java - 为什么我的 ListView 不显示来自 SQLite 数据库的数据？

Question

我是 android 开发的新手，所以这似乎是一个显而易见的问题，但我无法弄清楚。我正在尝试使用存储在 SQLite 数据库中的数据填充 ListView 。我已经设法填充了数据库。k 但是，我无法弄清楚为什么数据没有显示在 ListView 中。我的代码运行良好，没有错误。有人可以帮帮我吗？我将不胜感激!

主要 Activity .java:

import android.os.Bundle;
import android.support.v7.app.AppCompatActivity;
import android.widget.ListView;
import android.widget.SimpleCursorAdapter;
import android.database.Cursor;
import java.util.List;

public class MainActivity extends AppCompatActivity {

    SimpleCursorAdapter simpleCursorAdapter;
    SqlHelper sqlHelper;
    ListView list;

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

        list = (ListView) findViewById(R.id.listView);
        sqlHelper = new SqlHelper(this, null, null, 1);



        /** CRUD Operations **/
        // add Books
        sqlHelper.addBook(new Book("Professional Android 4 Application Development", "Reto Meier", 4));
        sqlHelper.addBook(new Book("Beginning Android 4 Application Development", "Wei-Meng Lee", 2));
        sqlHelper.addBook(new Book("Programming Android", "Wallace Jackson", 3));
        sqlHelper.addBook(new Book("Hello, Android", "Ben Wallace", 1));
        // get all books
        List<Book> list = sqlHelper.getAllBooks();
        //sqlHelper.getAllBooks();
        sqlHelper.getIds(list.get(0));

        displayBookList();

    }

    private void displayBookList() {
        try
        {
            Cursor cursor = sqlHelper.getBooks();
            if (cursor == null)
            {
                return;
            }
            if (cursor.getCount() == 0)
            {
                return;
            }
            String[] columns = new String[] {
                    sqlHelper.KEY_ID,
                    sqlHelper.KEY_TITLE,
                    sqlHelper.KEY_AUTHOR,
                    sqlHelper.KEY_RATING
            };
            int[] boundTo = new int[] {
                    R.id.booknum,
                    R.id.booktitle,
                    R.id.bookauthor,
                    R.id.ratingBar
            };
            simpleCursorAdapter = new SimpleCursorAdapter(getApplicationContext(),
                    R.layout.each_book,
                    cursor,
                    columns,
                    boundTo,
                    0);
            list.setAdapter(simpleCursorAdapter);
        }
        catch (Exception ex)
        {

        }
    }

}

书.java:

public class Book {
    private int id;
    private String title;
    private String author;
    private int rating;

    public Book() {
    }

    public Book(String title, String author, int rating) {
        super();
        this.title = title;
        this.author = author;
        this.rating = rating;
    }

    //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;
    }

    public int getRating() {
        return rating;
    }

    public void setRating(int rating) {
        this.rating = rating;
    }

    @Override
    public String toString() {
        return "Book [id=" + id + ", title=" + title + ", author=" + author + ", rating=" + rating + "]";
    }
}

SQLHelper.java:

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 = 1;
    // Database Name
    private static final String DATABASE_NAME = "Books1DB"; // Books table name
    private static final String TABLE_BOOKS = "books"; // Books Table Columns names
    public static final String KEY_ID = "id";
    public static final String KEY_TITLE = "title";
    public static final String KEY_AUTHOR = "author";
    public static final String KEY_RATING = "rating";

    public SqlHelper(Context context, String name,
                     SQLiteDatabase.CursorFactory factory, int version) {
        super(context, DATABASE_NAME, factory, 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 INTEGER )";
        // 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);
    }

    public Cursor getBooks() {
        SQLiteDatabase db = this.getReadableDatabase();
        Cursor cursor = db.query(TABLE_BOOKS, new String[] {KEY_ID, KEY_TITLE,
                KEY_AUTHOR, KEY_RATING}, null, null, null, null, null);
        if(cursor != null)
        {
            cursor.moveToFirst();
            return cursor;
        }
        else {
            return null;
        }
    }

    /*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
        values.put(KEY_RATING, book.getRating()); // get rating
        // 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));
                book.setRating(Integer.parseInt(cursor.getString(3)));
                // 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) {
        // 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", book.getTitle()); // get title
        values.put("author", book.getAuthor()); // get author
        // 3. updating row
        int i = db.update(TABLE_BOOKS, //table
                values, // column/value
                KEY_ID + " = ?", // selections
                new String[]{String.valueOf(book.getId())}); //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();
        String stringTotal = String.valueOf(total);
        Log.d("Total Count:", stringTotal);
        return total;
    }

}

activity_main.xml:

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    android:orientation="vertical"
    android:layout_width="match_parent"
    android:layout_height="wrap_content"
    tools:context=".MainActivity">

    <ListView
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:id="@+id/listView"
        android:layout_gravity="center_horizontal" />

</LinearLayout>

each_book.java:

<?xml version="1.0" encoding="utf-8"?>
<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:orientation="vertical" android:layout_width="match_parent"
    android:layout_height="match_parent">


    <TextView
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:textAppearance="?android:attr/textAppearanceMedium"
        android:text="Medium Text"
        android:id="@+id/booknum"
        android:layout_alignParentTop="true"
        android:layout_alignParentStart="true" />

    <TextView
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:textAppearance="?android:attr/textAppearanceLarge"
        android:text="Large Text"
        android:id="@+id/booktitle"
        android:layout_gravity="center_horizontal"
        android:layout_below="@+id/booknum"
        android:layout_centerHorizontal="true" />

    <TextView
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:textAppearance="?android:attr/textAppearanceMedium"
        android:text="Medium Text"
        android:id="@+id/bookauthor"
        android:layout_gravity="center_horizontal"
        android:layout_below="@+id/booktitle"
        android:layout_toEndOf="@+id/booktitle" />

    <RatingBar
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:id="@+id/ratingBar"
        android:layout_below="@+id/bookauthor"
        android:layout_alignParentStart="true" />

</RelativeLayout>

AndroidManifest.xml:

<?xml version="1.0" encoding="utf-8"?>
<manifest xmlns:android="http://schemas.android.com/apk/res/android"
    package="com.example.shaydoe.bookreviews">

    <application
        android:allowBackup="true"
        android:icon="@mipmap/ic_launcher"
        android:label="@string/app_name"
        android:supportsRtl="true"
        android:theme="@style/AppTheme">
        <activity android:name=".MainActivity">
            <intent-filter>
                <action android:name="android.intent.action.MAIN" />

                <category android:name="android.intent.category.LAUNCHER" />
            </intent-filter>
        </activity>
    </application>
</manifest>

Answer 1

CursorAdpater 需要一个名为 _id 的列。根据 Cursor 必须包含名为“_id”的列，否则此类将不起作用。 CursorAdapter .

因此您需要使游标有一个名为_id 的列。您可以通过几种方式做到这一点。例如在用于使用 AS _id 创建游标的查询中命名(重命名)或重命名实际列。我建议后者在这种情况下改变:

public static final String KEY_ID = "id";

到

public static final String KEY_ID = "_id";