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Java Certification Questions 1 - 10   «Prev  Next»
Java Questions 2
  1. What is the natural order for sorting objects?
    Answer:
    For a collection of String objects, the natural order is alphabetical. For Integer objects, the natural order is by numeric value. [Page 561 SCJP by K.S.]
  2. What are all 3 list implementations ordered by?
    Answer:
    All 3 list implementations are ordered by index position, a position that you determine either by setting an object at a specific index or by adding it without specifying position,
    in which case the object is added to the end.
  3. What is the difference between an ArrayList and a LinkedList?
    Answer:
    In a LinkedList the elements are doubly-linked to one another.
  4. What is characterisitc of the Set Interface?
    Answer:
    A set cares about uniqueness and does not allow duplicates.
  5. When should you use the HashSet class?
    Answer:
    Use this class when you want a collection with no duplicates and you do not care about order when you iterate through it.
  6. When should you use the LinkedHashSet?
    Answer:
    1. Use this class instead of HashSet when you care about the iteration order.
    2. A LinkedHashSet lets you iterate through the elements in the order in which they were inserted. [P563 SCJP by K.S.]
  7. What set does TreeSet implement?
    Answer:
    As of Java 6, TreeSet implements NavigableSet.
  8. Which method do Maps rely on to determine whether two keys are the same or different?
    Answer:
    equals()
  9. When should you use a HashMap?
    Answer:
    When you need a Map and do not care about the order (when you iterate through it), then you should select HashMap.
  10. What are the key differences between HashMap and Hashtable?
    Answer:
    Hashtable is the synchronized counterpart to HashMap.
    When we say Vector and Hashtable are synchronized, we mean that the key methods of the class are synchronized.
    A HashMap lets you have null values as well as one null key, a Hashtable does not allow anything that is null.