Clarify on nested class in Java (Refer: https://stackoverflow.com/questions/70324/java-inner-class-and-static-nested-class)

Java_Inner_Class.java

// There are 2 types of nested class in Java: 1. Static nested class 2. Non-static nested class, which is called inner class
// So, Inner class is non-static nested class.
// Example:
public OuterClass {

  public InnerClass {}
}

// An instance of InnerClass can only exists within an instance of OuterClass
// and has direct access (through reference) to the methods and fields of instance of OuterClass.

// To initiate an InnerClass, we need to initiate OuterClass first. 
// Then create the instance of InnerClass within the outer instance using this syntax:

OuterClass outerObj = new OuterClass();
OuterClass.InnerClass innerObj = ourterObj.new InnerClass();

Java_Static_Nested_Class.java

// Java does not have a top-level static class.
// However, it does have "nested static class".

// Example:
public OuterClass {

    public static InnerClass {
        private innerState;
        private void innerMethod();
        public static void staticInnerMethod();
    }
}

// A static nested class is a normal class, but does not implicitly have a reference to the instance of outer class.
// So it can only access the static members of outer (enclosing) class.
// Static nested class can have instance and static methods.