How to put/get values into/from Nested HashMap

Question

I want to create a nested HashMap that will take two keys of type float and give out value of type Integer.

 public static HashMap<Float, HashMap<Float, Integer>> hashX = new HashMap<Float,HashMap<Float, Integer>>();

Is there a simple method of putting/getting the values like an ordinary HashMap i.e.

  hashX.put(key, value);
  hashX.get(key);

or is it a more complicated method that must be used? I have searched around the web for a solution but am finding it tough to find a solution that applies to me. Any help would be appreciated!

Solution 1

Map<Float, Map<Float, Integer>> map = new HashMap<>();

map.put(.0F, new HashMap(){{put(.0F,0);}});
map.put(.1F, new HashMap(){{put(.1F,1);}});

map.get(.0F).get(.0F);

Solution 2

You have to get() the nested map out of the outer map and call can call put() and get() on it

float x = 1.0F;
HashMap<Float, Integer> innerMap = hashX.get(x);
if (innerMap == null) {
    hashX.put(x, innerMap = new HashMap<>()); // Java version >= 1.7
}
innerMap.put(2.0F, 5);

Solution 3

You can create a wrapper class with a method like this:

public class MyWrapper {
    private Map<Float, Map<Float, Integer>> hashX;
    // ...
    public void doublePut(Float one, Float two, Integer value) {
        if (hashX.get(one) == null) {
            hashX.put(one, new HashMap<Float, Integer>());
        }
      hashX.get(one).put(two, value);
    }
}

Please note that you should use interfaces instead of concrete implementations when you declare your fields. For example it would make easier to refactor HashMap into ConcurrentHashMap if the need arises.

Solution 4

You can do it like this:

HashMap<Float, Integer> h1 = new HashMap<Float, Integer>();
h1.put(1.0f,new Integer(1));
HashMap<Float, Integer> h2 = new HashMap<Float, Integer>();
h2.put(3.0f,new Integer(3));

hashX.put(1.0f, h1);
hashX.put(1.0f, h1);

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Comments

[jlordo]

Why don't you save the reference to the new map, to avoid the following lookup operation, like you saw in my answer?

[Adam Arold]

And the infamous instance initializer block appears. :)

[jlordo]

That's a NullPointerException, because map.get(0) will return null

[user1927105]

@isvforall thanks, is there anyway around using HashMap<>(), i'm using Android and I can't use level 1.7 as Android requires 5.0

[Adam Arold]

Well this looked somewhat a more idiomatic way to do it.

[isvforall]

@user1927105 Unfortunately, only HashMap<Float, Map<Float, Integer>>();

[Janac Meena]

@AdamArold why infamous?

[Adam Arold]

@JanacMeena this is subjective, but most people I know don't like it (including me) because it makes the code harder to read and reason about.