Question

我希望在片段中的列表中显示数据库表中的所有值。为此，我使用自定义列表适配器在片段列表中显示光标中存在的值。

片段类在我的DatabaseHandler类和适配器类中显示了一些与构造函数相关的错误（表示构造函数未定义）。

我猜错误是由于我在Databasehandler类中使用的上下文变量。而且我认为我们需要为片段声明一些其他变量类型。这也有助于其他人在片段中使用自定义列表适配器和listview，因为我几乎没有在网上找到任何好的教程。

所以当我在我的Handler类中创建一个像这样的新构造函数时 -

public  Database_Schema(Fragment_Java fragment_Java) {
        context = fragment_Java;
        dbHelper = new DatabaseHelper(context);
    }

编辑器再次抱怨要将上下文的类型更改为fragment_java。

这是我在我的应用程序中使用的DatabaseHandler类：

    package info.aea.database;

import android.content.ContentValues;
import android.content.Context;
import android.database.sqlite.SQLiteDatabase;
import android.database.sqlite.SQLiteOpenHelper;
import android.util.Log;

public class DatabaseHelper extends SQLiteOpenHelper{
    // Called when no database exists in disk and the helper class needs
    // to create a new one.
    SQLiteDatabase sqldb;
    public long r;
    public void onCreate(SQLiteDatabase _db)
    {
        //  _db.execSQL(LoginTable.DATABASE_CREATE);
        //  sqldb = this.getWritableDatabase();

    }
    /*  public LoginDatabaseHelper(Context context, String name,CursorFactory factory, int version) {
        super(context, name, factory, version);
}*/
    public DatabaseHelper(Context context) {
        super(context, "java.db", null, 1);
        sqldb = this.getWritableDatabase();

        try {
            sqldb.execSQL("CREATE TABLE if not exists SourceCodes (CodeID text PRIMARY KEY NOT NULL , " +
                    "CodeLang text NOT NULL, CodeTitle text NOT NULL , CodeSource text NOT NULL , " +
                    "CodeOutput text NOT NULL);");

            ContentValues cv = new ContentValues();

            cv.put("CodeID", "t1");
            cv.put("CodeLang", "java");
            cv.put("CodeTitle", "title1");
            cv.put("CodeSource", "source code here1");
            cv.put("CodeOutput", "output here1");

            r = sqldb.insert("SourceCodes", null, cv);

            //ContentValues cv2 = new ContentValues();

            cv.put("CodeID", "t2");
            cv.put("CodeLang", "java");
            cv.put("CodeTitle", "title2");
            cv.put("CodeSource", "source code here2");
            cv.put("CodeOutput", "output here2");

            r = sqldb.insert("SourceCodes", null, cv);

        } catch (Exception e) {
            // TODO: handle exception
        }
}



    // Called when there is a database version mismatch meaning that the version
    // of the database on disk needs to be upgraded to the current version.
    @Override
    public void onUpgrade(SQLiteDatabase _db, int _oldVersion, int _newVersion)
    {
        // Log the version upgrade.
        Log.w("TaskDBAdapter", "Upgrading from version " +_oldVersion + " to " +_newVersion + ", which will destroy all old data");


        // Upgrade the existing database to conform to the new version. Multiple
        // previous versions can be handled by comparing _oldVersion and _newVersion
        // values.
        // The simplest case is to drop the old table and create a new one.
        _db.execSQL("DROP TABLE IF EXISTS " + Database_Schema.DATABASE_CREATE);

        // Create a new one.
        onCreate(_db);
    }
}

下面是定义数据库模式并包含与表相关的CRUD操作方法的类 -

    package info.aea.database;

import java.util.ArrayList;
import java.util.List;

import android.content.Context;
import android.database.Cursor;
import android.database.SQLException;
import android.database.sqlite.SQLiteDatabase;

public class Database_Schema {

    static final String DATABASE_NAME = "login.db";
    static final int DATABASE_VERSION = 1;
    public static final int NAME_COLUMN = 1;
    // TODO: Create public field for each column in your table.
    // SQL Statement to create a new database.
    static final String DATABASE_CREATE = "create table "+" LOGIN "+
            "( " +"ID"+" integer primary key autoincrement," + "USERNAME  char unique,PASSWORD text, USERTYPE text); ";



    // Variable to hold the database instance
    public  SQLiteDatabase db;

    // Context of the application using the database.
    private final Context context;

    // Database open/upgrade helper
    private DatabaseHelper dbHelper;
    public  Database_Schema(Context _context) {
        context = _context;
        dbHelper = new DatabaseHelper(context);
    }



    public  Database_Schema open() throws SQLException {
        db = dbHelper.getWritableDatabase();
        return this;
    }

    public void close() {
        db.close();
    }

    public  SQLiteDatabase getDatabaseInstance(){
        return db;
    }





    //---------------------CRUD Operations---------------------//


    // get all query
    public List<SourceCode_Table> getall(String lang) {
        List<SourceCode_Table> codelist = new ArrayList<SourceCode_Table>();
        String selectQuery = "SELECT  * FROM SourceCodes where codelang='java'" ;    // Select All Query
        db = dbHelper.getWritableDatabase();
        Cursor cursor = db.rawQuery(selectQuery, null);

        // looping through all rows and adding to Vector
        if (cursor.moveToFirst()) {
            do {
                SourceCode_Table a = new SourceCode_Table();

                a.setCodeID(cursor.getString(0));
                a.setCodeTitle(cursor.getString(1));
                a.setCodeSource(cursor.getString(3));
                a.setCodeOutput(cursor.getString(4));

                codelist.add(a);
            } while (cursor.moveToNext());
        }
        return codelist;
    }
}

此外，我希望在单个DB_handler类中合并这些DB_related类。但它会产生一些奇怪的错误，或者如果没有错误编译则不会创建数据库。在这方面也需要帮助。我只是想删除那个额外的类，因为它在声明数据库模式和应用查询时会产生混淆。

对于名为源代码的表，我已经创建了一个适配器类。这是列表适配器类 -

    package info.aea.database;

import info.devey.java.R;
import android.content.Context;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;
import android.widget.ArrayAdapter;
import android.widget.TextView;

public class SourceCode_Adapter extends ArrayAdapter<String> {

    private final Context context;
    private final String[] CodeID;
    private final String[] CodeLang;
    private final String[] CodeTitle;
    private final String[] CodeSource;
    private final String[] CodeOutput;


    public SourceCode_Adapter(Context context, String[] codeid, String[] codelang, String[] codetitle, String[] codesource, String[] codeoutput) {
        super(context, R.layout.list_items, codeid);

        this.context = context;
        this.CodeID = codeid;
        this.CodeLang = codelang;
        this.CodeTitle = codetitle;
        this.CodeSource = codesource;
        this.CodeOutput = codeoutput;

    }



    @Override
    public View getView(int position, View convertView, ViewGroup parent) {
        LayoutInflater inflater = (LayoutInflater) context.getSystemService(Context.LAYOUT_INFLATER_SERVICE);
        View rowView = inflater.inflate(R.layout.list_items, parent, false);
        TextView textView = (TextView) rowView.findViewById(R.id.tvPat);
        String test=(" code ID: "+ CodeID[position]+ "\n code Language: "+ CodeLang[position]+ "\n Code Title: "+
                    CodeTitle[position]+ "\n code Source: "+ CodeSource[position]+ "\n code output: "+ CodeOutput[position]);
        textView.setText(test);
        System.out.println("list values ------->> " + CodeID[position] + CodeLang[position] + CodeOutput[position] + CodeSource[position] + CodeTitle[position] );
        return rowView;
    }

}

最后这里是Fragment类，我想在列表中显示数据库值。但是当我尝试实例化一个新的DB对象时，编辑器会显示一个错误标记，并建议更改构造函数声明和上下文类型。

猜测所有这些都是由于那些写得不好的数据库类。

    package info.aea.drawer;

import info.aea.database.Database_Schema;
import info.aea.database.SourceCode_Adapter;
import info.aea.database.SourceCode_Table;
import info.devey.java.R;

import java.util.List;

import android.app.AlertDialog;
import android.app.Fragment;
import android.content.DialogInterface;
import android.content.Intent;
import android.database.sqlite.SQLiteDatabase;
import android.os.Bundle;
import android.util.Log;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;
import android.widget.AdapterView;
import android.widget.AdapterView.OnItemClickListener;
import android.widget.ArrayAdapter;
import android.widget.EditText;
import android.widget.ListView;

public class Fragment_Java extends Fragment {

    String[] codeid;
    String codelang[];
    String codetitle[];
    String codesource[];
    String codeoutput[];

    ListView listview;

    SQLiteDatabase db;
    Database_Schema logindb;


    @Override
    public View onCreateView(LayoutInflater inflater,ViewGroup container,Bundle savedInstanceState)
    {       

        View view = inflater.inflate(R.layout.fragment_java, container,false);

        logindb=new Database_Schema(this);
        logindb=logindb.open();

        String lang="java";
        List<SourceCode_Table> ls   = logindb.getall(lang);

        codeid    = new String[ls.size()];
        codelang    = new String[ls.size()];
        codetitle    = new String[ls.size()];
        codesource    = new String[ls.size()];
        codeoutput = new String[ls.size()];

        for(int i = 0; i<ls.size();i++) {
            codeid[i]= ls.get(i).getCodeID();
            codelang[i]= ls.get(i).getCodeLang();
            codetitle[i]= ls.get(i).getCodeTitle();
            codesource[i]= ls.get(i).getCodeSource();
            codeoutput[i]= ls.get(i).getCodeOutput();

            Log.v("code id","-------"+ls.get(i).getCodeID());
            Log.v("code lang","-------"+ls.get(i).getCodeLang());
            System.out.println("patname==============+++++++++++++++++++"+ codeid);
            System.out.println("date==============+++++++++++++++++++"+ codelang);
        }


        SourceCode_Adapter adapter = new SourceCode_Adapter(this, codeid, codelang, codetitle, codesource, codeoutput);
        listview.setAdapter(adapter);  
          }
}

因此需要帮助来纠正这些错误。我愿意将整个项目作为导出文件共享，以便使调试变得简单。

Answer 1

哦，我发现我错了。我正在传递一个用于片段活动的上下文。片段需要片段上下文而不是活动上下文。 getActivity（）用于将上下文传递给片段，片段返回与片段关联的活动。

如果您的活动类扩展了Activity，则可以使用getApplicatoinContext（）获取应用程序上下文。但是当您的Activity从Fragment扩展时，此方法将不可用。当您的活动扩展Fragment时，使用getActivity（）来获取活动的上下文。

public class MainActivity extends Fragment {

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

        // Getting application context
        Context context = getActivity();
    }
}

所以我换了

logindb=new Database_Schema(this); 
   logindb=logindb.open();

带

 logindb = new Database_Schema(getActivity()); 
    logindb = logindb.open()

通过片段中的列表适配器获取并显示sqlite数据

1 个答案: