Collections FrameWork

All collections are iterable(for set it’s an advanced loop). 
And all collections are core interface. sync(means slow) not sync(means fast)

LIST: Can store duplicate values, maintains /keeps the insertion order. 
It allows any number of null values, from list we can read a certain value 
by index.
• ArrayList(not sync), LinkedList(not sync, can work as stack,queue and dequeue)
and Vector(sync)=>(all are class and it implements list)

SET: can only store unique values (no duplicates), and does not maintain order
• HashSet(not sync, no order, allow null), LinkedHashSet(no sync, ordered, 
allow null, and maintains double LinkedList) and 
TreeSet(sorted in ascending order(S-L))=>(all are class and it implements Set). 
Sorted set(not sync, ordered, comparable interface and it extends Set )
• I have used Tree set to print dropdown list in for non duplicate values and 
ascending order
• SortedSet<String> name = new TreeSet<>();

MAP : is a key+value format and keys are always unique, 
and value can be duplicated. Map read value by passing key, 
map can have single null key but multiple null values.
• HashTable(sync, no null key or value), 
LinkedHashMap,HasHMap(not sync, 1 null key, multiple null values), 
TreeMap(sorted in ascending order)=>(all implements Map)