Question about Abstract Classes when classes are nested

Question

因此，假设ClassB是在ClassA （嵌套类）中定义的类，我的问题是，如果ClassB被声明为abstract因为它包含抽象方法，那么 ClassA 是否也必须声明为Abstract ？

Answer 1

No. The methods of each class are its own. A class does not own the methods of other classes (or interfaces!) nested within, and containing a nested abstract class or a nested interface does not require the container class to be abstract.

Answer 2

Nope, it doesn't. It doesn't matter whether the outer or parent class is abstract , there is no rule in Java enforcing a class being abstract .

An example:

class classA {
    abstract class classB {      // can also be static
        abstract void foo();
    }
}

On the other hand, it's a bit different with the methods: abstract methods must be placed only inside the abstract classes or interfaces (implicitely abstract).