Core Java Interview Question Page 5
Question: What are different types of
inner classes ?
Answer: Inner classes nest within
other classes. A normal class is a direct member of a package. Inner
classes, which became available with Java 1.1, are four types
- Static member classes
- Member classes
- Local classes
- Anonymous classes
Static member classes - a
static member class is a static member of a class. Like any other
static method, a static member class has access to all static
methods of the parent, or top-level, class.
Member
Classes - a member class is also defined as a member of a
class. Unlike the static variety, the member class is instance
specific and has access to any and all methods and members, even the
parent's this reference.
Local Classes -
Local Classes declared within a block of code and these classes are
visible only within the block.
Anonymous Classes
- These type of classes does not have any name and its like
a local class Java Anonymous Class
Example public class SomeGUI extends JFrame { ... button member
declarations ... protected void buildGUI() { button1 = new
JButton(); button2 = new JButton(); ... button1.addActionListener(
new java.awt.event.ActionListener() <------ Anonymous Class {
public void actionPerformed(java.awt.event.ActionEvent e) { // do
something } } );
Question: What are the uses of
Serialization?
Answer: In some types of applications
you have to write the code to serialize objects, but in many cases
serialization is performed behind the scenes by various server-side
containers.
These are some of the typical uses of
serialization:
- To persist data for future use.
- To send data to a remote computer using
such client/server Java technologies as RMI or socket
programming.
- To "flatten" an object into array of bytes
in memory.
- To exchange data between applets and
servlets.
- To store user session in Web
applications.
- To activate/passivate enterprise java
beans.
- To send objects between the servers in a
cluster.
Question: what is a collection
?
Answer: Collection is a group of
objects. java.util package provides important types of collections.
There are two fundamental types of collections they are Collection
and Map. Collection types hold a group of objects, Eg. Lists and
Sets where as Map types hold group of objects as key, value pairs
Eg. HashMap and Hashtable.
Question: For concatenation of
strings, which method is good, StringBuffer or String ?
Answer: StringBuffer is faster than
String for concatenation.
Question: What is Runnable interface ?
Are there any other ways to make a java program as multithred java
program?
Answer: There are two ways to create
new kinds of threads:
- Define a new class that extends the
Thread class
- Define a new class that implements the Runnable
interface, and pass an object of that class to a Thread's
constructor.
- An advantage of the second approach is that the
new class can be a subclass of any class, not just of the Thread
class.
Here is a very simple
example just to illustrate how to use the second approach to
creating threads: class myThread implements Runnable { public void
run() { System.out.println("I'm running!"); } } public class
tstRunnable { public static void main(String[] args) { myThread my1
= new myThread(); myThread my2 = new myThread(); new
Thread(my1).start(); new Thread(my2).start(); }
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