如何从嵌套的 HashMaps 中检索键而不是其值

Question

I am trying to figure out a fast way of how to access the key of a nested hashmap.

I am quite new to java and i know how to access the value of the innermost hashmap (with the get() method) but I can't figure out how to easily access the key of an inner hashmap.The hashMap has its own class and is defined like this:

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

I can perfectly get the Value of the inner most HashMap by using this get() method I defined within the class:

int get(String a, String b, Double c){
        if (data.containsKey(a)){
            if (data.get(a).containsKey(b)){
                  if (data.get(a).get(b).containsKey(c){    
                            return data.get(a).get(b).get(c);
                  }
            }       
        } 
        return 0;

Now i want to define a method getk() for this class and HashMap that allows me to retrieve the key of the inner HashMap by entering the first two Keys like this:

double getk(String a, String b){
        if (data.containsKey(a)){
            if (data.get(a).containsKey(b)){
                return ***I don't know what to put here***;
            }       
        } 
        return 0;
    }

Edit additional info: The key "c" I want to get has a 1:1 relationship with key "b". Is there a possibility to easily solve this?

Thanks a lot for your time and help;)

Answer 1

You cannot get only one key of the inner Hashmap. Because Hashmap contains as Keys a set of keys. You can get the list of keys by calling:

map.keySet();

Your IDE should helps you find that.

[EDIT]

If you have a 1:1 relationship, just used a Pair Wrapper Object than HashMap. like this one:

public class KeyValue {
    private double key;
    private Integer value;

    public KeyValue(double key, Integer value) {
        this.key = key;
        this.value = value;
    }

    public double getKey() {
        return key;
    }

    public void setKey(double key) {
        this.key = key;
    }

    public Integer getValue() {
        return value;
    }

    public void setValue(Integer value) {
        this.value = value;
    }
}

You wil be able to get your key by calling getKey() of this Object.

Answer 2

You can call method keySet() on hashmap onject to get a Set of all keys in hashmap.

myMap.keySet();

https://docs.oracle.com/javase/8/docs/api/java/util/HashMap.html#keySet--

Answer 3

Edit:

Sorry i misunderstood your question. All that you have to do is just return the following

data.get(a).get(b).keySet().iterator().next();

As the b hash set has only one key c, by iterating the first element will return the key c

keySet() method returns set of all the keys.