Using Java Debugger "jdb" one gets information about
which thread currently executing threads

but if I want check/findout within my program itself 
How do I do that ?

You can check by putting following code in your program

Thread t = Thread.currentThread();
System.out.println("Current Thread: "+t);

Check executing/running current thread by

currentThread() [Method]

public static Thread currentThread() 

Find the currently executing thread. 

Returns: 

the currently executing thread

by using currentThread() Method ,we can find which thread 
is runing

We can check the current Thread name as 
Thread t= new Thread.currentThread();
System.out.println(t.getName());

Over loading is polymorphism implementation in same class, 
where two or more methods can share same name differing in 
parameters passed.
Overriding is polymorphism implementation in different 
classes having parent child relationship and the 
funtionality in parent class is over shadowed by the 
funtionality in subclass.

Overloading class has different returntype.
overloading comes with in class but overriding comes in two 
classes.
Overloading is resolved at compiletime
Overriding is resolved at runtime.

overloading means same method name but different parameters 
are passed but overriding means same method name with same 
parameters passed in different classes.

overloading : can only be done by changing the type or no.
of parameters for eg:a(int a);
a(int a,int b);


while in case of overriding we can give new definition
without changing the parameters.

In general in java language everything is object  but 
Primitive data types are not objects.We need a conversion 
from primitive data type to object .For this conversion 
java provides Wrapper classes.
datatype      Wrapper class
     int-------Integer
     float-----Float
     double----Double
     long------Long
     short-----Short
     byte------Byte
     char------Char

wraper classes that  allow  primitive types to be accessed
as objects.
These classes are similar to primitive data types but
starting with capital letter.
Number          Byte               Boolean         
Double          Short              Character 
Float           Integer

The Wrapper class represents the base class for a set of 
data sources. This class provides library initialization 
services and access to the data source servers that the 
wrapper supports. The Wrapper class maintains the following 
information: 
The wrapper name. 
The wrapper core library name. The returned name is the 
name of the native library that loaded the wrapper. 
A WrapperInfo object that contains all of the information 
that pertains to this wrapper. This information gets stored 
in the federated server's system catalog as a result of 
issuing the DDL statements CREATE WRAPPER or ALTER WRAPPER. 
The Wrapper class is a wrapper class for the Java API. 

JVM (Java Virtual Machine): A Java runtime environment, 
required for the running of Java programs, which includes a 
Java interpreter. A different JVM is required for each 
unique operating system (Linux, OS/2, Windows 98, etc.), 
but any JVM can run the same version of a Java program.

wrapper class is the one which will allow the primitive
datatypes to be used as objects.

Jvm is the java virtual machine which will convert the byte
code to user understandable code.

"Wrapper class" has two meanings.
A class with a similar name to a primitive type, which 
encapsulates the value of that primitive, and provides 
methods which relate to that sort of primitive.
A class which calls methods of something else, and adds an 
additional layer of functionality.
The first sort includes Integer, Double, etc, and there are 
(I think) also classes like Void. Also an interface called 
NullType.

In the 2nd sort you could have a List and surround it with 
a class which calls all the List methods, but makes sure 
that all the calls become thread-safe. Look in the 
Collections class for methods like synchronizedList.