Inner Classes and Nested Classes

Inner classes:

  • A “regular” inner class is declared inside the curly braces of another class, but outside my method or other code block.
  • A inner class is a full-fledged member of enclosing (outer) class, so it can be marked with an access modifier as well as the abstract or final modifiers. (Never both abstract and final together- remember that abstract must be sub classed, where as final cannot be sub classed).
  • An inner class instance shares a special relationship with an instance of the enclosing class. This relationship gives the inner class access to all of the outer class’s members including those marked private.
  • To instantiate an inner class, you must have a reference to an instance of the outer class.
  • From code within the enclosing class, you must have a reference to instance of outer class.
  • From cod within the enclosing class, you can instantiate the inner class using only the name of the inner class as follows:

MyInner mi = new MyInner();

  • From the code outside the encoding class’s instance methods, you can instantiate the inner class only by using both the inner and outer class names, and a reference to the outer class as follows:

MyOrder mo = new MyOrder();

MyOrder.MyInner inner = MyInner();

  • From code within the inner class, the keyword this holds a reference to the inner class instance. To reference the outer this(in other words, the instance of the outer class that this inner instance is tied to) precede the keyword this with the outer class name as follows: MyOuter.this;

Method-Local Inner Classes:

  • A method-local inner class is defined within the method of the enclosing class.
  • For the inner class to be used, you must instantiate it, and the instantiation must happen in the same method, but after the class definition code.
  • A method local inner class cannot use variables declared within the method. (Including the parameters) unless those variables marked as final.
  • The only modifiers you can apply to a method-local inner class are abstract and final (Never both at the same time).

Anonymous Inner Classes:

  • Anonymous inner classes have no name, and their type must be either a subclass if the named type or an implementer of the named interface.
  • An anonymous inner class is always created as part of a statement; don’t forget to close the statement after the class definition with a curly brace. This is a rare case in java, a curly brace followed by semicolon.
  • Because of polymorphism, the only methods you can on a anonymous inner class reference are those defined in the reference variable class (or interface), even though the anonymous class is really a subclass or implementer of the reference variable.
  • An anonymous inner class can extend one subclass or implement one interface, unlike non-anonymous classes (inner otherwise), an anonymous inner class cannot do both. In other words, it cannot both extend a class and implement an interface, not can implement more than one interface.
  • An argument-defined inner class id declared, defined and automatically instantiated as part of a method invocation.
  • The key to remember is that the class is being defined within a method argument, so the syntax will end the class definition with a curly brace, followed by a closing parenthesis to end the method call, followed by as semicolon to end the statement: });

Static Nested Classes:

  • Static nested class is inner classes marked with static modifier.
  • A static nested class is not an inner class; it’s a top-level nested class.
  • Because the nested class is static, it does not show any relationship with instance of outer class. In fact, you don’t need an instance of the outer class to instantiate the static nested class.
  • Instantiating a static nested class require using both the outer and nested class names a follows:

BigOuter.Nested n = new BigOuter.Nested();

  • A static nested class cannot access non-static members if the outer class. Since it doesn’t have an implicit reference to any outer instance (in other words, the nested class instance does not get an outer this reference).

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