Java Fill nested HashMap

Question

I have a problem. I created the following object:

HashMap<String, HashMap<String, HashMap<Integer, ArrayList<Slope>>>>
     usedSlopeList = new HashMap<>();

Then I have the following ArrayList<Slope>:

ArrayList<Slope> tempSlopeList = Slope.getSlopeList(
    agent, key, slope, startDateTimeWithExtraCandles);

But when I want to fill the usedSlopeList like this:

usedSlopeList.put("15m", 
new HashMap<String, HashMap<Integer, ArrayList<Slope>>>()
    .put("EMA15", new HashMap<Integer, ArrayList<Slope>>()
    .put(15, tempSlopeList)));

Unfortunately this gives me the error:

Required type: HashMap<Integer,java.util.ArrayList<com.company.models.Slope>>
Provided: ArrayList<Slope,

But I don't see why this is wrong... Can someone help me?

Nowhere Man · Accepted Answer · 2021-02-15 10:47:06Z

1

Map::put returns value while a map is expected.

That is, new HashMap<Integer, ArrayList<Slope>>().put(15, tempSlopeList) returns ArrayList<Slope> and so on.

The following code using Map.of available since Java 9 works fine:

usedSlopeList.put("15m", new HashMap<>(Map.of("EMA15", new HashMap<>(Map.of(15, tempSlopeList)))));

Update A cleaner solution not requiring Java 9 could be to implement a generic helper method which creates an instance of a HashMap and populates it with the given key/value:

static <K, V> HashMap<K, V> fillMap(K key, V val) {
    HashMap<K, V> map = new HashMap<>();
    map.put(key, val);
    return map;
}

ArrayList<Slope> tempSlopeList = new ArrayList<>(Collections.emptyList());
HashMap<String, HashMap<String, HashMap<Integer, ArrayList<Slope>>>>
     usedSlopeList2 = fillMap("15m", 
                          fillMap("EMA15", 
                              fillMap(15, tempSlopeList)
                          )
                      );
    
System.out.println(usedSlopeList2);

Output:

{15m={EMA15={15=[]}}}

edited Feb 15, 2021 at 10:47

answered Feb 15, 2021 at 10:26

Nowhere Man

19.7k9 gold badges21 silver badges45 bronze badges

Sign up to request clarification or add additional context in comments.

1 Comment

A. Vreeswijk Over a year ago

Thanks, very easy solution for me!

score 1 · Accepted Answer · 2021-02-15 10:15:29Z

1

You used the new HashMap().put() as the second parameter in your code, which causes the issue.

HashMap().put is not a builder method; it doesn't return the hashmap. It returns the previous value associated with the key, which in your case is an ArrayList.

edited Feb 15, 2021 at 10:15

answered Feb 15, 2021 at 10:10

user10992464

2 Comments

A. Vreeswijk Over a year ago

Then what could I use to create a filled hashmap?

user10992464 Over a year ago

If you don't have an other map with the values then you can't inline them. You have to create a new variable and fill them up manually.

Dragos Ionut · Accepted Answer · 2021-02-15 10:14:59Z

0

You have a map, which expects a string as key and a hashmap as value, but put() method doesn't return a hashmap, it return an V object(<K, V>), that is why you should create hashmap separately, add the object and than try to add it. Anyway i think you should reconsider your design.

answered Feb 15, 2021 at 10:14

Dragos Ionut

2571 silver badge8 bronze badges

Collectives™ on Stack Overflow

Java Fill nested HashMap

3 Answers 3

1 Comment

2 Comments

Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

3 Answers 3

1 Comment

2 Comments

Comments

Your Answer

Sign up or log in

Post as a guest

Related