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I wonder what is the difference in Java between an abstract class and a class that has all its methods abstract? I mean, is an abstract class just a class whose methods automatically get abstract?
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Abstract class: is a restricted class that cannot be used to create objects (to access it, it must be inherited from another class). Abstract method: can only be used in an abstract class, and it does not have a body. The body is provided by the subclass (inherited from).
An interface is a better choice for defining an abstract API. Java allows classes to extend at most one class, but implement multiple interfaces.
Abstract classes are similar to interfaces. You cannot instantiate them, and they may contain a mix of methods declared with or without an implementation. However, with abstract classes, you can declare fields that are not static and final, and define public, protected, and private concrete methods.
Absolutely not. Indeed, a class can be abstract without any methods being abstract, although that's relatively rare (see Mark's comment below for an example). On the other hand, if a class has any abstract methods, then it must be declared abstract.
Generally speaking, the purpose of an abstract class is to provide a skeleton with some non-abstract behaviour, but other bits still to be filled in by subclasses. This can be used with the template method pattern, for example.
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