hashCode() and equals()

Rule 1: The connection between hashCode() and equals():
If x.equals(y), then x.hashCode() must be same as y.hashCode().

Rule 2: Overwriting hashCode() or equals():
If you overwrite one of these functions, you have to overwrite the second too.

When you need to overwrite equals?

Collections questions

Differences between Iterator and Enumeration?
Iterator duplicate functionality of Enumeration with one addition of remove() method and both provide navigation functionally on objects of Collection.

Difference between Set and List in Java?
In Set the order of the elements in not saved.Set doesn't allowed duplicate while List does and List maintains insertion order while Set does not. Set uses the method equals() internally, not ==.

How do you Sort objects on collection?
The class Collections has functionality for sorting collections. There are two most used methods for this:
Collections.sort() - called without Comparator. It uses Comparable interface in Java. The order is the default , which is specified by CompareTo method;
Collections.sort(Comparator) will sort objects based on compare() method of Comparator. See Sorting in Java using Comparator and Comparable for more details.

Similarities and differences between HashMap, Hashtable, ConcurrentHashMap and synchronizedMap

There are several similarities and differences between HashMap, Hashtable, ConcurrentHashMap and synchronizedMap  in Java. (I name synchronizedMap the object retrieved by Collections.synchronizedMap(map) method.)

Fail-fast vs fail-safe iterators

Fail-fast iterators
"Fail-fast iterator" means that if the collection class is structurally modified at any time after the Iterator is created, in any way except through the Iterator's own remove or add methods, the Iterator will throw a ConcurrentModificationException.

 Throwing ConcurrentModificationException is not granted, so you have NOT to relay on it. ConcurrentModificationException is used to show the developer that his/her classes are not synced.

Most ifiterators in Java are fail-fast, these found in classes:
- LinkedList
- ArrayList
- Vector

Fail-safe iterators
Fail-safe iterator doesn't throw any Exception if Collection is modified structurally.

Advanced view over Concurrent collections

A nice article for java syncronization from IBM developers:
http://www.ibm.com/developerworks/java/library/j-jtp07233/index.html

LinkedList, ArrayList & Vector classes - differences

LinkedList, ArrayList & Vector are tree different implementations of the List interface. LinkedList implements it with a doubly-linked list. ArrayList & Vector classes implement it with a dynamically resizing array.