Inheritance

Inheritance

• Uses the Extends keyword

• Java uses Single Inheritance

  • Only one class can be inherited at a time

• ex) Dog extends Animal

  => The Dog class inherits from the Animal Class

• The parent class can be accessed with the super keyword

  => super();

• The implicitly called super() invokes the parent's default constructor

• The parent constructor of all objects is Object!!!!!!!!

• When loaded into memory, the parent is on top and the child is on the bottom

• When searching for data, it goes from the bottom child upward to the parent

  => It searches from the bottom and goes up if not found

• ex) If the data type is Dog, both Animal and Object are accessible

• Using super, you can skip one child class and search from the parent

• is-a relationship

  => The data type of all objects can be the parent

  • Data compatibility is only possible in parent-child relationships

    • Not possible in sibling relationships!

  • A parent type cannot access child data at the bottom

super();

• Calls the parent's constructor.

  • Even if removed, it is automatically included!

  • Even if absent, it is automatically called!

• If not visible, it is omitted!

• Like the this() method, it is only allowed in the first statement!

  • So you can tell that super() is absent in constructors where the this() method is used

  • super.______ allows searching from the parent area instead of the child!

• Both this. keyword and super. keyword can only be used in the heap area!!!!

img

Method Overloading

• Multiple methods with the same name can exist within a single class

  • However, the type or number of the method's parameters must be different!

Constructor

• A function created when an object is created

• The constructor function name is the same as the class name

• Return type must not be mentioned - method overloading applies

• Constructors can also be called by method name

  • this();

• A constructor with no parameters = default constructor

• public can be added or omitted

• Access modifiers can be specified

Overriding in Java

In any object-oriented programming language, Overriding is a feature that allows a subclass or child class to provide a specific implementation of a method that is already provided by one of its super-classes or parent classes. When a method in a subclass has the same name, same parameters or signature and same return type(or sub-type) as a method in its super-class, then the method in the subclass is said to override the method in the super-class.

• Redefining a function inherited from the parent!

• Just bring over the method declaration as is!

• Overriding is not mandatory!

  • So no error is shown.. fix it yourself~

overriding in java

• If inheritance is not mentioned, it can be seen that java.lang.Object is automatically inherited!

• Class Animal extends Object!

• Object = methods available to all objects

  • At the very top of any object is Object!

Is a

• The data type of all objects can be the parent!!

  • The data type of all objects is Object!

    • why? Because Object is at the very top of all objects' memory!

ex1)

Animal a = new Animal();
Object

• Since the parent object of Animal is Object, Object can also be used as the data type when declaring an object!

• However, if the type is Object, the data inside Animal cannot be accessed, and the accessible type is limited to the very top!

ex2)

Animal d = new Dog();
Dog
    Object

• Since Dog has Animal as a parent object, Animal or Object can be used when declaring an object!

  • When declared as Animal, Dog is not accessible; Animal and above (Animal + Object) are accessible

• All objects in Java can be of Object type, but the accessible information is limited!

• The parent object can be accessed through type casting!

  => Casting in objects -> Up casting

  Ex) ((Animal)d).kind;

  ​ => If you up-cast Dog, which is originally Dog, to Animal, you can directly access Animal

Annotations

: a tag that represents the metadata i.e. attached with class, interface, methods or fields to indicate some additional information which can be used by java compiler and JVM.

-> ex) @Override

Polymorphism

• the ability of an object to take on many forms

  • By incorporating overriding techniques, a single method can accept various types of objects!