Importing packages and using instance variables

Question

I've got two pretty rudimentary questions regarding Java:

First of all, why is it that importing java.awt.* doesn't also import java.awt.geom.*? Isn't everything within java.awt.geom inside of java.awt? I would assume so, judging by the name of the package.

Secondly, my professor told me for this rectangle maker project thing that accessing the instance variables directly was bad and that I should use the accessor method instead. Why is that? Is it just convention? For example:

Rectangle rectangle = new Rectangle(1, 2, 3, 4);

int tallness = rectangle.height; //"bad"

int tallness = rectangle.getHeight(); //"good"

Both of those are valid and do the exact same thing, to my understanding. The only difference is that the former feels quicker.

Furthermore, why does accessor come up as misspelled on this site's spellchecker? (Or maybe Chrome's spellchecker)

Answer 1

why is it that importing java.awt.* doesn't also import java.awt.geom.*? Isn't everything within java.awt.geom inside of java.awt? I would assume so, judging by the name of the package.

The name of the packages are misleading. They are given names like java.awt and java.awt.geom, but those are actually entirely different packages. In Java there isn't really a "package hierarchy."

 import java.awt.*;

imports all of the classes inside the java.awt package. Because java.awt.geom is a different package with different classes, you have to import that package too. The hierarchical names are mostly for organization!

accessing the instance variables directly was bad and that I should use the accessor method instead. Why is that? Is it just convention?

What Sotirios Delimanolis said about encapsulation was good, but I believe the best explanation is that in Object-Oriented Programming, you-the-programmer are responsible for deciding what a user (or a programmer using your package) is allowed to do with a class's member variables.

That's why it's convention to make member variables private; the data stored in the object is the objet's responsibility, and it's your job as a programmer to make sure that interfacing with that object through the methods you define work properly. So if you want someone to be allowed to change the data in an object directly, you can make a set method to do so.

Consider something like this:

 Pizza myPizza = new Pizza(18) //18-inch pizza. Yum!

When I do this, the pizza will calculate other variable members such as numSlices (currently 1, since it's uncut) and pizzaLeft (which is (18/2)^2 * pi inches). Which of these makes more sense?

 myPizza.pizzaLeft -= 50; //50 inches of pizza eaten
 myPizza.pizzaLeft; //returns about 204.47

or

 myPizza.slice(8); // cut into 8 pieces which are about 32 square inches each
 myPizza.eat(1); // eat one slice
 myPizza.getPizzaLeft(); //returns about 222.47

Since you're dealing with a pizza, it makes more sense to just cut into slices and eat pieces. The pizza object itself will be responsible for calculating the changes in things like pizzaLeft or something like slicesLeft, and those interfacing with the class don't see it.

It's because of THIS reason that member variables are conventionally set to private, which then means that you have to use an accessor method.

I hope this helps.

Answer 2

First of all, why is it that importing java.awt.* doesn't also import java.awt.geom.*? Isn't everything within java.awt.geom inside of java.awt?

No.

To get the all classes of package geom you have to write java.awt.geom.*

my professor told me for this rectangle maker project thing that accessing the instance variables directly was bad and that I should use the accessor method instead.

Yes your professor is correct.

The reason is data Encapsulation,

• A language mechanism for restricting access to some of the object's components.
• A language construct that facilitates the bundling of data with the methods (or other functions) operating on that data.

Read more about Encapsulation

Answer 3

When you write java.awt.*,then it only imports the classes that are present inside awt Package It does not import if some packages are present inside awt package. java.awt.geom in this case geom is another package,so to import all the classes you need to do java.awt.geom.*

click here for more info on packages

Answer 4

int tallness = rectangle.height; //"bad"

int tallness = rectangle.getHeight(); //"good"

That is because in the latter case, other programmers using your code won't be able to change the value of rectangle.height by assigning it random value through a code like rectangle.height= 5; as you would encapsulate the height variable through getter and setter methods by declaring it private.