Why Are Static Methods Not Allowed in Non-Static Inner Classes Before Java 16?

Why can't we declare static methods in non-static inner classes before Java 16?

public class Foo {
    class Bar {
        static void method() {} // Compiler error
    }
}

In Java, inner classes are tied closely to their outer classes, which is why static methods cannot be defined in non-static inner classes. This answer explores the reason behind this design choice and the changes introduced in Java 16.

public class Foo {
    static class Bar { // now static
        static void method() {}
    }
}

Causes

• Inner classes have a reference to their enclosing class, and thus they cannot be static because static members cannot hold non-static references to the instance of the enclosing class.
• The presence of a non-static inner class implies an instance of the outer class, making it necessary to maintain that relationship.

Solutions

• To have static methods, define the inner class as static, thus breaking the implicit connection with the outer class instance.
• Utilize a static nested class instead of an inner class; nested classes can contain static methods.

Mistake: Declaring a static method in a non-static inner class without understanding its implications.

Solution: Always ensure that the logic of the inner class aligns with Java's class structure rules.

Mistake: Assuming that inner classes can behave like top-level classes, including their ability to have static members.

Solution: Remember that non-static inner classes maintain a link to the outer class instance.