JAVA INTERVIEW QUESTIONS - Part 3

Question : Describe the wrapper classes in Java ?

Answer : Wrapper class is wrapper around a primitive data type. An instance of a wrapper class contains, or wraps, a primitive value of the corresponding type.

Following table lists the primitive types and the corresponding wrapper classes:

Primitive Wrapper
boolean java.lang.Boolean
byte java.lang.Byte
char java.lang.Character
double java.lang.Double
float java.lang.Float
int java.lang.Integer
long java.lang.Long
short java.lang.Short
void java.lang.Void



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.