Developer Vs Tester ?

Can the developer can also be a code tester; or it should be some other person?
To understand the code written by someone else is quite difficult task, what is the easier way to do it?

Yes, a developer can act as tester but he should be a tester because he is the one has written the code so he tends to unintentionally/unconsciously avoid to use the application in a way that might break it. Testing should be done only by some other person who did not have any role in the development.

In order to write easy and understandable code, the developing team must stick to some coding standard throughout the development phase so that it becomes easy for the testing team to understand and testing becomes easy.