When to use what???

What is the difference between Abstract class and an interface?
When to use what?
Why is it that abstract classes have constructors but not interfaces?
Thanks in advance...

sorry...yawmark..No prob - I was probably a little harsh, but laziness is rampant here. I've summarized some useful links below:
Java Glossary : interface vs abstract class
Abstract classes vs. interfaces: When does it make sense to choose an abstract class over an interface?
Abstract classes and interfaces practicum: Move from theory to practice on when to employ abstract classes vs. interfaces
Tech Tips: ABSTRACT CLASSES VS. INTERFACES

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