Normally developpers those who have  developed the code are 
responsible for the code testing.
This is called as "debugging".
Normally a developper will be asigned for certain module to 
development.
The module is composed of units or code blocks.
So the developper starts coding the code blocks and 
integrates the code blocks into Module.
You can view this this code block as each individual 
component of the Module.
After development of code block/unit/component the 
developper starts testing the unit/component,this is called 
as Unit testing.
After completion of unit/component testing the the 
components are integrated in to one module.
THis low level Integration is called as Component 
Integration.
After this developper conducts compoenet integration 
testing on the entire module.
   
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developers are responsible for code testing.if the 
developers code a perticular module mean the developer test 
the same module.this is called debugging.