Retrieve all values from HashMap keys in an ArrayList Java

Question

Good day, this is kind of confusing me now(brain freeze!) and seem to be missing something. Have an ArrayList which i populate with a HashMap. now i put in my HashMap and arraylist.

 Map.put(DATE, value1);
 Map.put(VALUE, value2);

 arraylist.put(Map);

Since am parsing a JSON, the arraylist increases in significant size. now my question is how do you get the values from both map keys in the arraylist? i have tried this

  if(!list.isEmpty()){   // list is an ArrayList

            for(int k = 0; k < list.size(); k++){
                map = (HashMap)list.get(k);
            }

        }

        Log.d(TAG, "map size is" + map.size());

        String [] keys = new String[map.size()];
        String [] date_value = new String[map.size()];

        String [] value_values = new String[map.size()];

        int i = 0;
        Set entries = map.entrySet();
        Iterator iterator = entries.iterator();

        while(iterator.hasNext()){

            Map.Entry mapping = (Map.Entry)iterator.next();
            keys[i] = mapping.getKey().toString(); 
            date_value[i] = map.get(keys[i]);

            if(keys[i].equals(DATE)){
                date_value[i] = map.get(keys[i]);

            } else if(keys[i].equals(VALUE)){
                value_values[i] = map.get(keys[i]);
            }

                   i++;
                 }

But i can't seem to get all the values. the Map size always return a value of 2, which is just the elements. how can i get all the values from the Map keys in the ArrayList? Thanks

Solution 1

Why do you want to re-invent the wheel, when you already have something to do your work. Map.keySet() method gives you a Set of all the keys in the Map.

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

for (String key: map.keySet()) {
    System.out.println("key : " + key);
    System.out.println("value : " + map.get(key));
}

Also, your 1st for-loop looks odd to me: -

   for(int k = 0; k < list.size(); k++){
            map = (HashMap)list.get(k);
   }

You are iterating over your list, and assigning each element to the same reference - map, which will overwrite all the previous values.. All you will be having is the last map in your list.

EDIT: -

You can also use entrySet if you want both key and value for your map. That would be better bet for you: -

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

    for(Entry<String, Integer> entry: map.entrySet()) {
        System.out.println(entry.getKey());
        System.out.println(entry.getValue());
    }

P.S.: -
Your code looks jumbled to me. I would suggest, keep that code aside, and think about your design one more time. For now, as the code stands, it is very difficult to understand what its trying to do.

Solution 2

List constructor accepts any data structure that implements Collection interface to be used to build a list.

To get all the keys from a hash map to a list:

Map<String, Integer> map = new HashMap<String, Integer>();
List<String> keys = new ArrayList<>(map.keySet());

To get all the values from a hash map to a list:

Map<String, Integer> map = new HashMap<String, Integer>();
List<Integer> values = new ArrayList<>(map.values());

Solution 3

Try it this way...

I am considering the HashMap with key and value of type String, HashMap<String,String>

HashMap<String,String> hmap = new HashMap<String,String>();

hmap.put("key1","Val1");
hmap.put("key2","Val2");

ArrayList<String> arList = new ArrayList<String>();

for(Map.Entry<String,String> map : hmap.entrySet()){

     arList.add(map.getValue());

}

Solution 4

Put i++ somewhere at the end of your loop.

In the above code, the 0 position of the array is overwritten because i is not incremented in each loop.

FYI: the below is doing a redundant search:

if(keys[i].equals(DATE)){                 
   date_value[i] = map.get(keys[i]);              
} else if(keys[i].equals(VALUE)){              
   value_values[i] = map.get(keys[i]);             
} 

replace with

if(keys[i].equals(DATE)){                 
   date_value[i] = mapping.getValue();
} else if(keys[i].equals(VALUE)){              
   value_values[i] = mapping.getValue()
} 

Another issue is that you are using i for date_value and value_values. This is not valid unless you intend to have null values in your array.

Solution 5

This is incredibly old, but I stumbled across it trying to find an answer to a different question.

my question is how do you get the values from both map keys in the arraylist?

for (String key : map.keyset()) {
  list.add(key + "|" + map.get(key));
}

the Map size always return a value of 2, which is just the elements

I think you may be confused by the functionality of HashMap. HashMap only allows 1 to 1 relationships in the map.

For example if you have:

String TAG_FOO = "FOO";
String TAG_BAR = "BAR";

and attempt to do something like this:

ArrayList<String> bars = ArrayList<>("bar","Bar","bAr","baR");
HashMap<String,String> map = new HashMap<>();
for (String bar : bars) {
  map.put(TAG_BAR, bar);
}

This code will end up setting the key entry "BAR" to be associated with the final item in the list bars.

In your example you seem to be confused that there are only two items, yet you only have two keys recorded which leads me to believe that you've simply overwritten the each key's field multiple times.

View more solutions

Comments

[John B]

Downvoter care to comment? in the above code, the 0 position of the array is overwritten because i is not incremented.

[John B]

entrySet is a better way to iterate the elements in a map if the code in the loop calls get

[toadzky]

he didn't post code where i is declared. if it's in a for-loop, it's probably already being incremented. you don't know what is going on with 'i' because he hasn't posted anything about 'i'.

[John B]

@toadzky recheck the post please. Look for int i = 0;. He DID post the code where i is declared.

[Rohit Jain]

Yeah thats fine now. But why are you adding all your map values to a list? Is it really neede? I think no. Anytime you want a value, you can get it from the Map itself.. For the time being, it is very unclear, what OP wants to do.

[Kumar Vivek Mitra]

@RohitJain I have added the value in the ArrayList as the OP wanted it this way..and thats what the heading of the question says....

[Rohit Jain]

@KumarVivekMitra.. Yeah it is correct according to what OP wants, but as this is not correct way to code, you can better add some suggestion. Just for improvement of this post I'm telling. :)

[Kumar Vivek Mitra]

@RohitJain i can, but then we should know what the programmer wants to do with that data..as its not very clear to me that what he wants to do with that data...i am not suggesting anything for now...

[Rohit Jain]

@KumarVivekMitra.. Hmm. Yeah that part is also right. Cheers :)

[Rohit Jain]

@KumarVivekMitra.. Well, You're Welcome. But what for? :-^

[irobotxx]

@RohitJain sorry for replying so late, didn't have a chance after posting the question. what am trying to do is get all the values from the map. As i said in the post, the Hashmap has 2 keys which is populated through a JSON parsing and then added to an ArrayList. I want to retrieve all the values for the 2 keys in the HashMap, which is inside the ArrayList. Yes, i know the first "for" loop look odd. but i was trying to get all the values in the Arraylist because the Hashmap was returning only a size of 2 values and there are more in the ArrayList.

[irobotxx]

@JohnB sorry for my late response. But i had that Value of i incremented. i didn't reallise i did not copy it in. Will edit my code now.

[Patricio Córdova]

@RohitJain is better if you show the part that explains the "entrySet" in the upper part of the answer, instead of the part that talks about the "keySet". Just swap the order of the sections.

[paulz]

This is the best answer I saw for this question! Directly to the point.