Java：实例化一个没有默认构造函数的泛型类 [英] Java: Instantiating a generic class with no default constructor

问题描述

我试图这样做：

  public class BaseTable< T extends TableEntry> 
 
 {
 
保护int mRows; 
保护int mCols; 
 protected ArrayList< T> mEntries; 
 
 public BaseTable（int rows，int cols）
 {
 mRows = rows; 
 mCols = cols; 
 mEntries = new ArrayList< T>（）; 
 for（int i = 0; i  {
 mEntries.add（new T（cols））; //这个obv。无效
 
 
 
 
 
 
实例化泛型这很难实现，但是更难的是这里 T 没有默认的构造函数，它只需要一个 int  
 
  
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 $ 
 
我已经询问了。如果你能回答这个问题，我将不胜感激。 
 
 
 问题是相关的，但只有在类被假定为具有默认构造函数时才是相关的。
 

已经说过，你不能用 new 创建一个T的实例，所以
我会使用Factory Pattern或一个原型模式

所以你的构造函数看起来像
public BaseTable（int rows，int cols，LineFactory factory）


在你的情况中，我更喜欢原型模式，因为你的TableEntry对象可能非常轻。您的代码如下所示：



  public BaseTable（int rows，int cols，T prototype）
 {
 mRows =行; 
 mCols = cols; 
 prototype.setColumns（cols）; 
 mEntries = new ArrayList< T>（）; 
 for（int i = 0; i  {
 @SuppressWarnings（unchecked）
 T newClone =（T）prototype.clone（）; 
 mEntries.add（newClone）; //这个obv。工作:) 
} 
} 
 
 public static void main（String [] args）
 {
新的BaseTable< SimpleTableEntry>（10，2 ，新的SimpleTableEntry（））; 
} 
  




I am trying to do this:

public class BaseTable<T extends TableEntry>

{

    protected int mRows;
    protected int mCols;
    protected ArrayList<T> mEntries;

    public BaseTable(int rows, int cols)
    {
        mRows = rows;
        mCols = cols;
        mEntries = new ArrayList<T>();
        for (int i = 0; i < rows; i++)
        {
            mEntries.add(new T(cols)); //this obv. doesn't work
        }
    }
}

Instantiating generics is hard enough as it is, but what makes this even harder is that T here does not have a default constructor, it takes a single int parameter in its constructor.

How can this be done?

---

I have asked a follow up question here too. I'd be grateful if you could answer that as well.

This question is related, but only is relevant where the classes are assumed to have a default constructor.

解决方案

It was already said, that you can't create an instance of T with new, so I would use the Factory Pattern or a Prototype Pattern

So your constructor would look like public BaseTable(int rows, int cols, LineFactory factory) with an appropriate instance of a factory.

In your case, I would prefer the Prototype Pattern, because your TableEntry objects are probably very light-weight. Your code would look like:

public BaseTable(int rows, int cols, T prototype)
{       
  mRows = rows;
  mCols = cols;
  prototype.setColumns(cols);
  mEntries = new ArrayList<T>();
  for (int i = 0; i < rows; i++)
  {
    @SuppressWarnings("unchecked")
    T newClone = (T)prototype.clone();
    mEntries.add(newClone); //this obv. does work :)
  }
}

public static void main(String[] args)
{
  new BaseTable<SimpleTableEntry>(10, 2, new SimpleTableEntry());
}

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