How to use HashMap with List<Number>

Question

How do I correctly instantiate a HashMap whose values are variants of List<Number> - i.e., the values could take the form of List<Long> and/or List<Double>. Here's what I've tried so far:

HashMap<String, List<Number>> foo = new HashMap<String, List<Number>>();
foo.put("Tenticles", new ArrayList<Long>());

The error message: The method put(String, List<Number>) ... is not applicable for the arguments (String, List<Long>).

I also want to be able to add a Long to a constituent List<Long> and likewise with any Double.

EDIT These answers don't show how to add a Long or a Double to one of the constituent lists.

This is going to be tricky to do if you can make it work at all, because the covariance stuff will get messy. You could totally make it work, though, with foo.put("Tenticles", new ArrayList<Number>()), though. — Louis Wasserman
– Louis Wasserman, Commented Oct 8, 2013 at 3:01

cmd · Accepted Answer · 2013-10-08 03:29:15Z

3

You can use wildcards

e.g.

Map<String, List<? extends Number>> foo = new HashMap<String, List<? extends Number>>();

If you are using Java 7, you can do the following to avoid some duplication

Map<String, List<? extends Number>> foo = new HashMap<>();

To add an item to the List

List<Long> longs= new ArrayList<>();
longs.add(10L);
foo.put("numbers", longs);

edited Oct 8, 2013 at 3:29

answered Oct 8, 2013 at 2:42

cmd

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3 Comments

user2763361 Over a year ago

How do I then add something to the arraylist that go inside

cmd Over a year ago

You can do it a number of ways. Here is an exmaple: List<Long> longs = new ArrayList<>(); longs.add(10L); List<Double> doubles = new ArrayList<>(); doubles.add(23.20); List<Number> numbers = new ArrayList<>(); numbers.addAll(longs); numbers.addAll(doubles); foo.put("numbers", numbers); I apologize for the formatting. :P

cmd Over a year ago

The answer now contains an example of adding elements to the list contained in the Map

Sotirios Delimanolis · Accepted Answer · 2013-10-08 03:16:49Z

2

Use a wildcard

HashMap<String, List<? extends Number>> foo = new HashMap<String, List<? extends Number>>();
LinkedList<Long> list = new LinkedList<>();
list.add(10L);
foo.put("Tenticles", list);

Your List objects now must be a sub-type of Number.

edited Oct 8, 2013 at 3:16

answered Oct 8, 2013 at 2:41

Sotirios Delimanolis

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6 Comments

user2763361 Over a year ago

How do I then add something to the arraylist that go inside

Sotirios Delimanolis Over a year ago

@user2763361 Just keep a reference to the List and call its add method with the element you want to add. You can always retrieve the List with get(int).

user2763361 Over a year ago

It doesn't work. The method add(capture#7-of ?) in the type ... not applicable for the arguments long.

Sotirios Delimanolis Over a year ago

@user Please update your question with how you are trying to use it. It works fine for me.

user2763361 Over a year ago

foo.get("Tenticles").add(10L).

|

WW. · Accepted Answer · 2013-10-08 02:44:15Z

0

Here's what you need to do:-

    final HashMap<String, List<? extends Number>> foo = new HashMap<>();
    foo.put( "Tenticles", new ArrayList<Long>() );

The syntax List<? extends Number> means a list of objects where each object implements Number.

answered Oct 8, 2013 at 2:44

WW.

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1 Comment

user2763361 Over a year ago

How do I then add something to this arraylist? It's not working :s

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