通用：扩展了数量和可比性的基本知识

Question

class  MyTest<K,V extends Number & Comparable> implements Comparable<MyTest>{

    private K key;
    private V value;

}

What does this mean K, V extends Number & Comparable ? In above statement MyTest is implementing a comparable interface, But I am not able to figure out K,V and why there is Number & Comparable. k,v are not an interface. Can some one enlighten me on this.

Answer 1

This class accepts two generic arguments, first of them is named as K (which is likely to be used as "key"), and the second of them is named as V (which is likely to be used as "value").

V extends Number & Comparable part means that the class accepts only those V values which both extend Number abstract class and implement Comparable interface. Examples of such classes are standard JDK Integer , Float , Long , Double or BigInteger . If you want to use your own class as V , you should extend Number and implement Comparable at the same time. No restriction for the K is applied: you may use any class for K . For example, this type is valid:

MyTest<String, Integer> myTest; // K = String, V = Integer

But this one will result in compilation error:

MyTest<Integer, String> myTest; // K = Integer, V = String, does not implement Number

Answer 2

We can subtype a generic class or interface by extending or implementing it. The relationship between the type parameters of one class or interface and the type parameters of another are determined by the extends and implements clauses.

For example, ArrayList implements List that extends Collection, so ArrayList is a subtype of List and List is subtype of Collection. Here K represents key and V represents value. then from your code the Java compiler replaces the type parameter K with the Number and V with the Comparable, as below code:

private Number key;
private Comparable value;