Question

我的SQLiteOpenHelper课程有问题。我有一个打印机制造商的数据库和任何类型的打印机的详细信息。我尝试使用此代码从我的数据库中获取所有制造商并将其返回到arraylist。

// Read database for All printer manufacturers and return a list
public ArrayList<String> getPrManufacturer(){
    ArrayList<String> manufacturerList = new ArrayList<String>();
    SQLiteDatabase db = getReadableDatabase();
    Cursor cursor = db.query(CoDeskContract.Printer.TABLE_NAME,
                             printerManuProjection, null, null, null, null, null,null);

    // If cursor is not null and manufacturer List 
    // does not contains Manufacturer in the list then add it!
    if ((cursor != null) && (cursor.getCount()>0)){
        cursor.moveToFirst();
        do {
            String cursorManufacturer = cursor.getString(0);
            //Checking for manufacturer in the list
            for(String manufacturerInList:manufacturerList){
                if (!manufacturerInList.equals(cursorManufacturer))
                    manufacturerList.add(cursorManufacturer);               
            }                   
        }while(cursor.moveToNext());
    }

    // Return list of manufacturers from database
    return manufacturerList;
}

我希望每个制造商都在列表中。不知怎的，我不能让它工作。我还是新手。

感谢您的帮助。

Answer 1

您还可以在SQLite中使用distinct关键字（http://www.sqlite.org/lang_select.html）。使用SQLiteDatabase.rawQuery（String query，String [] args）。

db.rawQuery("SELECT DISTINCT name FROM " + CoDeskContract.Printer.TABLE_NAME,null);

Answer 2

有两个问题：

一开始，当您的列表manufacturerInList为空时，它不会进入for(String manufacturerInList:manufacturerList){循环内部，因此它永远不会在列表中添加任何条目。
修复问题1后，仍然无法对if (!manufacturerInList.equals(cursorManufacturer))检查列表中的每个条目，并且可能多次在列表中添加非匹配条目。

要解决此问题，您有两种选择。

选项1 ：使用contains作为：

            if (!manufacturerList.contains(cursorManufacturer)) {
                 manufacturerList.add(cursorManufacturer); 
            }

Option2 ：使用matchFound布尔标志：

        String cursorManufacturer = cursor.getString(0);
        boolean matchFound = false;

        //Checking for manufacturer in the list
        for(String manufacturerInList:manufacturerList){
            if (manufacturerInList.equals(cursorManufacturer)){
                 matchFound = true;
                 break;
            }
        }                   
        if(!matchFound){ // <- doesn't exit in the list
            manufacturerList.add(cursorManufacturer); 
        }

Answer 3

使用ArrayList.contains(Object elem)检查ArrayList中是否存在项目。将代码更改为：

  // does not contains Manufacturer in the list then add it!
    if ((cursor != null) && (cursor.getCount()>0)){
        cursor.moveToFirst();
        do {
            String cursorManufacturer = cursor.getString(0);
            //Checking for manufacturer in the list
           if (!manufacturerList.contains(cursorManufacturer)) {
                     manufacturerList.add(cursorManufacturer); 
                } else {
                    System.out.println("cursorManufacturernot found");
                }              
        }while(cursor.moveToNext());
    }

使用SQLITEOpenHelper获取ArrayList不起作用

3 个答案: