| Question : |
Describe the wrapper classes in Java ? |
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| 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 |
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| Question : |
What are different types of inner classes ? |
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| 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
}
}
);
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| Question : |
What are the uses of Serialization? |
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| 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.
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| Question : |
what is a collection ? |
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| 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.
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| Question : |
For concatenation of strings, which method is good, StringBuffer or String ? |
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| Answer : |
StringBuffer is faster than String for concatenation.
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