We can use logical operators AND (&&) and OR (||) in the logical expressions. The AND operator returns true if both conditions are true while OR returns true if only one of the conditions is true. 

Short circuit operators

1. The logical operators && and || are called short circuit operators.

2. Short circuiting is the process of stop evaluating remainder of a logical expression once the result becomes obvious.

HashMap is a Hash table based implementation of the Map interface. For more details on hashing, you can refer to http://javajee.com/hashing-basics.

Important properties of HashMap

1. Provides all of the optional map operations

2. Permits null values and the null key.

3. Roughly equivalent to Hashtable, except that it is unsynchronized and permits nulls.

Map is an object that maps keys to values.

Important properties of a Map

1. A map cannot contain duplicate keys

2. Each key can map to at most one value.

3. Map interface takes the place of the abstract Dictionary class.

4. The order of a map is the order in which the iterators on the map's collection views return their elements.

Important properties of LinkedList

1. Doubly-linked list implementation

2. Implements all optional list operations

3. permits all elements (including null)

4. is not synchronized

   • could be "wrapped" using the Collections.synchronizedList method

     • best done at creation time, to prevent accidental unsynchronized access to the list

List is an ordered collection.

Important points about List interface

1. also known as a sequence.

2. typically allow duplicate

   • typically allow multiple null elements if they allow null elements at all.

3. Places additional stipulations on the contracts of the iterator, add, remove, equals, and hashCode methods.