Java：Class< T>和类：当从类内部/外部使用时的差异 [英] Java: Class<T> and Class: Difference when used from within/ outside of the class

问题描述

我有一个类，需要使用类< T> 参数（参见我以前的半相关问题）。它是：

  public class BaseTable< T extends TableEntry> 
 {
 protected Class< T> mClass; 
 ... 
 
 public BaseTable（int rows，int cols，Class< T> clasz）
 {
 ... 
 mClass = clasz; 
} 
 
 public BaseTable（int rows，int cols）
 {
 this（rows，cols，StringTableEntry.class）; 
 //不编译：
 // expected [int，int，Class< T>]，但得到
 // [int，int，Class< blah.blah.StringTableEntry> 
} 
 ... 
} 
  

构造函数（有2个参数）不工作，但是当我从外部类调用完全相同的事情，如下：

  mSomeTable = new BaseTable< StringTableEntry>（2，2，StringTableEntry.class）; 
  

它编译和运行没有抱怨。

解决方案

p>具有通用类型参数的类的构造函数可以用于构造具有任何类型（适合类声明中的边界）的类。那么，如果有人说：

  new BaseTable< SomeTypeUnrelatedToStringTableEntry>（rows，cols）
  
 
 
 
？
 
 
 
解决方法是使用静态工厂方法：
  public static BaseTable< StringTableEntry> newStringTableEntryBaseTable（
 int rows，int cols）
 {
 return new BaseTable< StringTableEntry>（rows，cols，StringTableEntry.class）; 
} 
  
或者您可以创建一个子类：
  class StringTableEntryBaseTable extends BaseTable< StringTableEntry> 
 {
 StringTableEntryBaseTable（int rows，int cols）
 {
 super（rows，cols，StringTableEntry.class）; 
} 
} 
  
 
I have a class that needs to use a Class<T> parameter (see my previous semi-related question). It is:
public class BaseTable<T extends TableEntry>
{
    protected Class<T> mClass;
    ...

    public BaseTable(int rows, int cols, Class<T> clasz)
    {
        ...
        mClass = clasz;
    }

    public BaseTable(int rows, int cols)
    {
        this(rows, cols, StringTableEntry.class);
        //Does NOT compile:
        //expected [int, int, Class<T>], but got
        //[int, int, Class<blah.blah.StringTableEntry>]
    }
...
}
I am wondering why the constructor (with 2 parameters) does not work, but when I am invoking the exact same thing from an external class, like this:
    mSomeTable = new BaseTable<StringTableEntry>(2, 2, StringTableEntry.class);
It compiles and runs without complaining. Why so, and how to work around this?

Thanks!
解决方案
A constructor for a class that has generic type parameters can be used to construct that class with any types (that fit the bounds in the class declaration). So what happens if someone says:
new BaseTable<SomeTypeUnrelatedToStringTableEntry>(rows, cols)
?

As a workaround, use a static factory method:
public static BaseTable<StringTableEntry> newStringTableEntryBaseTable(
    int rows, int cols)
{
    return new BaseTable<StringTableEntry>(rows, cols, StringTableEntry.class);
}
Or you can create a subclass:
class StringTableEntryBaseTable extends BaseTable<StringTableEntry>
{
    StringTableEntryBaseTable(int rows, int cols)
    {
        super(rows, cols, StringTableEntry.class);
    }
}


                        
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