Learn JUnit! My very first advice! The best advice!
I have seen developers who start the hard way, by developing classes of 3000 lines and then they compile everything, deploy their stuff in a server and start debugging. Once one of my colleagues said "you should remove the debugger from eclipse in order to make them better developers". And he was right!
And if you make little changes, you will not fear that you forgot to test some part manually. The JUnit will tell you if it still works.
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