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Java - Things to know before your OCJP exam - Collections

1. TreeSet and TreeMap are sorted collections i.e they store the elements in an order.

2. compareTo(Obj obj1) method needs to be implemented if the class implements comparable interface whereas compare(Obj obj1, Obj obj2) needs to be implemented if it implements comparator inyterface.

3. After Java 1.5, TreeSet implements NavigableSet which in turn extends SortedSet. Earlier version used to have TreeSet implementing SortedSet.

4. A Class must override hashCode method if its overridding equal method. Not doing the same may create trouble in with hash collections and hash search.

5. A Class can override hashCode even if it's not overridding equals method.

6. If equals method is not overridden, Two objects are treated equal only if they are same i.e obj1 == obj2.

7. HashCode method should be implemeted such that it should generate same code for an object unless its state is changed. There shouldn't be use of random functions to generate the hashcode.

8. Collection is an interface whereas Collections is a utility class.

9. Set doesn't allow duplicate values.

10. Set can have only one null value but not more than one as duplicates are not allowed.

11. Collection<SuperClass> cannot hold Collection<BaseClass>

      Collection<SuperClass> = new ArrayList<SubClass>; 

      results in compile time error.

12. Collections can be of objects only and not of primitives.

13. Arrays.sort only takes arrays as arguments whereas Collections.sort only takes list as argument. TreeSet is used for sorting Set and TreeMap is used for sorting Map.

14. LinkedHashMap maintains the entries in the order in which they were last accessed.

15. HashTable is thread safe whereas HashMap is not. That's why HashMap is faster than HashTable.

16. HashMap can have null keys as well as null values whereas HashTable cannot have either.

17. Sorted collections ( TreeMap , TreeSet ) cannot have heterogeneous objects. Reason being heterogenous object cannot be compared and hence elements cannot be sorted. It throws ArrayStoreException if we try to add non compatible object.

18. Map interface doesn't extend the Collection interface. List, Set and Queue extends Collection interface.

19. If an instance of a class needs to be added to a sorted collection, either the class implements comparable interface or Comparator should be specified during construction of sorted collection.

20. peek() method of PriorityQueue accesses the highest priority element but doesn't remove it , whereas poll() removes the highest priority element from the queue.

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