DJ retains the object-oriented feature of Java. A class A can extends another class B. In this case, class A is called a subclass of B, and class B is called the super class of A. By using inheritance, we can write the HelloWorldButton as follows:
class HelloWorldButton extends Button {
text=="Hello World!";
}
Although the two HelloWorldButton classes specify the same
sub-panel, their definitions are different. For the previous
definition, for example, we reference the size of the button as
hw.bt.size. For this declaration, however, we reference the size of
the button as hw.size. The attribute size in HelloWorldButton is
inherited from its super class Button.