What are the features of using static and inner nested classes in Java? What is the difference between them?

Static nested classes:

• Do not have an implicit reference to an instance of the outer class.
• Can only access the static members of the outer class (fields and methods, including private ones).
• Are created using the static keyword.
• Can be declared as public, protected, package-private, or private.
• Are used to group helper classes that do not depend on a specific instance of the outer class.

Example:

class Outer {
    private static String staticField = "Static Outer";
    private String instanceField = "Instance Outer";

    static class StaticNested {
        void display() {
            // Access only static members of Outer
            System.out.println(staticField);
            // Error: No access to instanceField
            // System.out.println(instanceField); 
        }
    }
}

Inner classes:

• Have an implicit reference to an instance of the outer class.
• Can access any members of the outer class (static and non-static, including private).
• Are created without the static keyword.
• Can be declared as public, protected, package-private, or private.
• Are used to create tightly coupled helper classes that require access to the outer class's instance members.

Example:

class Outer {
    private static String staticField = "Static Outer";
    private String instanceField = "Instance Outer";

    class Inner {
        void display() {
            // Access all members of Outer
            System.out.println(staticField);
            System.out.println(instanceField);
        }
    }
}

Differences:

Feature	Static nested class	Inner class
Implicit reference to outer class	No	Yes
Access to outer class members	Only static	Static and non-static
Use of static	Yes	No
Connection to an instance of outer class	No	Yes
Usage	Helper classes without dependency on an instance	Helper classes with dependency on an instance