CODEWITHSUNDEEP
javahashmapnested
Tanner Quinn
Tanner Quinn

Reputation: 23

Nested Hashmaps overwriting other keys

I have a problem where when I want to change a value in a nested HashMap, the other HashMaps are being overwritten.

for example I have a key name Alligator, which stores a HashMap that contains the keys Weight and Size. I want to be able to change the value associated with Size, but not affect Weight at all. In a yml file, this is what it would look like.

Alligator:
  Weight: 100.0
  Size: 10.0

And this is what I would like to happen.

Alligator:
  Weight: 150.0
  Size: 10.0

This was Some Code that I found earlier that lets me change the values, but with overwriting...

HashMap< String, HashMap< String, Double>> data = new HashMap<>();

       data.put("Alligator", new HashMap() {
            {
                put("Size", 10.0
            }
        });

HashMap< String, HashMap< String, Double>> data = new HashMap<>();

       data.put("Alligator", new HashMap() {
            {
                put("Weight", 100.0
            }
        });

Upvotes: 2

Views: 1569

Answers (4)

SHAH NIRAV
SHAH NIRAV

Reputation: 35

Crate a inner HashMap with different name, so you never get confused between two maps.

Map<String,Map<String,Integer>> outermap = new HashMap<String,Map<String,Integer>>();
        Map<String,Integer> innermap1 = new HashMap<String,Integer>();


        innermap1.put("Weight",100);
        innermap1.put("Size", 10);
        outermap.put("Alligator",innermap1);

Upvotes: 0

Eran
Eran

Reputation: 393771

   data.put("Alligator", new HashMap() {
        {
            put("Weight", 100.0);
        }
    });

would overwrite the internal Map of the "Alligator" key if it's already in the outer Map.

You must check for existence first :

Map<String, Double> inner = data.get("Alligator");
if (inner != null) {
    inner.put("Weight", 100.0);
} else {
    data.put ("Alligator", new HashMap() {
        {
            put("Weight", 100.0);
        }
    });
}

BTW, I wouldn't use an anonymous sub-class of HashMap instance for the inner Map. You can replace it with this simpler code :

Map<String, Double> inner = data.get("Alligator");
if (inner == null) {
    inner = new HashMap<>();     
    data.put ("Alligator",inner);
}
inner.put("Weight", 100.0);

Upvotes: 2

Luffy
Luffy

Reputation: 99

A 

new HashMap(){ // blablabla

means a new Object instance in the memery,

and a 

data.put("Alligator", new HashMap() { // blablabla

means adding(key not exists) or overwriting(key exists) a KV pair in the data map.

As @Eran suggests, 

inner = data.get("Alligator")

checking if inner exists

Upvotes: 1

dting
dting

Reputation: 39287

First handle what you want to happen if there is no "Alligator" in your data HashMap, possibly put in a new HashMap for "Alligator":

if (!data.containsKey("Alligator")) {
  data.put("Alligator", new HashMap<>());
}

Then use get to grab the nested HashMap and use put to change the values:

HashMap<String, Double> alligator = data.get("Alligator");
alligator.put("Weight", 150.0);

Upvotes: 1

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