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As the title suggests I would like to understand why static classes can have only static members. I hope it is same in all the programming languages. So the explanation would be the same for all the languages, I believe.
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Static classes cannot be instantiated hence it will not have any instance, Non-static members would require an instance of their class to access. Since static class cannot have any instance you cannot access the non-static members if there are any. Hence static classes can have only static members.
Can a class be static in Java? The answer is Yes, some classes can be made static in Java. Java supports Static Instance Variables, Static Methods, Static Block and Static Classes. Java allows a class to be defined within another class. These are called Nested Classes. The class in which the nested class is defined is known as the Outer Class.
Creating a static class is therefore basically the same as creating a class that contains only static members and a private constructor. A private constructor prevents the class from being instantiated. The advantage of using a static class is that the compiler can check to make sure that no instance members are accidentally added.
To access static methods there is no need to instantiate the class, you can do it just using the class name as − Static Fields − You can create a static field by using the keyword static. The static fields have the same value in all the instances of the class.
Static classes cannot be instantiated hence it will not have any instance, Non-static members would require an instance of their class to access. Since static class cannot have any instance you cannot access the non-static members if there are any.
Hence static classes can have only static members
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It's not a design decision, so much as a logical one. The easiest place to start is by looking at the relevant definitions of the concepts:
A static class is one that cannot be instantiated. That means you cannot create objects that are of that class's type.
Non-static members are tied to a specific instance of a class. They contain data that is associated exclusively with one single object of that class type.
So, if a static class contained non-static members, you could never access that data or call that method because you could never instantiate an object of that static class's type. Instead, you must have all static members that can be called directly from a static instance of the class.
However, you can have non-static classes that contain static members. This way, you can access the data or call the methods exposed as static members without instantiating an object of that class. However, you could also instantiate an object of that class's type and access non-static (or instance) members. For example, if you had a class Circle, you could have static members like a CalculateArea function and a PI field. These members are applicable to all circles, generally, just by virtue of the fact that they are circles. But you could also have non-static members that are associated with specific instances of that class, because they describe specific circle objects. These could be the fields Diameter and Circumference. You could also have non-static functions that calculate the area of the circle, given the data stored in the non-static fields for that particular instance.
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