How to mock a method call in junit when the method is inside a base class and the testing method extends it?

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Java

I need to write unit test in junit for a method, the method is inside class A and class A extends Class B. My testing method methodA() calls another methodB() which is inside class B. So how should I mock this methodB() call so that when I call methodA() from my testCase, it get passed?
Which class should I mock, classA or Class B?

What I have tried:

ClassA obj=new ClassA();
ClassB mockClsB=mock(ClassB.class);
when(mockClsB.methodB()).thenReturn(someValue);
int expected=10;
int result=obj.methodA();