1. Visibilities and Static-ness
Aaron Bloomfield
CS 101-E

2. field is not initialized
Our Circle class
We are going to write a Circle class
Somebody else is going to use it
public class Circle {
double radius;
double Pi = ; }
Note the radius
field is not initialized
Remember that a
variable of a class
is called a field
Note the fields
are not static
We’re ignoring the
public for now

3. Using our Circle class public class CircleTest {
Note it’s a different class
public class CircleTest {
public static void main (String[] args) { int x = 5;
Circle c = new Circle(); }
public class CircleTest {
public static void main (String[] args) { int x = 5;
Circle c = new Circle(); }
Remember that a
variable of a method is
just called a variable
c is the object
Circle is the class
Note the new

4. Accessing our Circle object
Any field or method in an object can be accessed by using a period
Example: System.in
Example: c.radius = 10;
Example: c.Pi = 4;
This is bad – Pi should
have been declared final
(this will be done later)

5. What’s the output? public class Circle { double radius;
double Pi = ; }
public class CircleTest {
public static void main (String[] args) { int x;
Circle c = new Circle();
System.out.println (x);
Java will give a
“variable not
initialized” error

6. What’s the output now? public class Circle { double radius;
double Pi = ; }
public class CircleTest {
public static void main (String[] args) { int x;
Circle c = new Circle();
System.out.println (c.radius);
Java outputs 0.0!

7. What’s going on?
A (method) variable needs to be initialized before it is used
A field (class variable) is automatically initialized by Java
All doubles are initialized to 0.0, ints to 0, etc.
This is a bit counter-intuitive…

8. What happens in memory Consider: Circle c = new Circle()
A double takes up 8 bytes in memory
Thus, a Circle object takes up 16 bytes of memory
As it contains two doubles
radius = 0.0
Pi =
Circle c

9. Consider the following code
public class CircleTest {
public static void main (String[] args) { Circle c1 = new Circle();
Circle c2 = new Circle();
Circle c3 = new Circle();
Circle c4 = new Circle(); }

10. What happens in memory There are 4 Circle objects in memory
Taking up a total of 4*16 = 64 bytes of memory
radius = 0.0
Pi =
Circle c1
radius = 0.0
Pi =
Circle c2
radius = 0.0
Pi =
Circle c3
radius = 0.0
Pi =
Circle c4

11. Consider the following code
public class CircleTest {
public static void main (String[] args) { Circle c1 = new Circle();
//...
Circle c = new Circle(); }
This program creates 1 million Circle objects!

12. … What happens in memory There are 1 million Circle objects in memory
Taking up a total of 1,000,000*16 = 16 Mb of memory
radius = 0.0
Pi =
Circle c1
radius = 0.0
Pi =
Circle c2
radius = 0.0
Pi =
Circle c …
Note that the final Pi field is repeated 1 million times

13. The use of static for fields
If a field is static, then there is only ONE of that field for ALL the objects
Total memory usage: 8 Mb + 8 bytes (1,000,000+1=1,000,001 doubles)
Total memory usage: 16 bytes (1+1=2 doubles)
Total memory usage: 40 bytes (4+1=5 doubles)
radius = 0.0
Pi =
Circle c1
Circle c2
Circle c …
radius = 0.0
Pi =
Circle c1
Circle c2
Circle c3
Circle c4
radius = 0.0
Pi =
Circle c

14. More on static fields What does the following print
Note that Pi is not final
Circle c1 = new Circle();
Circle c2 = new Circle();
Circle c3 = new Circle();
Circle c4 = new Circle();
c1.Pi = 4.3;
System.out.println (c2.Pi);
Note you can refer
to static fields by
object.field

15. Even more on static fields
There is only one copy of a static field no matter how many objects are declared in memory
Even if there are zero objects declared!
The one field is “common” to all the objects
Thus, you can refer to a static field by using the class name:
Circle.Pi

16. Even even more on static fields
This program also prints 4.3:
Circle c1 = new Circle();
Circle c2 = new Circle();
Circle c3 = new Circle();
Circle c4 = new Circle();
Circle.Pi = 4.3;
System.out.println (c2.Pi);

17. Even even even more on static fields
We’ve seen static fields used with their class names:
System.in (type: InputStream)
System.out (type: OutputStream)
Math.PI (type: double)
Integer.MAX_VALUE (type: int)
Non-static fields are generally not called dynamic fields
Just “non-static”, or no qualifier at all

18. A bit of humor…

19. Back to our Circle class
public class Circle {
double radius;
final static double Pi = ; }
But it doesn’t do much!
Note that Pi is now final and static

20. Adding a method public class Circle { double radius;
final static double Pi = ;
double computeArea () {
return Pi*radius*radius; }
Note that a (non-static) method can use all the fields

21. Using that method public class CircleTest {
public static void main (String[] args) {
Circle c = new Circle();
c.radius = 2.0;
System.out.println (c.computeArea()); }
Prints

22. Adding another method
double oneOverRadius() {
return 1.0/radius; }
I couldn’t think of a good reason to divide something by the radius…

23. What happens now? Code in class CircleTest’s main() method
Circle c = new Circle(); // c.radius = 0.0
System.out.println (c.oneOverRadius());
Java won’t crash, but many other programming languages (C and C++, especially) will
Java prints “Infinity”
Not what we wanted, though!

24. One way to fix this… public class Circle { double radius = 1.0;
Note that the radius field
is now initialized to 1.0
public class Circle {
double radius = 1.0;
final static double Pi = ;
double computeArea () {
return Pi*radius*radius; }
double oneOverRadius() {
return 1.0/radius;

25. Back to our program… This code will now run properly:
Circle c = new Circle(); // c.radius = 1.0
System.out.println (c.oneOverRadius());
But this code will “crash”:
c.radius = 0.0;

26. Where the “crash” occurs
public class CircleTest {
public static void main
(String[] args) {
Circle c = new Circle();
// c.radius = 1.0
c.radius = 0.0;
System.out.println
(c.oneOverRadius()); }
public class Circle {
double radius = 1.0;
final static double Pi = ;
double computeArea () {
return Pi*radius*radius; }
double oneOverRadius() {
return 1.0/radius;
Here is the badly written code
Here is where the “crash” occurs

27. Motivation for private fields
Problem: We do not want people using our Circle class to be able to modify the fields on their own
Solution: Don’t allow other code to modify the radius field
Give it private visibility
private means that only code within the class can modify the field

28. One way to fix this… public class Circle {
Note that the radius field
is now private
public class Circle {
private double radius = 1.0;
final static double Pi = ;
double computeArea () {
return Pi*radius*radius; }
double oneOverRadius() {
return 1.0/radius;

29. Back to our program… This code will now not compile:
Circle c = new Circle(); // c.radius = 1.0
c.radius = 0.0;
System.out.println (c.oneOverRadius());
Java will give a compile-time error:
radius has private access in Circle

30. Back to our program… This code will also not compile:
Circle c = new Circle(); // c.radius = 1.0
System.out.println (c.radius);
Java will give the same compile-time error:
radius has private access in Circle

31. The problem now…
But now you can’t have a Circle with a radius other than 1.0!
Solution: Use a get/set methods in Circle:
void setRadius (double r) {
radius = r; }
double getRadius () {
return radius;

32. Our Circle class so far public class Circle { double radius = 1.0;
final static double Pi = ;
double computeArea () {
return Pi*radius*radius; }
double oneOverRadius() {
return 1.0/radius;
void setRadius (double r) {
radius = r;
double getRadius () {
return radius;

33. Using the get/set methods
public class CircleTest {
public static void main
(String[] args) {
Circle c = new Circle();
c.setRadius (1.0);
System.out.println
(c.computeArea());
(c.getRadius()); }
public class Circle {
private double radius = 1.0;
final static double Pi = ;
double computeArea () {
return Pi*radius*radius; }
double oneOverRadius() {
return 1.0/radius;
void setRadius (double r) {
radius = r;
double getRadius () {
return radius;
Here a method is invoked
Here the change to radius occurs

34. Wait! Another problem! Here is the problem now…
public class CircleTest {
public static void main (String[] args) {
Circle c = new Circle();
c.setRadius (0.0);
System.out.println (c.oneOverRadius()); }
Here is the problem now…

35. This problem is easily fixed
Change the setRadius method to the following
void setRadius (double r) {
if ( r > 0.0 )
radius = r; }
Now there is no way for code outside the Circle class to change the radius to zero

36. Visibilities in Java There are four visibilities:
private: Only code within the same class can access the field or method
Note: “access” means reading or writing the field, or invoking the method
public: Any code, anywhere, can access the field or method
protected: Used with inheritance
We won’t get to that this semester
package: Almost the same as public
This is the default
Note that it can’t be specified like the others

37. A few notes on visibilities
You can NOT specify visibilities for method variables
Any method variable can only be accessed within that method
Think of it as public within the method (after it’s defined) and private outside the method
You can also specify visibilities for methods and classes
More on that later in the course (or next course)

38. Constructors
When you create a object, you often have to do some initialization
You have to “construct” the object
public class Circle {
String name;
public Circle() {
Scanner scanner = new Scanner(System.in);
System.out.println (“Enter the Circle’s name:”);
name = scanner.next(); }
//...
Note the different method declaration

39. Constructors, take 2 Now when a Circle is created:
Circle c = new Circle();
Java will ask for the Circle’s name, and put it in the name field
Okay, so this isn’t the greatest example…

40. Note that the constructor
Constructors, take 3
More useful constructors for our Circle class:
public Circle() {
radius = 1.0; }
public Circle (double r) {
radius = r;
Note there is no return
type for constructors
Note that the constructor
name is the EXACT same
as the class name
Note that there are two “methods” with the same name!

41. Overriding methods (and constructors)
Consider the following code:
Circle c1 = new Circle ();
Circle c2 = new Circle (2.0);
Java knows which constructor to call by the list of parameters
This is called “overloading”
Creates a Circle
of radius 1.0
Creates a Circle
of radius 2.0

42. Overriding methods (and constructors), take 2
The following Circle constructors would not be allowed:
We are assuming Pi is not final for this example
public Circle() {
radius = 1.0; }
public Circle (double r) {
radius = r;
public Circle (double p) {
Pi = p;
When Circle(1.0)
is called, which
one is meant?

43. Constructors, take 4 Our second constructor has a problem:
public Circle (double r) {
radius = r; }
Consider the following code:
Circle c = new Circle (0.0);
System.out.println (c.oneOverRadius());
The method is dividing 1 by zero (again)

44. Constructors, take 5 Our revised constructors: public Circle() {
radius = 1.0; }
public Circle (double r) {
if ( r <= 0.0 )
else
radius = r;

45. A bit of humor…

46. Back to the static discussion
Remember that there is one (and only one) static Pi field, regardless of how many objects are declared
Consider the following method:
double getPi() {
return Pi; }

47. The getPi() method It doesn’t read or modify the “state” of any object
In this example, it doesn’t read/write the radius
In fact, that particular method doesn’t care anything about the objects declared
It’s only accessing a static field

48. Make getPi() static Consider the following:
static double getPi() {
return Pi; }
As the method is static, it can ONLY access static fields
A static method does not care about the “state” of an object
Examples: Math.sin(), Math.tan(), Math.cos()
They don’t care about the state of the Math class
They only perform the computation

49. Invoking static methods
As with static fields, they can be called using either an object or the class name:
Circle c = new Circle();
System.out.println (c.getPi());
System.out.println (Circle.getPi());

50. static methods and non-static fields
Consider the following (illegal) Circle method:
static double getRadius() {
return radius; }
And the code to invoke it:
public static void main (String[] args) {
Circle c1 = new Circle();
Circle c2 = new Circle();
Circle c3 = new Circle();
Circle c4 = new Circle();
System.out.println (Circle.getRadius());

51. What happening in memory
There are no Circle objects in memory
There are 4 Circle objects in memory
Which radius field does Circle.getRadius() want?
There are 1 million Circle objects in memory
radius = 0.0
Pi =
Circle c1
Circle c2
Circle c …
radius = 0.0
Pi =
Circle c1
Circle c2
Circle c3
Circle c4
Pi =

52. The main static lesson
A static method cannot access or modify the state of the object

53. static and non-static rules
Non-static fields and methods can ONLY be accessed by the object name
Static fields and methods can be accessed by Either the class name or the object name
Non-static methods can refer to BOTH static and non-static fields
Static methods can ONLY access static fields

54. Back to our main() method
public static void main (String[] args)
The method does not return a value
Any code anywhere
can call this method
It’s a static method:
Can’t access non-static fields
Can be called only by the class name

55. A bit of humor…