Java 天地·Core Java·Lecture Notes·Unitt 07 - Inheritance and Interfaces

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Java keyword 慹xtends?

In Java, the keyword extends denotes full inheritance. If a definition states that a new class 'extends' another, this new class will inherits all the non-private methods (and variables) of the class it extends

To change a method in some way is called overriding the method.

For example, suppose the class Shape has a method called draw() . If we define Line as being a type of Shape as below:

class Line extends Shape
{
// variables (none at present)
// methods (none at present)
} This means that class Line is a direct subclass of class Shape .

Then we could issue the instructions:

Line aLine = new Line();
aLine.draw();

this would send a message to the object aLine , which will invoke the draw() method in Shape , because we have not defined a draw() method for Line (in fact, we have not defined any methods for Line !)

However, we now define a draw method in the class Line , class Line extends Shape
{
public void draw()
{
// some actions to draw a line
}
// other methods go here...
} When we do: aLine.draw()

this message will result in the invocation of the draw() method defined in class Line , not the one in Shape . Thus we have overridden the inherited method Shape.draw() in the class Line .

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