menu.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();

    //noinspection SimplifiableIfStatement
    if (id == R.id.action_settings) {
        return true;
    }

    return super.onOptionsItemSelected(item);
}
}

---

Book.java

package com.bookreviews.hernandez.bookreviews;

public class Book {
private int id;
private String title;
private String author;
private String rating;
public void setRating(String rating) {
    this.rating = rating;
}
public String getRating() {
    return rating;
}
public Book(){}
public Book(String title, String author) {
    super();
    this.title = title;
    this.author = author;
}
//getters & setters
public int getId() {
    return id;
}
public void setId(int id) {
    this.id = id;
}
public String getTitle() {
    return title;
}
public void setTitle(String title) {
    this.title = title;
}
public String getAuthor() {
    return author;
}
public void setAuthor(String author) {
    this.author = author;
}
@Override
public String toString() {
    return "Book [id=" + id + ", title=" + title + ", author=" + author
            + ", rating=" + rating + "]";
}
}

---

SqlHelper.java

package com.bookreviews.hernandez.bookreviews;

import java.util.LinkedList;
import java.util.List;
import android.content.ContentValues;
import android.content.Context;
import android.database.Cursor;
import android.database.sqlite.SQLiteDatabase;
import android.database.sqlite.SQLiteOpenHelper;
import android.util.Log;

public class SqlHelper extends SQLiteOpenHelper {
// Database Version
private static final int DATABASE_VERSION = 6;
// Database Name
private static final String DATABASE_NAME = "BookDB";
// Books table name
private static final String TABLE_BOOKS = "books";

// Books Table Columns names
private static final String KEY_ID = "id";
private static final String KEY_TITLE = "title";
private static final String KEY_AUTHOR = "author";
public SqlHelper(Context context) {
    super(context, DATABASE_NAME, null, DATABASE_VERSION);
}
@Override
public void onCreate(SQLiteDatabase db) {
// SQL statement to create book table
    String CREATE_BOOK_TABLE = "CREATE TABLE books ( " +
            "id INTEGER PRIMARY KEY AUTOINCREMENT, " +
            "title TEXT, "+
            "author TEXT, "+
            "rating TEXT)";
    // create books table
    db.execSQL(CREATE_BOOK_TABLE);
}
@Override
public void onUpgrade(SQLiteDatabase db, int oldVersion, int newVersion) {
// Drop older books table if existed
    db.execSQL("DROP TABLE IF EXISTS books");
// create fresh books table
    this.onCreate(db);
}
/*CRUD operations (create "add", read "get", update, delete) */
public void addBook(Book book){
    Log.d("addBook", book.toString());
    // 1. get reference to writable DB
    SQLiteDatabase db = this.getWritableDatabase();
    // 2. create ContentValues to add key "column"/value
    ContentValues values = new ContentValues();
    values.put(KEY_TITLE, book.getTitle()); // get title
    values.put(KEY_AUTHOR, book.getAuthor()); // get author
    // 3. insert
    db.insert(TABLE_BOOKS, // table
            null, //nullColumnHack
            values); // key/value -> keys = column names/values
    // 4. Close dbase
    db.close();
}
// Get All Books
public List<Book> getAllBooks() {
    List<Book> books = new LinkedList<Book>();
// 1. build the query
    String query = "SELECT * FROM " + TABLE_BOOKS;
// 2. get reference to writable DB
    SQLiteDatabase db = this.getWritableDatabase();
    Cursor cursor = db.rawQuery(query, null);
    // 3. go over each row, build book and add it to list
    Book book = null;
    if (cursor.moveToFirst()) {
        do {
            book = new Book();
            book.setId(Integer.parseInt(cursor.getString(0)));
            book.setTitle(cursor.getString(1));
            book.setAuthor(cursor.getString(2));
// Add book to books
            books.add(book);
        } while (cursor.moveToNext());
    }
    Log.d("getAllBooks()", books.toString());
    return books; // return books
}
// Updating single book
public int updateBook(Book book, String newTitle, String newAuthor) {
// 1. get reference to writable DB
    SQLiteDatabase db = this.getWritableDatabase();
// 2. create ContentValues to add key "column"/value
    ContentValues values = new ContentValues();
    values.put("title", newTitle); // get title
    values.put("author", newAuthor); // get author
// 3. updating row
    int i = db.update(TABLE_BOOKS, //table
            values, // column/value
            KEY_ID+" = ?", // selections
            new String[] { String.valueOf(159) }); //selection args
// 4. close dbase
    db.close();
    Log.d("UpdateBook", book.toString());
    return i;
}
// Deleting single book
public void deleteBook(Book book) {
// 1. get reference to writable DB
    SQLiteDatabase db = this.getWritableDatabase();
// 2. delete
    db.delete(TABLE_BOOKS,
            KEY_ID+" = ?",
            new String[] { String.valueOf(book.getId()) });
    // 3. close
    db.close();
    Log.d("deleteBook", book.toString());
}
public int getIds(Book book)
{
    String selectQuery = "SELECT id FROM books";
    SQLiteDatabase database = this.getReadableDatabase();
    Cursor c = database.rawQuery(selectQuery, null);
    c.moveToFirst();
    int total = c.getCount();
    Log.d("getIds", "Count=" + total);

    return total;

}
}

---

ListAdapter.java

package com.bookreviews.hernandez.bookreviews;

import java.util.List;
import android.content.Context;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;
import android.widget.ArrayAdapter;
import android.widget.RatingBar;
import android.widget.TextView;

public class ListAdapter extends ArrayAdapter<Book> {
private List<Book> items;
public ListAdapter(Context context, int textViewResourceId) {
    super(context, textViewResourceId);
}
public ListAdapter(Context context, int resource, List<Book> items) {
    super(context, resource, items);
    this.items = items;
}
@Override
public View getView(int position, View convertView, ViewGroup parent) {
    View v = convertView;
    if (v == null) {
        LayoutInflater vi;
        vi = LayoutInflater.from(getContext());
        v = vi.inflate(R.layout.itemlistrow, null);
    }
    Book p = getItem(position);
    if (p != null) {
        TextView tt = (TextView) v.findViewById(R.id._id);
        TextView tt1 = (TextView) v.findViewById(R.id.title);
        TextView tt3 = (TextView) v.findViewById(R.id.author);
        RatingBar rb = (RatingBar) v.findViewById(R.id.rating);
        if (tt != null) {
            tt.setText("" + p.getId());
        }
        if (tt1 != null) {
            tt1.setText(p.getTitle());
        }
        if (tt3 != null) {
            tt3.setText(p.getAuthor());
        }
        if (rb != null) {
            float rating = Float.parseFloat(p.getRating());
            rb.setRating(rating);
        }
    }
    return v;
}
}

---

itemlistrow.xml

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
android:layout_width="fill_parent"
android:layout_height="fill_parent"
android:orientation="vertical" >
<TextView android:textColor="#000"
    android:id="@+id/_id"
    android:layout_width="fill_parent"
    android:layout_height="wrap_content"
    android:text="id" android:textStyle="bold"
    android:gravity="left"
    android:layout_weight="1"
    android:typeface="monospace"
    android:height="40sp" />
<TextView android:textColor="#000"
    android:id="@+id/title"
    android:layout_width="fill_parent"
    android:layout_height="wrap_content"
    android:text="title"
    android:layout_weight="1"
    android:height="20sp" />
<TextView android:layout_height="wrap_content"
    android:layout_width="fill_parent"
    android:layout_weight="1"
    android:textColor="#000"
    android:gravity="right"
    android:id="@+id/author"
    android:text="author"
    android:height="20sp" />
<RatingBar
    android:id="@+id/rating"
    style="?android:attr/ratingBarStyleSmall"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:paddingTop="6dip"
    android:stepSize="0.25"
    android:numStars="5"
    />
</LinearLayout>

---

activity_main.xml

<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:tools="http://schemas.android.com/tools"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:baselineAligned="false"
android:orientation="vertical" >
<ListView
    android:id="@+id/list"
    android:layout_width="250dp"
    android:layout_marginTop="10dp"
    android:layout_marginBottom="10dp"
    android:layout_height="fill_parent"
    android:layout_gravity="center" >
</ListView>
</LinearLayout>

The Logcat showing DB entries. The ratings still show as "null" despite my having updated them through the SQLite Manager.

19:32:07.426 2433-2433/? D/getAllBooks(): [Book [id=1, title=Professional Android 4 Application Development, author=Reto Meier, rating=null], Book [id=2, title=Beginning Android 4 Application Development, author=Wei-Meng Lee, rating=null], Book [id=3, title=Programming Android, author=Wallace Jackson, rating=null], Book [id=4, title=Hello, Android, author=Wallace Jackson, rating=null]]

The Problem:

My SQLite Manager changes are not reflected in in the Logcat and the app crashes

What I've already tried:

Uninstalling and re-installing the app, upgrading the DB version, deleting the database and starting over

What I'm using:

The latest version of Android Studio

Firefox's SQLite Manager

Any suggestions or advice on how to resolve these errors and reflect the database changes would be greatly appreciated.

Answer 1

好像您没有在SQLiteHelper类的getAllBooks()方法中SQLiteHelper等级。