What is the best way to store three attributes entry sets in Java?

Question

HashMaps allow for mapping key value pairs; what is the recommendation if there is an extra value?

i.e HashMap maps key-value pairs: Key, Value

What is the best way to map: Key, Value, Value1?

Thanks.

Answer 1

You can :

• Use a list as value

  Map<String,List<String>> map = new HashMap<String,List<String>>();
  map.put("key",Arrays.asList("one","two" );
  map.get("key").get(0);
  map.get("key").get(1);
  

• Use an array as value and refer as

  Map<String,String[]> map = new HashMap<String,String[]>();
  map.put( "key", new String[]{"one", "two"});
  map.get("key")[0];
  map.get("key")[1];
  

• Use a custom generic class to hold two values

  Map<String,Pair> map = new HashMap<String,Pair>();
   map.put( "key", new Pair<String,String>("one","two"));
   map.get("key").val1();
   map.get("key").val2();
  

Answer 2

A nice, clean solution I like is to write up a utility class that matches a pair:

public class Pair<U, V> {

    private U first;

    private V second;

public Pair(U first, V second) {

 this.first = first;
 this.second = second;
}

// getters for "first" and "second"

and then have that be the Value in your map:

Map<Key, Pair<U,V>> map;

However, this would be in the case that you would always have only two values. If you have a feeling that there might be more in the future, a List<Object> or Set<Object> will be better to use as a Value in the map.

EDIT you could have a static creator-method too:

public static <U, V> Pair<U, V> newInstance(U first, V second) {
        return new Pair<U, V>(first, second);
    }

Answer 3

The standard solution is to encapsulate the two values in their own data structure.

The data structure could be specific to your situation, or you could hack together a simple Pair class.

You'd then use an instance of the structure as your list value, e.g. map.put(key, new Pair(foo, bar));

There's also the option of using a specialised Map implementation, e.g. http://commons.apache.org/collections/api-3.1/org/apache/commons/collections/MultiMap.html

Answer 4

You can use Map< key,List< MyVOClazz > >

    public class MyVOClazz{
        private String value1;
        private String value2;

        //getters, setters for value1 and value2 
}