Android Studio- ClassCastException 错误？

Question

我想快速提及这是一项家庭作业，但我只是在寻找方向。我的教授以前没有应用过 SQLite，所以他不确定该怎么做。 class 正在执行 SharedPreference（我无法获取），但这是用于存储待办事项列表数据的另一种选择。

现在，当我点击按钮 "Add" AddDo class 下的任务时，它会抛出以下错误：

              java.lang.RuntimeException: Unable to instantiate activity ComponentInfo{net.example.name.dotoday/net.name.example.dotoday.AddDo}: java.lang.ClassCastException: net.example.name.dotoday.AddDo cannot be cast to android.app.Activity

编辑：这是实际按钮的 class。这是我的主要任务列表所在的位置，因为 XML 已准备好数据并且有一个用于添加新任务的按钮。

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


    Button addMainTask = (Button) findViewById(R.id.addMainTask);

    addMainTask.setOnClickListener(new View.OnClickListener() {

        @Override
        public void onClick(View view) {
            //This is being flagged as of now because the class type IS not Activity, but serializable.
            startActivity(new Intent(main.this, AddDo.class));

        }
    });

这是我的 class 用于添加任务（这是在早期阶段）：

    package net.example.name.dotoday;

import java.io.Serializable;

public class AddDo implements Serializable {

    //This is what will be transferred to the actual DB when I make a task.
    public String doParentID, doTaskTitle, doStatus, doDueDate, doCompletedTask, doCalculatedProgress = "";

}

这是我的 DBHandler：

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;

import net.example.name.dotoday.AddDo;

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

public class DBHelper extends SQLiteOpenHelper {

    private static final String TAG = "DbHelper";

    // Database Info
    private static final String TheDB = "TheDB";
    private static final int DATABASE_VERSION = 1;

    //DB Table Names
    private static final String TABLE_ParentTask = "ParentTask";
    private static final String TABLE_SubTask = "SubTask";


    //ParentTask Table Columns
    private static final String parentIDColumn = "_parentID";
    private static final String taskTitleColumn = "_taskTitle";
    private static final String statusColumn = "_status";
    private static final String dueDateColumn = "_dueDate";
    private static final String completedColumn = "_completedTask"; //When completion is 100%
    private static final String calculatedProgressColumn = "_calculatedProgress"; //LONG VALUE WHEN CONVERTED.

    //SubTask Table Columns
    private static final String foreignkeyColumn = "_foreignkey";
    private static final String childIDColumn = "_childID";
    private static final String subTaskDescColumn = "_subTaskDesc";
    private static final String subCompletedColumn = "_subCompleted"; // BOOL for individual tasks -- may not be implemented


    /*  -- Here comes the real fun, ya'll! -- */

    private static DBHelper mDbHelper;


    public static synchronized DBHelper getInstance(Context context) {
        // Use the application context, which will ensure that you
        // don't accidentally leak an Activity's context.

        if (mDbHelper == null) {
            mDbHelper = new DBHelper(context.getApplicationContext());
        }
        return mDbHelper;
    }


    /**
     * Constructor should be private to prevent direct instantiation.
     * Make a call to the static method "getInstance()" instead.
     */
    private DBHelper(Context context) {
        super(context, TheDB, null, DATABASE_VERSION);
    }

    @Override
    public void onCreate(SQLiteDatabase db) {
        String CREATE_ParentTask_TABLE = "CREATE TABLE " + TABLE_ParentTask +
                "(" +
                parentIDColumn + " INTEGER PRIMARY KEY ," +
                taskTitleColumn + " TEXT," +
                statusColumn + " BOOL," +
                dueDateColumn+ " TEXT," +
                completedColumn + " BOOL," +
                calculatedProgressColumn + " REAL" +
                ")";

        //CREATE THE PHYSICAL TABLE
        db.execSQL(CREATE_ParentTask_TABLE);

        /*String CREATE_SubTask_TABLE = "CREATE TABLE " + TABLE_SubTask +
                "(" +
                childID + " INTEGER PRIMARY KEY ," +
                foreignkey + " INT," +
                subTaskDesc + " TEXT," +
                subCompleted + " BOOL" +
                ")";

        //CREATE THE PHYSICAL TABLE
        db.execSQL(CREATE_SubTask_TABLE);*/
    }

    @Override
    public void onUpgrade(SQLiteDatabase db, int oldVersion, int newVersion) {
        if (oldVersion != newVersion) {
            // Simplest implementation is to drop all old tables and recreate them
            db.execSQL("DROP TABLE IF EXISTS " + TABLE_ParentTask);
            db.execSQL("DROP TABLE IF EXISTS " + TABLE_SubTask);

            onCreate(db);
        }
    }

    public void insertTaskDetails(AddDo userData) {

        SQLiteDatabase db = getWritableDatabase();

        db.beginTransaction();

        try {
            ContentValues values = new ContentValues();
            values.put(parentIDColumn, userData.doParentID);
            values.put(taskTitleColumn, userData.doTaskTitle);
            values.put(statusColumn, userData.doStatus);
            values.put(dueDateColumn, userData.doDueDate);
            values.put(completedColumn, userData.doCompletedTask);
            values.put(calculatedProgressColumn, userData.doCalculatedProgress);

            db.insertOrThrow(TABLE_ParentTask, null, values);
            db.setTransactionSuccessful();
        } catch (SQLException e) {
            e.printStackTrace();
            Log.d(TAG, "Error while trying to add post to database");
        } finally {

            db.endTransaction();
        }


    }

   /*
   fetch all data from UserTable
    */

    public List<AddDo> getAllMainTasks() {

        List<AddDo> usersdetail = new ArrayList<>();

        String USER_DETAIL_SELECT_QUERY = "SELECT * FROM " + TABLE_ParentTask;

        SQLiteDatabase db = getReadableDatabase();
        Cursor cursor = db.rawQuery(USER_DETAIL_SELECT_QUERY, null);

        try {
            if (cursor.moveToFirst()) {
                do {
                    AddDo userData = new AddDo();
                    userData.doParentID = cursor.getString(cursor.getColumnIndex(parentIDColumn));
                    userData.doTaskTitle = cursor.getString(cursor.getColumnIndex(taskTitleColumn));
                    userData.doStatus = cursor.getString(cursor.getColumnIndex(statusColumn));
                    userData.doDueDate = cursor.getString(cursor.getColumnIndex(dueDateColumn));
                    userData.doCompletedTask = cursor.getString(cursor.getColumnIndex(completedColumn));
                    userData.doCalculatedProgress = cursor.getString(cursor.getColumnIndex(calculatedProgressColumn));

                    usersdetail.add(userData);

                } while (cursor.moveToNext());
            }
        } catch (Exception e) {
            Log.d(TAG, "Error while trying to get posts from database");
        } finally {
            if (cursor != null && !cursor.isClosed()) {
                cursor.close();
            }
        }

        return usersdetail;

    }

    /*
   Delete single row from ParentTask
     */
    void deleteRow(String name) {
        SQLiteDatabase db = getWritableDatabase();


        try {
            db.beginTransaction();
            db.execSQL("delete from " + TABLE_ParentTask + " where parentid ='" + parentIDColumn + "'");
            db.setTransactionSuccessful();
        } catch (SQLException e) {
            Log.d(TAG, "Error while trying to delete task detail");
        } finally {
            db.endTransaction();
        }


    }
}

我想知道这是否与 AddDo 实现可序列化有关？我可以用标准 activity 将它换成不同的 XML，它会起作用。

如有任何指示，我们将不胜感激。我和我的教授不知道为什么这是不想表现的，直到现在我们都没有使用过 SQLite。

非常感谢！

Answer 1

错误描述：

Unable to instantiate activity 
ComponentInfo{net.example.name.dotoday/net.name.example.dotoday.AddDo}:
java.lang.ClassCastException: net.example.name.dotoday.AddDo cannot be cast to android.app.Activity

意思是

Android is unable to create an object of Activity , 
the reason is related with AddDo class 
android can't convert AddDo class to Activity because AddDo is not a child of
Activity but android need an activity parent-child relationship 
between AddDo and Activity

Addo 是一个简单的 class 所以你必须扩展 Activity 或 AppCompactActivity

class Addo  extends AppCompactActivity

别忘了在您的 manifest.xml

中注册它

加上您使用 Serializable 的原因，您应该创建一个单独的 class 以将其用作您的应用程序的模型，例如

class Data implements Serializable 
{
 public String doParentID, doTaskTitle, doStatus, doDueDate, doCompletedTask, doCalculatedProgress = "";

  public Data (){} // suitable default and parameterized constructor  

  .. fields and getter and setters
}

Answer 2

AddDo 未扩展 Activity class

使用下面的代码扩展使用 Activity 或 AppCompactActivity

public class AddDo extends Activity implements Serializable {

Android Studio- ClassCastException 错误？

Android Studio- ClassCastException error?

java

android

serializable

android-sqlite

android-studio