java - 在 Java 中从 SQLite 值创建菜单

Question

我正在尝试使用 SQLite 数据库中的数据创建一个 ListMenu 来定义每个 MenuItem 的名称。所以在一个名为 menu.java 的类中，我定义了数组

String classes [] = {};

其中应包含每个菜单项名称。

在 DBAdapter 类中，我创建了一个函数，以便用户可以将信息插入表中(顺便说一句，这一切都很好)。

public long insertContact(String name, String code, String location, String comments, int days, int start, int end, String type) 
    {
        ContentValues initialValues = new ContentValues();
        initialValues.put(KEY_NAME, name);
        initialValues.put(KEY_CODE, code);
        initialValues.put(KEY_LOCATION, location);
        initialValues.put(KEY_COMMENTS, comments);
        initialValues.put(KEY_DAYS, days);
        initialValues.put(KEY_START, start);
        initialValues.put(KEY_END, end);
        initialValues.put(KEY_TYPE, type);
        return db.insert(DATABASE_TABLE, null, initialValues);
    }

这将是插入到 KEY_NAME 中的 String，我需要用它来填充 String 数组。有谁知道这是否可能？

非常感谢大家的帮助。

如果我通过 Sam/Mango 实现该函数，程序就会崩溃，是我使用不当还是由于数组大小未知而导致错误？

DBAdapter db = new DBAdapter(this);
String classes [] = db.getClasses();

编辑:我应该提到，如果我手动定义数组:

String classes [] = {"test1", "test2", "test3", etc};

效果很好。错误是 NullPointerException

这是 logcat(抱歉格式问题)。我没有用 db = helper.getReadableDatabase(); 初始化在 getClasses() 函数中，但不幸的是它没有解决问题。

11-11 22:53:39.117: D/dalvikvm(17856): Late-enabling CheckJNI
11-11 22:53:39.297: D/TextLayoutCache(17856): Using debug level: 0 - Debug Enabled: 0
11-11 22:53:39.337: D/libEGL(17856): loaded /system/lib/egl/libGLES_android.so
11-11 22:53:39.337: D/libEGL(17856): loaded /system/lib/egl/libEGL_adreno200.so
11-11 22:53:39.357: D/libEGL(17856): loaded /system/lib/egl/libGLESv1_CM_adreno200.so
11-11 22:53:39.357: D/libEGL(17856): loaded /system/lib/egl/libGLESv2_adreno200.so
11-11 22:53:39.387: I/Adreno200-EGLSUB(17856): <ConfigWindowMatch:2078>: Format RGBA_8888.
11-11 22:53:39.407: D/memalloc(17856): /dev/pmem: Mapped buffer base:0x5c66d000 size:36593664 offset:32825344 fd:65
11-11 22:53:39.417: E/(17856): Can't open file for reading
11-11 22:53:39.417: E/(17856): Can't open file for reading
11-11 22:53:39.417: D/OpenGLRenderer(17856): Enabling debug mode 0
11-11 22:53:39.477: D/memalloc(17856): /dev/pmem: Mapped buffer base:0x5ecd3000 size:40361984 offset:36593664 fd:68
11-11 22:53:40.507: D/memalloc(17856): /dev/pmem: Mapped buffer base:0x61451000 size:7254016 offset:3485696 fd:71
11-11 22:53:41.077: I/Adreno200-EGLSUB(17856): <ConfigWindowMatch:2078>: Format RGBA_8888.
11-11 22:53:41.077: D/memalloc(17856): /dev/pmem: Mapped buffer base:0x61c4c000 size:7725056 offset:7254016 fd:74
11-11 22:53:41.097: D/memalloc(17856): /dev/pmem: Mapped buffer base:0x623aa000 size:8196096 offset:7725056 fd:80
11-11 22:53:41.937: D/memalloc(17856): /dev/pmem: Mapped buffer base:0x62b7b000 size:8667136 offset:8196096 fd:83
11-11 22:53:41.977: D/memalloc(17856): /dev/pmem: Unmapping buffer base:0x61c4c000 size:7725056 offset:7254016
11-11 22:53:41.977: D/memalloc(17856): /dev/pmem: Unmapping buffer base:0x623aa000 size:8196096 offset:7725056
11-11 22:53:41.977: D/memalloc(17856): /dev/pmem: Unmapping buffer base:0x62b7b000 size:8667136 offset:8196096
11-11 22:53:42.167: I/Adreno200-EGLSUB(17856): <ConfigWindowMatch:2078>: Format RGBA_8888.
11-11 22:53:42.177: D/memalloc(17856): /dev/pmem: Mapped buffer base:0x61c5d000 size:17084416 offset:13316096 fd:74
11-11 22:53:42.317: D/memalloc(17856): /dev/pmem: Mapped buffer base:0x63853000 size:20852736 offset:17084416 fd:80
11-11 22:53:42.357: D/OpenGLRenderer(17856): Flushing caches (mode 0)
11-11 22:53:42.357: D/memalloc(17856): /dev/pmem: Unmapping buffer base:0x5c66d000 size:36593664 offset:32825344
11-11 22:53:42.357: D/memalloc(17856): /dev/pmem: Unmapping buffer base:0x5ecd3000 size:40361984 offset:36593664
11-11 22:53:42.367: D/memalloc(17856): /dev/pmem: Unmapping buffer base:0x61451000 size:7254016 offset:3485696
11-11 22:53:42.757: D/memalloc(17856): /dev/pmem: Mapped buffer base:0x5c56d000 size:24621056 offset:20852736 fd:65
11-11 22:53:44.247: D/AndroidRuntime(17856): Shutting down VM
11-11 22:53:44.247: W/dalvikvm(17856): threadid=1: thread exiting with uncaught exception (group=0x40ac3210)
11-11 22:53:44.257: E/AndroidRuntime(17856): FATAL EXCEPTION: main
11-11 22:53:44.257: E/AndroidRuntime(17856): java.lang.RuntimeException: Unable to instantiate activity ComponentInfo{x.x.timetable/x.x.timetable.menu}: java.lang.NullPointerException
11-11 22:53:44.257: E/AndroidRuntime(17856):    at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:1891)
11-11 22:53:44.257: E/AndroidRuntime(17856):    at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:1992)
11-11 22:53:44.257: E/AndroidRuntime(17856):    at android.app.ActivityThread.access$600(ActivityThread.java:127)
11-11 22:53:44.257: E/AndroidRuntime(17856):    at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1158)
11-11 22:53:44.257: E/AndroidRuntime(17856):    at android.os.Handler.dispatchMessage(Handler.java:99)
11-11 22:53:44.257: E/AndroidRuntime(17856):    at android.os.Looper.loop(Looper.java:137)
11-11 22:53:44.257: E/AndroidRuntime(17856):    at android.app.ActivityThread.main(ActivityThread.java:4441)
11-11 22:53:44.257: E/AndroidRuntime(17856):    at java.lang.reflect.Method.invokeNative(Native Method)
11-11 22:53:44.257: E/AndroidRuntime(17856):    at java.lang.reflect.Method.invoke(Method.java:511)
11-11 22:53:44.257: E/AndroidRuntime(17856):    at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:823)
11-11 22:53:44.257: E/AndroidRuntime(17856):    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:590)
11-11 22:53:44.257: E/AndroidRuntime(17856):    at dalvik.system.NativeStart.main(Native Method)
11-11 22:53:44.257: E/AndroidRuntime(17856): Caused by: java.lang.NullPointerException
11-11 22:53:44.257: E/AndroidRuntime(17856):    at android.content.ContextWrapper.openOrCreateDatabase(ContextWrapper.java:221)
11-11 22:53:44.257: E/AndroidRuntime(17856):    at android.database.sqlite.SQLiteOpenHelper.getWritableDatabase(SQLiteOpenHelper.java:157)
11-11 22:53:44.257: E/AndroidRuntime(17856):    at x.x.timetable.DBAdapter.getClasses(DBAdapter.java:151)
11-11 22:53:44.257: E/AndroidRuntime(17856):    at x.x.timetable.menu.<init>(menu.java:15)
11-11 22:53:44.257: E/AndroidRuntime(17856):    at java.lang.Class.newInstanceImpl(Native Method)
11-11 22:53:44.257: E/AndroidRuntime(17856):    at java.lang.Class.newInstance(Class.java:1319)
11-11 22:53:44.257: E/AndroidRuntime(17856):    at android.app.Instrumentation.newActivity(Instrumentation.java:1023)
11-11 22:53:44.257: E/AndroidRuntime(17856):    at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:1882)
11-11 22:53:44.257: E/AndroidRuntime(17856):    ... 11 more
11-11 22:53:46.527: I/Process(17856): Sending signal. PID: 17856 SIG: 9



package x.x.timetable;

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.SQLiteOpenHelper;
import android.util.Log;

public class DBAdapter {
    static final String KEY_ROWID = "_id";
    static final String KEY_NAME = "name";
    static final String KEY_CODE = "code";
    static final String KEY_LOCATION = "location";
    static final String KEY_COMMENTS = "comments";
    static final String KEY_DAYS = "days";
    static final String KEY_START = "start";
    static final String KEY_END = "end";
    static final String KEY_TYPE = "type";


    static final String TAG = "DBAdapter";

    static final String DATABASE_NAME = "MyDB";
    static final String DATABASE_TABLE = "timetable";
    static final int DATABASE_VERSION = 2;

    static final String DATABASE_CREATE =
        "create table timetable (_id integer primary key autoincrement, "
        + "name text not null, code text not null, location text not null, comments text not null, days INTEGER not null, start INTEGER not null, end INTEGER not null, type text not null);";

    final Context context;

    DatabaseHelper DBHelper;
    SQLiteDatabase db;

    public DBAdapter(Context ctx)
    {
        this.context = ctx;
        DBHelper = new DatabaseHelper(context);
    }

    private static class DatabaseHelper extends SQLiteOpenHelper
    {
        DatabaseHelper(Context context)
        {
            super(context, DATABASE_NAME, null, DATABASE_VERSION);
        }

        @Override
        public void onCreate(SQLiteDatabase db)
        {
            try {
                db.execSQL(DATABASE_CREATE);
            } catch (SQLException e) {
                e.printStackTrace();
            }
        }

        @Override
        public void onUpgrade(SQLiteDatabase db, int oldVersion, int newVersion)
        {
            Log.w(TAG, "Upgrading database from version " + oldVersion + " to "
                    + newVersion + ", which will destroy all old data");
            db.execSQL("DROP TABLE IF EXISTS timetable");
            db.execSQL("DROP TABLE IF EXISTS contacts");
            onCreate(db);
        }
    }

    //---opens the database---
    public DBAdapter open() throws SQLException 
    {
        db = DBHelper.getWritableDatabase();
        return this;
    }

    //---closes the database---
    public void close() 
    {
        DBHelper.close();
    }

    public int count(){
        db = DBHelper.getWritableDatabase();
        String count = "SELECT count(*) FROM table";
        Cursor mcursor = db.rawQuery(count, null);
        int icount = mcursor.getInt(0);
        return icount;
    }

    //---insert a contact into the database---
    public long insertContact(String name, String code, String location, String comments, int days, int start, int end, String type) 
    {
        ContentValues initialValues = new ContentValues();
        initialValues.put(KEY_NAME, name);
        initialValues.put(KEY_CODE, code);
        initialValues.put(KEY_LOCATION, location);
        initialValues.put(KEY_COMMENTS, comments);
        initialValues.put(KEY_DAYS, days);
        initialValues.put(KEY_START, start);
        initialValues.put(KEY_END, end);
        initialValues.put(KEY_TYPE, type);
        return db.insert(DATABASE_TABLE, null, initialValues);
    }

    //---deletes a particular contact---
    public boolean deleteContact(long rowId) 
    {
        return db.delete(DATABASE_TABLE, KEY_ROWID + "=" + rowId, null) > 0;
    }

    //---retrieves all the Timetable---
    public Cursor getAllTimetable()
    {
        return db.query(DATABASE_TABLE, new String[] {KEY_ROWID, KEY_NAME,
                KEY_CODE, KEY_LOCATION, KEY_COMMENTS, KEY_DAYS, KEY_START, KEY_END, KEY_TYPE}, null, null, null, null, null);
    }

    //---retrieves a particular contact---
    public Cursor getContact(long rowId) throws SQLException 
    {
        Cursor mCursor =
                db.query(true, DATABASE_TABLE, new String[] {KEY_ROWID,
                KEY_NAME, KEY_CODE, KEY_LOCATION, KEY_COMMENTS, KEY_DAYS, KEY_START, KEY_END, KEY_TYPE}, KEY_ROWID + "=" + rowId, null,
                null, null, null, null);
        if (mCursor != null) {
            mCursor.moveToFirst();
        }
        return mCursor;
    }

    //---updates a contact---
    public boolean updateContact(long rowId, String name, String code, String location, String comments, int days, int start, int end, String type) 
    {
        ContentValues args = new ContentValues();
        args.put(KEY_NAME, name);
        args.put(KEY_CODE, code);
        args.put(KEY_LOCATION, location);
        args.put(KEY_COMMENTS, comments);
        args.put(KEY_DAYS, days);
        args.put(KEY_START, start);
        args.put(KEY_END, end);
        args.put(KEY_TYPE, type);
        return db.update(DATABASE_TABLE, args, KEY_ROWID + "=" + rowId, null) > 0;
    }

    public String[] getClasses() {

        db = DBHelper.getWritableDatabase();
        String[] columns = {KEY_NAME};
        Cursor c = db.query(DATABASE_TABLE, columns, null, null, null, null, null);
        String[] classes = new String [c.getCount()];
        int i = 0;
        while (c.moveToNext()) {
            classes[i] = c.getString(0);
            i++;
        }
        c.close();
        return classes; 
    }
}

Answer 1

绝对有可能。您可以在数据库类中创建一个方法，以字符串数组形式返回值。

public String[] getClasses() {
    String[] columns = {KEY_NAME};
    Cursor c = db.query(DATABASE_TABLE, columns, null, null, null, null, null);
    String[] classes = new String [c.getCount()];
    int i = 0;
    while (c.moveToNext()) {
        classes[i] = c.getString(0);
        i++;
    }
    c.close();
    return classes; 
}