Getting the newly added , deleted , modified values in hashmap at a particular state

Question

I have an application , which operates on the hashmap at stages , in the sense it adds/deletes/modifies keys in different classes . Thought of creating wrapper class by extending the Map class . And Hacking the predefined put and remove methods .

STAGE 1 :

HashMap<String, String> hashMap = new HashMap<>(); hashMap.put("Key1","Value1"); hashMap.put("Key2","Value2"); hashMap.put("Key3","Value3"); hashMap.put("Key4", "Value4");

Desired Result :
Added:
Key1 : Value1
Key2 : Value2
Key3 : Value3
Key4 : Value4

STAGE 2:

hashMap.remove("Key1");

Desired Result :
Removed:
Key1 : Value1

STAGE 3:

hashMap.put("Key2", "ChangedValue");

Desired Result :
Modified :
Key2 : ChangedValue

What would be the best way or best logic to get only the diff ? The dataStructure HASHMAP is fixed .

Yes create a class which extends Map, and override the methods to print what you want — azro
– azro, Commented Aug 10, 2017 at 13:28
You could just extend HashList to record your changes as they come, print them or store them. I added a basic implementation below. — Assafs
– Assafs, Commented Aug 10, 2017 at 15:55
@shashantrika, if the answer below was useful, may I ask you accept it? — Assafs
– Assafs, Commented Aug 17, 2017 at 10:42
@shashantrika, why did you un-accept? Is there anything I can do to add to the solution? — Assafs
– Assafs, Commented Aug 17, 2017 at 12:56

Assafs · Accepted Answer · 2017-08-10 15:54:50Z

The simplest way is to extend HashMap to your own class, and record the changes:

class RecordHashMap extends HashMap<String,String> { private List<String[]> changes; public RecordHashMap() { super(); changes = new ArrayList<String[]>(); } @Override public String put(String key, String value) { if (containsKey(key)) { changes.add(new String[]{"modified",key,value}); } else { changes.add(new String[]{"added",key,value}); } return super.put(key, value); } @Override public String remove(Object key) { if (containsKey(key)) { String value = get(key); changes.add (new String[]{"removed",(String)key,value}); } return super.remove(key); } public List<String[]> getChanges() { return changes; } }

This way you can always check the last change, as they are all recorded. You can of course print them out as you record them - or later. You can add an index counter (to allow to only look at x recent changes), since you store them in an array list.

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