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Hashcode of Object class returns a value by converting the internal address of the object into an integer,

You want to override these methods whenever you want to use your class with a Hashtable.

map collection in java requires that hashcode and equals be overridden to retrieve values from the map (using get()). public int HashCode(), public boolean Equals()

Check this link . very nicely expained :
technologiquepanorama.wordpress.com/2009/02/12/use-of-hashcode-and-equals/

One needs to override hashCode() and equals() method when one needs to use a custom class as a key into HashMap (or Hashtable). following rules apply
a) Same members must be used in both hashCode() and equals().
b) hashCode() must different values as possible for different objects, this will ensure objects are better placed in different buckets of the map.
c) If one uses different members within hashCode and equals method then when map.get(o1) will return null even after we have zeroed in on the bucket.

hashCode() is a native method which returns the address at which this object is stored in heap.hashCode() in Object returns the hexadecimal representation of the address at which this object is stored in heap. toString() returns classname + "@" + hashCode().

if two objects are equal their hashcodes must be equal, opposite is not true though