How to have a key with multiple values in a map?

Question

I have a map like this

Map map=new HashMap();//HashMap key random order.
map.put(\"a\",10);
map.put(\"a\",20);
map.put(\"a\",30);
map.put(\"b\",10);

System.         


        

Answer 1

To implement what you want using the Java standard library, I would use a map like this:

Map> multiValueMap = new HashMap>();

Then you can add values:

multiValueMap.put("a", new ArrayList());
multiValueMap.get("a").add(new Integer(10));
multiValueMap.get("a").add(new Integer(20));
multiValueMap.get("a").add(new Integer(30));

If this results uncomfortable for you, consider wrapping this behaviour in a dedicated Class, or using a third-party solution, as others have suggested here (Guava Multimap).