My Java Learnings


set - no duplicate elements



The iterators returned by this class's iterator method are fail-fast.

ListIterator can be used to add/modify elements in a list

Suppose you have a Collection, c, and you want to create another Collection containing the same elements,

but with all duplicates eliminated. The following one-liner does the trick

Collection noDups = new HashSet(c);

TreeSet , alphabetically stores the contents.

Vector is synchronized

ArrayList is not synchronized

Collections.synchronizedList (new ArrayList or any collection object for that case)

Map : No duplication

HashMap : no duplicates allowed

The HashMap class is roughly equivalent to Hashtable, except that it is unsynchronized and permits nulls


Actually String and Date both implement the Comparable

Comparator: method compare



error messages , GUI labels , date , currencies ,


NumberFormat , to format numbers , currencies, date formats, time formats as per current locale


MessageBundle , all messages specific to a locale can be placed here.

For Example :

Some More Questions

How does java internally behave ?

System.out.println what is out ? PrintStream

new file(), Transient variables are not persisted.

Suppose If we have variable 'I' in run method,

If I can create one or more thread each thread will occupy a separate copy or same variable will be shared ?
Why there are some null interface in java ? What does it mean ? Give me some null interfaces in JAVA ?

When is the JVM invoked ?
JVM is involved when the "JAVA" command is executed by the application / java program

Why does EJB needs two interface( Home and Remote Interface) ?


What do mean stateless protocol ? HTTP ? What do we do for maintaining the state

What is the difference in between doPost and doGet methods ?

Difference between rmi and ejbs ?

What is iiop ?

Why cant we invoke methods on ejbs directly ?

Why define Remote Interfaces ?

What is the functionality stubs and skeletons ?
JDBC insert call ..what is the default behaviour.