Java泛型从类创建数组 [英] Java Generics Creating Array from Class

问题描述

我有一个层次结构，Square，Triangle和Circle都从Shape延伸出来。我有一个工作方法：

  public void someMethod（）{
 File file = new File（File_with_squares） ; 
 ThirdPartyClass foo = new ThirdPartyClass（）; 
 Square [] squares = foo.someMajicMethod（Square []。class，file）; 
 for（Square square：squares）
 square.draw（）; 
 
 
 $ / code> 

现在我想让这个方法是通用的，接受任何形状。我希望能够将它称为 someMethod（Triangle.class，new File（File_with_triangles）或 someMethod（Circle.class，new File（ File_with_circles）。我想这样：

  public void someMethod（Class< ;? extends Shape> type，File shapeFile）{
 ThirdPartyClass foo = new ThirdPartyClass（）; 
 ####这里有什么??? #### 
 for（Shape shape：shapes）
 shape.draw（）; 
} 
  

## ## ##这里有什么??? #### ???

解决方案

假设ThirdPartClass.someMajicMethod有一个这样的签名：



  public  T someMajicMethod（Class< T> class1，File file）; 
  

然后你应该可以这样做：



  public void someMethod（Class< ;? extends Shape> type，File shapeFile）{
 ThirdPartyClass foo = new ThirdPa rtyClass（）; 
 
 @SuppressWarnings（unchecked）
 Class <？ extends Shape []> arrayType = 
（Class< ;? extends Shape []>）Array.newInstance（type，0）.getClass（）; 
 assert Shape []。class.isAssignableFrom（arrayType）; 
 
 Shape [] shapes = foo.someMajicMethod（arrayType，shapeFile）; 
 
 for（Shape shape：shapes）
 shape.draw（）; 
 
 
 
 
 
 $ b 
所以如果你调用 someMethod（Triangle.class，文件），然后 arrayType 将在 Triangle []。class  code> someMajicMethod 。
 
 
 
尽管您可能会发现使someMethod将数组类型作为参数而不是元素类型所以你可以避免这一步。
 
I have a hierarchy where Square, Triangle and Circle all extend from Shape. I have a working method:
public void someMethod() {
   File file = new File("File_with_squares");
   ThirdPartyClass foo = new ThirdPartyClass();
   Square[] squares = foo.someMajicMethod(Square[].class,file);
   for (Square square: squares) 
      square.draw();

}
Now I want to make this method generic so that it can accept any shape. I want to be able to call it someMethod(Triangle.class,new File("File_with_triangles") or someMethod(Circle.class, new File("File_with_circles"). I am trying like this:
public void someMethod(Class<? extends Shape> type, File shapeFile) {
   ThirdPartyClass foo = new ThirdPartyClass();
   #### What goes here??? ####
   for (Shape shape: shapes)
       shape.draw();
}
What should be there at #### What goes here??? #### ???
解决方案
Assuming ThirdPartClass.someMajicMethod has a signature something like this:
public <T> T someMajicMethod(Class<T> class1, File file);
Then you should be able to do something like this:
public void someMethod(Class<? extends Shape> type, File shapeFile) {
    ThirdPartyClass foo = new ThirdPartyClass();

    @SuppressWarnings("unchecked")
    Class<? extends Shape[]> arrayType = 
        (Class<? extends Shape[]>) Array.newInstance(type, 0).getClass();
    assert Shape[].class.isAssignableFrom(arrayType);

    Shape[] shapes = foo.someMajicMethod(arrayType, shapeFile);

    for (Shape shape: shapes)
        shape.draw();
}
So if you call someMethod(Triangle.class, file), then arrayType will be Triangle[].class in the call to someMajicMethod.

Though you may find it simpler to have someMethod take the array type as a parameter instead of the element type so you can avoid that step.

                        
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