如何为唯一标识符设置地图？

Question

We have an database table and the unique index on that table is a composite key (String, int, int). I want to add a field in my Java class that will hold this key. I tried:

Map<String, Integer, Integer>

but the IDE told me I cannot do that as a map is just Map K,V.

My question is what is the best way to represent this key?

Answer 1

Map<String, Integer, Integer>

That does not work, because Map takes only two type parameters (for the key and the value), not three.

Create a class that represents the key. Use that as a key in the map. Note: The key class must implement the hashCode and equals methods correctly.

Answer 2

Why don't you just create a custom key like this -

class MyKey{
   int key1;
   int key2;
   String key3;
   // override hashCode & equals
}

You can then use this class as your key

Map<MyKey, Value>

Answer 3

It depends what you want to do with it:

You can create a bean as key:

public class MyCompositeKey {
    private String field1;
    private Integer field2, field3;

    //getters and setters here


    @Override
    public int hashCode() {
    //your implementation here, you can use here the hashCode of Integer and String
    }

    @Override
    public boolean equals(Object o) {
        if (o instanceof MyCompositeKey) {
            MyCompositeKey that=(MyCompositeKey) o;
            //compare here the fields  
        }
        return false;
    }
}

Or you can do something like:

String key=field1+"-"+field2+"-"+field3;

and use that String as key in your map.

Or maybe you just need a List<Comparable> where you add the three elements in a prefixed order.