< E extensions Number>之间的区别是什么？和< Number&gt ;? [英] What is the difference between <E extends Number> and <Number>?

问题描述

这个方法声明有什么区别：

  public static< E extends Number>列表与LT E  - 代替;进程（列表< E>数量）{
  

和

  public static List< Number> process（List< Number> nums）{
  

您会在哪里使用前者？

解决方案

第一个允许 List c进程 / code>，a 列表< Double> 等等。第二个不是。

泛型在Java中是不变的。它们不像数组那样是协变的。

即在Java中， Double [] 是子类型 Number [] ，但是 List< Double> 不是 List< Number>的子类型。 。然而，列表< Double> 是列表< ;?扩展数字> 。

泛型不变的原因很多，但这也是为什么扩展和 super 类型通常是子类型灵活性所必需的。 另见

• Java教程/泛型/子类型
  
  
  
  
  • 解释为什么泛型不变是一件好事
  
  


• 更多有趣的通配符
  
  
  
  
  • 解释 super 和 extends 限制通配符

  
  
  • 
    
    
    
    • 这讨论了Producer extends 消费者 super p原理
    
    
    • 有效Java第二版，项目28：使用有界通配符提高API灵活性
    
    
    
    

    What is the difference between this method declaration:

    public static <E extends Number> List<E> process(List<E> nums){
    

    and

     public static List<Number> process(List<Number> nums){
    

    Where would you use the former?

    解决方案

    The first allows process of a List<Integer>, a List<Double>, etc. The second doesn't.

    Generics in Java are invariant. They're not covariant like arrays.

    That is, in Java, Double[] is a subtype of Number[], but a List<Double> is NOT a subtype of List<Number>. A List<Double>, however, is a List<? extends Number>.

    There are good reasons for generics being invariant, but that's also why the extends and super type are often necessary for subtyping flexibility.

    See also

    • Java Tutorials/Generics/Subtyping
      • Explains why generics invariance is a good thing
    • More fun with wildcards
      • Explains some uses of super and extends for bounded wildcards
    • Java Generics: What is PECS?
      • This discusses the "Producer extends Consumer super" principle
      • Effective Java 2nd Edition, Item 28: Use bounded wildcards to increase API flexibility

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