在泛型类中扩展Comparable

Question

I am new to Java and I have a problem. I need a class to store two values, one called key and one called value . The class can store the objects, return value and allow comparison between two objects by comparing the key value.

public class Entry <K,V> implements Comparable<Entry<K,V>> 
{
    private K key;
    private V value;

    public Entry(K key,V value)
    {
        this.key=key;
        this.value=value;
    }

    public void setValue(V value)
    {
        this.value=value;
    }

    public String toString()
    {
        return "key= "+key+" value=" +value;
    }
}

He is now asking me to add the Comparable method. I am allowed to use equalsTo(Object) as well. How can I implement Comparable ? I tried

public Comparable(k key)
{
    if (this.k < k)
        return -1; 

    if (this.k > k)
        return 1;

    else return 0;
}

but I got an error saying that I am not allowed to use > or < .

Answer 1

The other answers here seem to be very confused as to generics and the Comparable interface .

Here is some code that will compile:

public class Entry<K extends Comparable<K>, V> implements Comparable<Entry<K, V>> {

    private final K key;
    private V value;

    public Entry(K key, V value) {
        this.key = key;
        this.value = value;
    }   

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

So you force the type of your key to also extend Comparable to itself; you then use its compareTo method to compare your Entry to the other Entry based on the keys.