<E extends Number>和<Number>之间有什么区别？

Question

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?

Answer 1

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> 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

Answer 2

The latter method (the one without <E extends Number> ) will only accept a parameter of exactly type List<Number> and will it always return a List<Number> . For example, it will not accept List<Integer> .

The former method (the one with <E extends Number> ) is a generic method , meaning it can accept different types of List s and it will return the same type of List , as long as the List s are lists of something that extends Number , eg List<Integer> .

Example:

import java.util.ArrayList;
import java.util.List;

public class ProcessGenerics {

    List<Number>  listNumber  = new ArrayList<Number>();
    List<Integer> listInteger = new ArrayList<Integer>();
    List<Double>  listDouble  = new ArrayList<Double>();


    public static
        List<Number> processWithoutExtends(List<Number> nums){ return nums; }

    List<Number>  resultN = processWithoutExtends(listNumber);  // OK
  //List<Integer> resultI = processWithoutExtends(listInteger); // compile-error - method not applicable
  //List<Double>  resultD = processWithoutExtends(listDouble);  // compile-error - method not applicable


    public static <E extends Number>
        List<E> processWithExtends(List<E> nums){ return nums; }

    List<Number>  resultN2 = processWithExtends(listNumber);  // OK
    List<Integer> resultI2 = processWithExtends(listInteger); // OK
    List<Double>  resultD2 = processWithExtends(listDouble);  // OK

}

See a similar explanation in the Wildcards chapter in the Generics Lesson in the Java Tutorials:
http://java.sun.com/docs/books/tutorial/java/generics/subtyping.html

See also How to cast a list of inheriting objects to a collection of objects in Java? Both questions are really about generics and subtypes, eg whether List<Integer> is a subtype of List<Number> (it's not!!!).