Text: Submit either of the documentation sets from the work on page 36 below
Exercise 1.6 Invoke the changeColor method, and write the color into the parameter field without the quotes. What happens?
Pitfall: A common error for beginners is to forget the double quotes when typing in a data value of type String. If you type green instead of "green", you will get an error message saying something like "Error: cannot find symbol - variable green."
Java supports several other data types, including decimal numbers and characters. We shall not discuss all of them right now, but rather come back to this issue later. If you want to find out about them now, look at Appendix B.
1.6 Multiple instances
Exercise 1.7 Create several circle objects on the object bench. You can do so by selecting new circle( from the pop-up menu of the circle class. Make them visible, then move them around on the screen using the "move" methods. Make one big and yellow; make another one small and green. Try the other shapes too: create a few triangles, squares, and persons. Change their positions, sizes, and colors.
Concept: Multiple instances. Many similar objects can be created from a single class
Once you have a class, you can create as many objects (or instances) of that class as you want. Every one of those objects has its own position, color, and size. You change an attribute of an object (such as its size) by calling a method on that object. This will affect this particular object, but not others.
You may also notice an additional detail about parameters. Have a look at the changeSize method of the triangle. Its signature is
void changeSize(int newHeight, int newWidth)
Here is an example of a method with more than one parameter. This method has two, and a comma separates them in the signature. Methods can, in fact, have any number of parameters.
1.7 State
The set of values of all attributes defining an object (such as x-position, y-position, color, diameter, and visibility status for a circle) is also referred to as the object's state. This is
Concept:
Objects have state. The state is represented by storing values in fields.
In BlueJ, the state of an object can be inspected by selecting the Inspect function from the object's pop-up menu. When an object is inspected, an object inspector is displayed. The object inspector is an enlarged view of the object that shows the attributes stored inside it (Figure 1.6)
