Heritage

• Inheritance is one of the three basic principles of object-oriented programming.

• Allows the creation of hierarchical classifications.

• With inheritance, you can create a general class that defines characteristics common to a set of related items.

• This general class can be inherited by other more specific classes, which add exclusive characteristics.

• In Java jargon, the inherited class is called a superclass.

• The class that inherits is called a subclass.

• The subclass is a specialized version of the superclass.

• The subclass inherits all variables and methods from the superclass and adds its own unique elements.

Basic aspects of inheritance

• Java supports inheritance, allowing one class to incorporate another in its declaration.

• This is done with the extends keyword.

• The subclass adds functionality (extends) to the superclass.

• An example of this is the creation of a superclass called TwoDShape, which stores the width and height of a two-dimensional object.

• A subclass called Triangle is created to extend TwoDShape.

• The extends keyword is used to create the subclass.

• TwoDShape defines attributes of a generic two-dimensional shape, such as square, rectangle, triangle, etc.

• The Triangle class is a subclass of TwoDShape and creates a specific type: the triangle.

• Triangle inherits everything from TwoDShape and adds the style field, the area() method, and the showStyle() method.

• style stores the style of the triangle, which can be any description such as "full", "contour", or "isosceles".

• The area() method calculates and returns the area of the triangle.

• The showStyle() method displays the style of the triangle.

• Triangle can access the width and height members of TwoDShape, both in area() and directly in the t1 and t2 objects.

• TwoDShape, despite being a superclass of Triangle, is an independent class and can be used separately.

TwoDShape shape = new TwoDShape();
shape.width = 10;
shape.height = 20;
shape.showDim();

• An object of the TwoDShape class does not know about or access any subclass of TwoDShape.

• Declaration of a class that inherits a superclass:

class subclass-name extends superclass-name {
// class body
}

• In Java, you can only specify a single superclass for any subclass you create.

• Java does not support multiple inheritance (inheritance from multiple superclasses in the same subclass), unlike C++.

• It is possible to create an inheritance hierarchy where a subclass becomes the superclass of another subclass.

• No class can be a superclass of itself.

• An advantage of inheritance is that a superclass can define attributes common to multiple objects, allowing the creation of numerous specific subclasses.

• Each subclass can define its own specific classification based on the superclass.

• An example of this is creating a subclass of TwoDShape to encapsulate rectangles.

• The Rectangle class is a subclass of TwoDShape.

• Adds the isSquare() method, which determines whether the rectangle is a square (when the width is equal to the height).

• Also adds the area() method, which calculates the area of the rectangle.

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