接受两种类型的通用类 [英] Generic class that accepts either of two types

问题描述

  class MyGenericClass< T extends Number> 
我想创建一个这种形式的泛型类： {} 
  

问题是，我希望T可以被接受为Integer或Long，但不能双。因此，只有两个可接受的声明是：

  MyGenericClass< Integer>实例; 
 MyGenericClass< Long>实例; 
  

有没有办法做到这一点？

解决方案

答案是否定的。至少没有办法使用泛型类型来完成它。我会推荐泛型和工厂方法的组合来做你想做的事。

  class MyGenericClass< T extends Number> {
 public static MyGenericClass< Long> newInstance（Long value）{
 return new MyGenericClass< Long>（value）; 
} 
 
 public static MyGenericClass< Integer> newInstance（Integer value）{
返回新的MyGenericClass< Integer>（value）; 
} 
 
 //隐藏构造函数，所以你必须使用工厂方法
 private MyGenericClass（T value）{
 //实现构造函数
} 
 // ...实现类
 public void frob（T number）{
 //用T 
做一些事情} 
} 
  MyGenericClass< Integer> 和  $ b>可以创建MyGenericClass< Long> 实例。尽管您仍然可以声明 MyGenericClass< Double> 类型的变量，它只需要为空。
 
I want to make a generic class of this form:
class MyGenericClass<T extends Number> {}
Problem is, I want to be acceptable for T to be either Integer or Long, but not Double. So the only two acceptable declarations will be:
MyGenericClass<Integer> instance;
MyGenericClass<Long> instance;
Is there any way to do that?
解决方案
The answer is no.  At least there is no way to do it using generic types.  I would recommend a combination of generics and factory methods to do what you want.
class MyGenericClass<T extends Number> {
  public static MyGenericClass<Long> newInstance(Long value) {
    return new MyGenericClass<Long>(value);
  }

  public static MyGenericClass<Integer> newInstance(Integer value) {
    return new MyGenericClass<Integer>(value);
  }

  // hide constructor so you have to use factory methods
  private MyGenericClass(T value) {
    // implement the constructor
  }
  // ... implement the class
  public void frob(T number) {
    // do something with T
  }
}
This ensures that only MyGenericClass<Integer> and MyGenericClass<Long> instances can be created.  Though you can still declare an variable of type MyGenericClass<Double> it will just have to be null.

                        
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