如何使关联对象仅可通过一种通用参数类型进行比较

Question

I'm trying to understand what <K, V extends Comparable<? super V>> <K, V extends Comparable<? super V>> actually means.

If compareTo() is called to compare two Association<K,V> objects; which of the two generic types K or V will be used to make the comparison?

What will happen when <K, V extends Comparable<? super K>> <K, V extends Comparable<? super K>> is used instead?

 public class Association<K, V extends Comparable<? super V>> { K key; V value; //--------------------------------------- public int compareTo(Association<K, V> object) { return this.key.compareTo(object.key); } } 

edited the compareTo() method.

Answer 1

If compareTo() is called to compare two Association objects; which of the two generic types K or V will be used to make the comparison?

As Association is implemented right now, neither will be used. Instead, your compareTo method will always throw a StackOverflowError , because it's incorrectly defined.

(The implication is that it's the responsibility of the Association class and its definition of compareTo to decide what the answer to your question is. For example, you might write public int compareTo(Association<K, V> other) { return value.compareTo(other.value); } . )

Answer 2

First to explain your current definition:

public class Association<K, V extends Comparable<? super V>>

This means you are defining a public class called Association This class has 2 type parameters: K and V . Of these, V has the additional restraint that it must be comparable to ? super V ? super V (= V or a super type of V ).

Since you want ot compare by the key, it should be K that is Comparable . Additionally, you want to compare the Association s themselves, so it too must be Comparable .

So change your class definition to the following:

public class Association<K extends Comparable<K>, V> implements Comparable<Association<K, V>>

Your compareTo would then be:

public int compareTo(Association<K, V> other) {
    return key.compareTo(other.key);
}