java-从抽象类创建对象的实例

Question

I have an abstract Record class that represents database records which has two abstract methods: getTable() and getColumns(). I then have a Customer class that extends Record and i implement those abstract methods in this class.

I'm trying to figure out how i can get a list of all Customers but keep the method as reusable as possible so i'd prefer a getAllRecords(Record record) method than getAllCustomers() method.

Here's what i have thus far. I can't create a new Record() object because it's abstract and need to create an instance of the class that was passed in.

//i'd like to do something like this to get all of the Customers in the db 
// datasource.getAllRecords(new Customer());

public List<Record> getAllRecords(Record record) {
    List<Record> records = new ArrayList<Record>();

    Cursor cursor = database.query(record.getTable(),
        record.getColumns(), null, null, null, null, null);

    cursor.moveToFirst();
    while (!cursor.isAfterLast()) {
      Record record = cursorToRecord(cursor, record);
      records.add(record);
      cursor.moveToNext();
    }
    // Make sure to close the cursor
    cursor.close();
    return records;
  }

  private Record cursorToRecord(Cursor cursor, Record record) {


    Record record = new Record(); <-- somehow clone a new instance of the record that was passed in

    record.setId(cursor.getLong(0));
    record.setValue("aKey",cursor.getString(1));
    return record;
  }

would having some kind of RecordRegistry object make sense instead of having individual factory classes for each subclass of Record?

class RecordRegistry{

    private static final List<Record> RECORDS;

    static {
            final List<Record> records = new ArrayList<Record>();
            records.add(new Customer());
            records.add(new Company());

            RECORDS = Collections.unmodifiableList(records);
    }

    public List<Record> allRecords(){

        return RECORDS;
    }

    public Record buildRecord(Class cClass){

        String className = cClass.getName().toString();

        if(className.equalsIgnoreCase("customer")){
            return new Customer();
        }else if(className.equalsIgnoreCase("company")){
            return new Company();
        }
        return null;
    }
}

Answer 1

You can get the class of the Record , provided that all the subclasses of Record will have a no-arg constructor.

Record newRecord = record.getClass().newInstance();

Note that you can just pass the class instead of the object itself.

You can also pass a factory that will be responsible to instanciate the right class.

interface RecordFactory {
    Record create();
}

class CustomerFactory implements RecordFactory {
    Record create() {
        return new Customer();
    }
}

public List<Record> getAllRecords(RecordFactory factory) {
    ...
    for(...) {
        ...
        Record record = factory.create();
        ...
    }
    ...
}

Answer 2

An odd use case. I would make the cursorToRecord method abstract. That pushes the logic into each class, which knows how to build itself from a Cursor .

public abstract Record cursorToRecord(Cursor cursor, Record record);

Answer 3

Not directly answering your Q, but take a look at the Android guides on Content Providers ( https://developer.android.com/guide/topics/providers/content-providers.html ). They allow for flexible data access amongst other benefits. I found them worth the initial learning and setup effort (especially when used w/ Loaders)

Answer 4

那反射呢？

Record newRecord = record.getClass().newInstance();