用确定的默认成员类型扩展ArrayList [英] Extending ArrayList with determined default member type

问题描述

我是java新手，所以我不知道这个技术叫什么，而且我可能在解释事情上很穷，但是我知道你懂。

假设我有两个类Item，它可以扩展为ExtendedItem

  public class Item {} 
 
 public class ExtendedItem extends Item {} 
  

我想为它创建集合包装，所以我创建一个从ArrayList扩展的类...（场景1）

  public class DataSet extends ArrayList< Item> {} 
  

然后初始化它

  DataSet dataset1 = new DataSet（）; 
 DataSet< ExtendedItem> dataset2 = new DataSet< ExtendedItem>（）; 
 
 dataset1.get（0）// yield Item实例...在这种情况下，正确的
 dataset2.get（0）//也产生Item实例
  

现在，当我使用泛型类型定义DataSet时（方案2）

  public class DataSet< T extends Item>扩展ArrayList< Item> {} 
  

收益

  DataSet dataset1 = new DataSet（）; 
 DataSet< ExtendedItem> dataset2 = new DataSet< ExtendedItem>（）; 
 
 dataset1.get（0）// yield对象实例....不要这个
 dataset2.get（0）//生成ExtendedItem实例...正确
  

是否有任何方法（场景），因此两个数据集都会生成正确的类型，其中Item为默认类型？像这样：

  DataSet dataset1 = new DataSet（）; 
 DataSet< ExtendedItem> dataset2 = new DataSet< ExtendedItem>（）; 
 
 dataset1.get（0）// yield Item实例
 dataset2.get（0）// yield ExtendedItem实例
  

---------------------------编辑------ ---------------------

我可能会找到解决方案：

  public class DataSet< E extends Item>扩展ArrayList< E>实现List< E> ;, RandomAccess，Cloneable，Serializable {
 @Override 
 public E get（int i）{
 return super.get（i）; 
} 
 
 @Override 
 public int size（）{
 return super.size（）; 
 
 
 $ / code $ / pre 
 
 
：
 
 
  DataSet dataset1 = new DataSet（）; 
 DataSet< ExtendedItem> dataset2 = new DataSet< ExtendedItem>（）; 
 
 dataset1.get（0）// yield Item实例...正确
 dataset2.get（0）//生成ExtendedItem实例...正确
  $ b 
解决方案
不要使用原始类型。
 
 
 
更改
  DataSet dataset1 = new DataSet（）; 
  
至
 数据集<项目> dataset1 = new DataSet< Item>（）; 
  
然后 dataset1.get（0） will返回项目。
 
 
 
您还应该更改 
 
 
  public class DataSet< T extends Item>扩展ArrayList< Item> {} 
  
到
  public class DataSet< T extends Item>扩展ArrayList< T> {} 
  
否则 dataset2.get（0）也会返回 Item 而不是 ExtendedItem 。
 
i am new at java, so i don't know what this technique called, and i may be poor at explain things, but i hpe you understand.

Assume i have this two class, Item and it might be extended to ExtendedItem
public class Item { }

public class ExtendedItem extends Item {}
i want to create collection wrapper for it, so i create a class extended from ArrayList... (Scenario 1)
public class DataSet extends ArrayList<Item> {}
then initialize it
DataSet dataset1 = new DataSet();
DataSet<ExtendedItem> dataset2  = new DataSet<ExtendedItem >();

dataset1.get(0) // yield Item instance... which is in this case, correct
dataset2.get(0) // also yield Item instance
now, when i define DataSet with generic type... (Scenario 2)
public class DataSet<T extends Item> extends ArrayList<Item> {}
it yielding
DataSet dataset1 = new DataSet();
DataSet<ExtendedItem> dataset2  = new DataSet<ExtendedItem >();

dataset1.get(0) // yield Object instance.... dont't want this
dataset2.get(0) // yield ExtendedItem instance... correct
is there any way (scenario) so both data set yield correct type with "Item" as the default type?, like this: 
DataSet dataset1 = new DataSet();
DataSet<ExtendedItem> dataset2  = new DataSet<ExtendedItem >();

dataset1.get(0) // yield Item instance
dataset2.get(0) // yield ExtendedItem instance
--------------------------- EDIT ---------------------------

I might found the solution: 
public class DataSet<E extends Item> extends ArrayList<E> implements List<E>, RandomAccess, Cloneable, Serializable {
    @Override
    public E get(int i) {
        return super.get(i);
    }

    @Override
    public int size() {
        return super.size();
    }
}
with that:
DataSet dataset1 = new DataSet();
DataSet<ExtendedItem> dataset2  = new DataSet<ExtendedItem >();

dataset1.get(0) // yield Item instance... correct
dataset2.get(0) // yield ExtendedItem instance... correct

解决方案
Don't use the raw type.

Change
DataSet dataset1 = new DataSet();
to
DataSet<Item> dataset1 = new DataSet<Item>();
Then dataset1.get(0) will return Item.

You should also change
public class DataSet<T extends Item> extends ArrayList<Item> {}
to
public class DataSet<T extends Item> extends ArrayList<T> {}
Otherwise dataset2.get(0) will also return Item instead of ExtendedItem.

                        
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