How to remove the dulpicates from ArrayList<HashMap<String,Object>> in java?

Question

I'm having a Arraylist of type HashMap. I need to eliminate the duplicate entries from the same. I was using the set to remove the duplicate as shown below.

Set<Object> valueSet = new HashSet<Object>();
        for(HashMap<String, Object> map: mDataList){
                valueSet.addAll(map.values());
        }

But in sets we cannot store HashMap. I want the HashMap with key,value pair as the return value which will be added to the list. That is the final value should be in the format of ArrayList>.

Can you please help me in this regard. Thanks.

Answer 1

Set should be able to contain a HashMap.

I assume the duplicate here means key, value pair is the same between 2 HashMap. In that case, you can try

Set<HashMap<String, Object>> abc = new HashSet<HashMap<String, Object>>();
HashMap<String, Object> a1 = new HashMap<String, Object>();
a1.put("a", "a");

HashMap<String, Object> a2 = new HashMap<String, Object>();
a2.put("a", "a");

HashMap<String, Object> a3 = new HashMap<String, Object>();
a3.put("c", "c");

abc.add(a1);
abc.add(a2);
abc.add(a3);

for (HashMap<String, Object> each : abc){
    System.out.println(each.toString()); //{a=a}, {c=c}
}

EDITED based on your comment

You can reverse the value to be the key of a new HashMap. Check out the code below

String yourkey = "key";
List<HashMap<String, Object>> oldList = new ArrayList<HashMap<String, Object>>();

HashMap<String, Object> a1 = new HashMap<String, Object>();
a1.put(yourkey, "1");

HashMap<String, Object> a2 = new HashMap<String, Object>();
a2.put(yourkey, "1");

HashMap<String, Object> a3 = new HashMap<String, Object>();
a3.put(yourkey, "2");

oldList.add(a1);
oldList.add(a2);
oldList.add(a3);

HashMap<Object, Object> newMap = new HashMap<Object, Object>();
for (HashMap<String, Object> each : oldList){
    // Put the value as key. Duplicate will be overridden
    newMap.put( each.get(yourkey), each);
}

List<HashMap<String, Object>> noDuplicateList = new ArrayList<HashMap<String, Object>>();
for (Object each : newMap.keySet()) {
    noDuplicateList.add( (HashMap)newMap.get(each) );
}

System.out.println(noDuplicateList); // [{key=2}, {key=1}]

You must make sure that the 'value' is comparable (Here we use String). Otherwise, create your own equals method.

Hope it helps