Question

我正在开发一个Android应用程序，我想在数据库中保存一些数据，然后通过按下按钮来检索它们，但由于我的构造函数方法的签名，我无法通过这种方式检索数据。我怎样才能实现这个目标？

这是我的DBAdapter.java

package com.example.databaseandroid10;

import android.content.ContentValues;

public class DBAdapter 
{
    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 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) 
        {
            db.execSQL(DATABASE_CREATE);
        }

        @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 titles");
            onCreate(db);
        }
    }    

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

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

    //---insert a title into the database---
    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);
    }

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

    //---retrieves all the titles---
    public Cursor getAllTitles() 
    {
        return db.query(DATABASE_TABLE, new String[] {
                KEY_ROWID, 
                KEY_ISBN,
                KEY_TITLE,
                KEY_PUBLISHER}, 
                null, 
                null, 
                null, 
                null, 
                null);
    }

    //---retrieves a particular title---
    public Cursor getTitle(long rowId) throws SQLException 
    {
        Cursor mCursor =
                db.query(true, DATABASE_TABLE, new String[] {
                        KEY_ROWID,
                        KEY_ISBN, 
                        KEY_TITLE,
                        KEY_PUBLISHER
                        }, 
                        KEY_ROWID + "=" + rowId, 
                        null,
                        null, 
                        null, 
                        null, 
                        null);
        if (mCursor != null) {
            mCursor.moveToFirst();
        }
        return mCursor;
    }

    //---updates a title---
    public boolean updateTitle(long rowId, String isbn, 
    String title, String publisher) 
    {
        ContentValues args = new ContentValues();
        args.put(KEY_ISBN, isbn);
        args.put(KEY_TITLE, title);
        args.put(KEY_PUBLISHER, publisher);
        return db.update(DATABASE_TABLE, args, 
                         KEY_ROWID + "=" + rowId, null) > 0;
    }
}

这是我的主要类我在OnClick方法体中收到错误：

package com.example.databaseandroid10;

import android.os.Bundle;

public class DatabaseAndroid10 extends Activity {

    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_database_android10);




        //---btnshow---
        Button btnShow=(Button)findViewById(R.id.btnShowInfo);

        btnShow.setOnClickListener(new View.OnClickListener() {

            @Override
            public void onClick(View arg0) {

                DBAdapter db = new DBAdapter(this); 

      //---get all titles---


        db.open();
        Cursor c = db.getAllTitles();
        if (c.moveToFirst())
        {
            do {          
                DisplayTitle(c);
            } while (c.moveToNext());
        }
        db.close(); 
            }
        });


        //---add 3 titles---
      /*  db.open();        
        long id;
        id = db.insertTitle(
                "0470285818",
                "C# 2008 Programmer's Reference",
                "Wrox");        
        id = db.insertTitle(
                "047017661X",
                "Professional Windows Vista Gadgets Programming",
                "Wrox");


        id=db.insertTitle(
                "224436998",
                "Guide",
                "Apress");


        db.close();
        */




    }



    //---Display Method---
    public void DisplayTitle(Cursor c)
    {
        Toast.makeText(this, 
                "id: " + c.getString(0) + "\n" +
                "ISBN: " + c.getString(1) + "\n" +
                "TITLE: " + c.getString(2) + "\n" +
                "PUBLISHER:  " + c.getString(3),
                Toast.LENGTH_LONG).show();        
    } 

    @Override
    public boolean onCreateOptionsMenu(Menu menu) {
        getMenuInflater().inflate(R.menu.activity_database_android10, menu);
        return true;
    }
}

非常感谢

Answer 1

在课程级别移动DBAdapter db = new DBAdapter(this);：

public class DatabaseAndroid10 extends Activity {
 DBAdapter db ;
@Override
public void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_database_android10);

  db = new DBAdapter(DatabaseAndroid10.this); 



    //---btnshow---
    Button btnShow=(Button)findViewById(R.id.btnShowInfo);

    btnShow.setOnClickListener(new View.OnClickListener() {

        @Override
        public void onClick(View arg0) {

           //your code here

问题： -

因为在创建数据库或执行数据库操作的当前代码中，您正在传递按钮视图上下文而不是当前活动或应用上下文。

解决方案： -

将当前活动上下文传递给应用程序上下文到DBAdapter，以便在应用程序级别创建或管理数据库

按下按钮显示数据库记录

1 个答案: