java - 每次我重新启动 Activity 时，ListView 都会重复项目

Question

我觉得这是一个非常简单的修复，但总的来说我对 android 和 sqlite 很陌生。我有一个数组，它从数据库中获取数据，然后将它们显示在列表中。但是，每次我重新启动应用程序时，列表都会再次将项目添加到列表中。我怎样才能让它不这样做？

package com.example.assignmenttracker;

import java.util.ArrayList;
import java.util.List;
import android.content.Intent;
import android.util.Log;
import android.view.View;
import android.view.View.OnClickListener;
import android.widget.AdapterView;
import android.widget.AdapterView.OnItemClickListener;
import android.widget.ListView;
import com.example.assignmenttracker.MySQLiteHelper;
import android.os.Bundle;
import android.widget.ArrayAdapter;
import android.widget.Button;
import android.app.Activity;

public class MainActivity extends Activity {
 private ListView ListView;
 private Button addbutton;
 public final static String ID_EXTRA="com.example.assignmenttracker._ID";
 public static ArrayList<String> ArrayofName = new ArrayList<String>();
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        this.addbutton = (Button)this.findViewById(R.id.button1);
        this.addbutton.setOnClickListener(new OnClickListener() {
          @Override
          public void onClick(View v) {
          Intent intent=new Intent(MainActivity.this, AddAssign.class);
          startActivity(intent);
          }
        });
    }
    private void populateListView() {
        MySQLiteHelper db = new MySQLiteHelper(this);

        List<tasklist> contacts = db.getSometasklist();       

        for (tasklist cn : contacts) {
            Log.d("Reading: ", "Reading all contacts..");
            String log = "Id: "+cn.getID()+" ,Task: " + cn.getTask() + " ,Date: " + cn.getDate() + " ,Status: " + cn.getStatus();
            Log.d("Name: ", log);
            }
        ListView = (ListView) findViewById(R.id.listView1);
        ArrayAdapter<String> adapter = new ArrayAdapter<String>(this,
                android.R.layout.simple_list_item_1, ArrayofName);
        ListView.setAdapter(adapter);

        ListView.setOnItemClickListener(new OnItemClickListener() {
            public void onItemClick(AdapterView<?> parent, View v,
                int position, long id) {
                Intent i = new Intent(MainActivity.this, ViewAssign.class);
                i.putExtra(ID_EXTRA, String.valueOf(id));
                startActivity(i);}
        });

    }
    @Override
    public void onResume() {
        super.onResume();
        populateListView();
    }
}

这是mysqlitehelper的代码

package com.example.assignmenttracker;

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

import android.database.Cursor;
import android.database.sqlite.SQLiteOpenHelper;
import android.database.sqlite.SQLiteDatabase;
import android.content.ContentValues;
import android.content.Context;


public class MySQLiteHelper extends SQLiteOpenHelper {
    // Contacts Table Columns names
    private static final String TABLE_CONTACTS = "tasklist1";
    private static final String KEY_ID = "id";
    private static final String KEY_TASK = "task";
    private static final String KEY_DESC = "desc";
    private static final String KEY_MOD = "module";
    private static final String KEY_DATE = "date";
    private static final String KEY_TYPE = "type";
    private static final String KEY_STATUS = "status";
    private static final int DATABASE_VERSION = 3;
    private static final String DATABASE_NAME = "tasklist1";

    public MySQLiteHelper(Context context) {
        super(context, DATABASE_NAME, null, DATABASE_VERSION);  
    }

    public void onCreate(SQLiteDatabase db) {
        String CREATE_TASKLIST_TABLE = "CREATE TABLE tasklist1 ( " +
                "id INTEGER PRIMARY KEY AUTOINCREMENT, " + 
                "task TEXT, "+
                "desc TEXT, "+
                "module TEXT, "+
                "date TEXT, "+
                "type TEXT, "+
                "status INTEGER )";

        db.execSQL(CREATE_TASKLIST_TABLE);
    }



    @Override
    public void onUpgrade(SQLiteDatabase db, int oldVersion, int newVersion) {
        // Drop older task list table if existed
        db.execSQL("DROP TABLE IF EXISTS " + TABLE_CONTACTS);

        // create fresh task list table
        this.onCreate(db);
    }       

    public void insertTask(tasklist tasklist){

        // 1. get reference to writable DB
        SQLiteDatabase db = this.getWritableDatabase();

        ContentValues values = new ContentValues();
        values.put(KEY_TASK, tasklist.getTask()); 
        values.put(KEY_DESC, tasklist.getDesc());
        values.put(KEY_MOD, tasklist.getModule());
        values.put(KEY_DATE, tasklist.getDate()); 
        values.put(KEY_TYPE, tasklist.getType()); 
        values.put(KEY_STATUS, tasklist.getStatus());


        // Inserting Row
        db.insert(TABLE_CONTACTS, null, values);
        db.close(); // Closing database connection
    }

    public List<tasklist> getAlltasklist(int passedid) {
        List<tasklist> tasklistList = new ArrayList<tasklist>();
        // Select All Query
        String selectQuery = "SELECT  * FROM " + TABLE_CONTACTS +
                " where id = " + passedid;

        SQLiteDatabase db = this.getWritableDatabase();
        Cursor cursor = db.rawQuery(selectQuery, null);

        // looping through all rows and adding to list
        if (cursor.moveToFirst()) {
            do {
                tasklist contact = new tasklist();
                contact.setTask(cursor.getString(1));
                contact.setDesc(cursor.getString(2));
                contact.setModule(cursor.getString(3));
                contact.setDate(cursor.getString(4));
                contact.setType(cursor.getString(5));
                contact.setStatus(cursor.getInt(6));

                String name = cursor.getString(1);
                String desc =cursor.getString(2);
                String module =cursor.getString(3);
                String date  = cursor.getString(4);
                String type = cursor.getString(5);
                int status = cursor.getInt(6);
                ViewAssign.task= name;
                ViewAssign.desc=desc;
                ViewAssign.module=module;
                ViewAssign.date=date;
                ViewAssign.type=type;
                ViewAssign.status=status;
                // Adding contact to list
                tasklistList.add(contact);
                db.close();
            } while (cursor.moveToNext());
        }
        return tasklistList;

}

    public List<tasklist> getSometasklist() {
        List<tasklist> tasklistList = new ArrayList<tasklist>();
        // Select All Query
        String selectQuery = "SELECT  * FROM " + TABLE_CONTACTS;

        SQLiteDatabase db = this.getWritableDatabase();
        Cursor cursor = db.rawQuery(selectQuery, null);

        // looping through all rows and adding to list
        if (cursor.moveToFirst()) {
            do {
                tasklist contact = new tasklist();
                contact.setTask(cursor.getString(1));
                contact.setDate(cursor.getString(4));

                String name = cursor.getString(1) +"\n"+ cursor.getString(4);
                MainActivity.ArrayofName.add(name);
                // Adding contact to list
                tasklistList.add(contact);
                db.close();
            } while (cursor.moveToNext());
        }
        return tasklistList;


}

    public void deleteTask(long row) {

        // Deletes a row given its rowId, but I want to be able to pass
        // in the name of the KEY_NAME and have it delete that row.
        SQLiteDatabase db = this.getWritableDatabase();
        db.delete(TABLE_CONTACTS, KEY_ID + "=" + row, null);
        db.close();
}
}

Answer 1

我怀疑罪魁祸首是这样的:

public static ArrayList<String> ArrayofName = new ArrayList<String>();

它是一个静态变量，意味着in some circumstances ，当您第二次运行它时，它仍然会包含您在应用程序第一次运行时添加的对象。

可能的解决方案:

• 删除“static”关键字，或
• 在填充之前清除 ArrayOfName，例如。 ArrayOfName.clear()