1. Classes and Objects Introduced
Java Programming
Classes and Objects Introduced

2. © Matt B. Pedersen (matt@cs.unlv.edu)
Introduction
A class is a named collection of
Fields, that hold data values.
Methods, that operate on the fields.
Classes are the most important reference type (4 others exist)
Classes define a new datatype
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3. © Matt B. Pedersen (matt@cs.unlv.edu)
Simple Class Example
Example:
class Point {
double x; // x coordinate
double y; // y coordinate }
Defines a 2 dimensional point in Cartesian coordinates. (No methods yet)
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4. Data types versus data values
It is important to distinguish between data types and data values:
int is a data type, 42 is a data value of type int.
char is a data type, is a character value (of char type).
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5. If classes are data types ….
A class is a data type, and data types define a range of values. So what are the values of class type?
Objects
An object is an instance of a class. E.g., where Point is a class representing all possible points a Point object represents a specific point in 2 dims.
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6. © Matt B. Pedersen (matt@cs.unlv.edu)
Just like blueprints
Classes are like blueprints of a ‘building’.
You cannot live in a ‘building’ blueprint.
You can use the blueprint to make a specific ‘building’ (or instantiate a ‘building’)
Every ‘building’ instantiated is its own new ‘building’ and is physically not the same as the other ‘buildings’
‘not the same’ does not mean they do not look the same, they are separate buildings.
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A Bigger Example
public class Point {
public double x, y;
public Point (double x, double y) {
this.x = x;
this.y = y; }
public double distanceFromOrigin() {
return Math.sqrt (x*x + y*y);
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A Bigger Example
public class Point {
public double x, y;
public Point (double x, double y) {
this.x = x;
this.y = y; }
public double distanceFromOrigin() {
return Math.sqrt (x*x + y*y);
Name of the class is Point
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A Bigger Example
public class Point {
public double x, y;
public Point (double x, double y) {
this.x = x;
this.y = y; }
public double distanceFromOrigin() {
return Math.sqrt (x*x + y*y);
It has 2 fields named x and y, both of double type.
They are public, i.e., directly available to anyone holding a reference to an object of type Point.
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A Bigger Example
public class Point {
public double x, y;
public Point (double x, double y) {
this.x = x;
this.y = y; }
public double distanceFromOrigin() {
return Math.sqrt (x*x + y*y);
It has a constructor that takes in 2 parameters (also called x and y)
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A Bigger Example
public class Point {
public double x, y;
public Point (double x, double y) {
this.x = x;
this.y = y; }
public double distanceFromOrigin() {
return Math.sqrt (x*x + y*y);
this.x refers to the field x, where as x (on the right hand side of the assignment) is the x from the parameter list).
this is a reference to the object being instantiated (almost true ;-), but good enough for now).
We only need this because the field and the parameter is called the same.
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12. © Matt B. Pedersen (matt@cs.unlv.edu)
A Bigger Example
public class Point {
public double x, y;
public Point (double x, double y) {
this.x = x;
this.y = y; }
public double distanceFromOrigin() {
return Math.sqrt (x*x + y*y);
A method called distanceFromOrigin, which takes in no parameters and returns _____ √x2+y2
Math.sqrt is the square root function.
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13. What should I call my file?
All classes with the modifier public must be in separate files that are named the same as the class.
For the example, it must be saved in a file called Point.java
When compiled it generates Point.class
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Creating Objects
Recall a declaration of an integer variable:
int myInt;
What value does myInt hold?
None, it must be assigned: myInt = 42;
We can declare variable of class type in the same way:
Point p;
but p do not hold a value until one has been assigned to it.
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Creating Objects
How do we get an object value?
By instantiating a class!
Use the new keyword.
Point p;
p = new Point (3.14, 2.72);
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16. Instantiating an Object
A number of important things happen when instantiating an object using new.
An actual object is created based on the class being instantiated
The object has the fields that the class lists.
The constructor is executed with the parameters passed to it (if any).
An object reference value is returned.
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17. Types of Fields and Methods
There are 4 types of fields and methods:
Class Fields
Class Methods
Instance Fields
Instance Methods
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Instance Fields
An instance field is a field associated with an object:
public class Point {
int x;
int y;
Every instance of Point has its own set of fields x and y.
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19. Class Fields (or static fields)
Where instance of a class (I.e., object) has instances of instance fields, a class field is shared between instances:
public class Point {
static int counter = 0; … }
We use the word static to denote this.
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20.  This class field lives here Public class Point { int x, y;
static int count = 0;
Point (int x, int y) {
this.x = x;
this.y = y;
count += 1; }
}
A CLASS
This class field lives here
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21. © Matt B. Pedersen (matt@cs.unlv.edu)
x = 4
y = 3
new Point(4,3);
Public class Point {
int x, y;
static int count = 1;
Point (int x, int y) {
this.x = x;
this.y = y;
count += 1; }
}
A CLASS
AN OBJECT
this.x = x initializes the instance field x to the value of the parameter x (4)
this.y = y initializes the instance field y to the value of the parameter y (3)
count += 1 increments the class field by one.
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x = 4
y = 3
new Point(4,3);
Public class Point {
int x, y;
static int count = 2;
Point (int x, int y) {
this.x = x;
this.y = y;
count += 1; }
}
A CLASS
AN OBJECT
x = 10
y = 98
new Point(10,98);
ANOTHER OBJECT
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Static fields
A static field can be initialized at declaration time:
static int count = 0;
It should never be initialized in a constructor:
Point (int x, int y) { …
count = 0; }
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Static Fields
Within the class in which a static field is declared, it may be referenced by its name:
public class Point {
static int counter = 0;
int getCouner() { return counter; } }
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Field Modifiers
The static modifier makes fields into class fields.
Other important modifiers:
public, private, protected
Determines if a field can be seen/used outside of the class in which it is defined.
final
Once initialized it can never be written to again.
transient, volatile
Don’t worry about these.
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Public Static Fields
Public static fields like counter:
public class Point {
public static int counter = 0 … }
may be referenced like this:
Inside the class: counter (by its name)
Outside the class:
Point.counter (by <class>.<fieldname>)
p.counter, where p is an instance of Point (<object>.<fieldname>) (This one is not advisable)
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Private Static Field
The counter might be private:
public class Point {
private static int counter = 0 … }
Now, Point.counter is no longer legal, neither is p.counter.
Only within Point can counter be accessed.
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Example
public class Point {
public int x;
public int y;
private static int counter = 0;
Point(int x, int y) {
this.x = x;
this.y = y;
counter = counter + 1; }
public static int getInstanceCount() {
return counter;
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29. © Matt B. Pedersen (matt@cs.unlv.edu)
Example
public class Point {
public int x;
public int y;
private static int counter = 0;
Point(int x, int y) {
this.x = x;
this.y = y;
counter = counter + 1; }
public static int getInstanceCount() {
return counter;
Instance Fields
Each object of class Point instantiated by
new Point(..,..)
as a copy of int x and int y.
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Example
public class Point {
public int x;
public int y;
private static int counter = 0;
Point(int x, int y) {
this.x = x;
this.y = y;
counter = counter + 1; }
public static int getInstanceCount() {
return counter;
Class Field
Each instance of Point
shares the counter field.
Class fields or static
fields are shared between instances.
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31. © Matt B. Pedersen (matt@cs.unlv.edu)
Example
public class Point {
public int x;
public int y;
private static int counter = 0;
Point(int x, int y) {
this.x = x;
this.y = y;
counter = counter + 1; }
public static int getInstanceCount() {
return counter;
Constructor
A constructor is invoked as the first thing that happens at the new Point(…,…) call. It is used to set up fields in the object. new Point(…,…) does not return a reference to the new object until the constructor has run.
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32. © Matt B. Pedersen (matt@cs.unlv.edu)
Example
public class Point {
public int x;
public int y;
private static int counter = 0;
Point(int x, int y) {
this.x = x;
this.y = y;
counter = counter + 1; }
public static int getInstanceCount() {
return counter;
this is a special reference
value that always referes to the object that we are currently operating inside.
Here we use it to get to the fields as they are overshadowed by the parameters.
Object Context
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33. © Matt B. Pedersen (matt@cs.unlv.edu)
Example
public class Point {
public int x;
public int y;
private static int counter = 0;
Point(int x, int y) {
this.x = x;
this.y = y;
counter = counter + 1; }
public static int getInstanceCount() {
return counter;
Since counter is private in Point,
we cannot access it through Point.counter or p.counter for any object p of class Point, so we need a way to get to the value of counter.
This is called an accessor method..
Accessor Method
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34. © Matt B. Pedersen (matt@cs.unlv.edu)
Example
public class Point {
public int x;
public int y;
private int counter = 0;
Point(int x, int y) {
this.x = x;
this.y = y;
counter = counter + 1; }
public static int getInstanceCount() {
return counter;
Point p1 = new Point(1,2);
Point p2 = new Point(3,4); …
System.out.println(
“# of times Point was instantiated:” +
Point.getInstanceCount());
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35. © Matt B. Pedersen (matt@cs.unlv.edu)
Example
public class Point {
public int x;
public int y;
private static int counter = 0;
Point(int x, int y) {
this.x = x;
this.y = y;
counter = counter + 1; }
public static int getInstanceCount() {
return counter;
public String toString() {
return “(“+x+“,”+y+“)”;
Point p1 = new Point(1,2);
Point p2 = new Point(3,4);
System.out.println(
“# of times Point was instantiated: ” +
Point.getInstanceCount());
System.out.println(p1);
System.out.println(p2);
Would produce:
# of times Point was instantiated: 2
(1,2)
(3,4)
Special method called toString; takes no arguments and returns a String. Automatically called when an
object is used in a print statement.
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36. © Matt B. Pedersen (matt@cs.unlv.edu)
Example
public class Point {
private int x;
private int y;
private static int counter = 0;
Point(int x, int y) {
this.x = x;
this.y = y;
counter = counter + 1; }
public static int getInstanceCount() {
return counter;
String toString() {
return “(“+x+“,”+y+“)”;
int getX() {
return x; // or this.x }
int getY() {
return y; // or this.y
void setX(int x) {
this.x = x;
void setY(int y) {
this.y = y;
Might be nice to make x and y private as well, but then we need accessors (also called ‘getters’) for them, and if we want to change them we need mutators (also called ‘setters’) as well.
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Methods
Like fields, we can have
Instance methods (class methods)
Again, we use the static keyword to denote this.
Object methods (regular methods)
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Non-static methods
A regular method (non-static) can be thought of as ‘belonging to the object’
It may reference both static and non-static fields.
Can be public, or private, and final
A final method cannot be re-implemented in subclasses.

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39. © Matt B. Pedersen (matt@cs.unlv.edu)
Example
public class Circle {
// a class field
public static final double π = ;
// an instance field
public double r; // the radius of the circle
public Circle(double radius) {
r = radius; }
public double area() {
return π * r * r;
public double circumference () {
return 2 * π * r;
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40. © Matt B. Pedersen (matt@cs.unlv.edu)
Example
public class Circle {
// a class field
public static final double π = ;
// an instance field
public double r; // the radius of the circle
public Circle(double radius) {
r = radius; }
public double area() {
return π * r * r;
public double circumference () {
return 2 * π * r;
Can I add this method:
public static getRadius() {
return r; }
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41. © Matt B. Pedersen (matt@cs.unlv.edu)
class
class Circle {
public static final π = ;
public double r;
public double area() { … } public double circumference() { … } }
// Create a circle with radius 5
Circle c1 = new Circle(5);
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class
class Circle {
public static final π = ;
public double r;
public double area() { … } public double circumference() { … } }
// Create a circle with radius 5
Circle c1 = new Circle(5);
c1 is a reference to an object of class Circle.
A new Circle object is created with the new keyword,
and the value 5 is passed to the constructor.
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43. © Matt B. Pedersen (matt@cs.unlv.edu)
class
object
r = 5
class Circle {
public static final π = ;
public double r;
public double area() { … } public double circumference() { … } }
// Create a circle with radius 5
Circle c1 = new Circle(5);
C1 is a reference to an object of class Circle.
A new Circle object is created with the new keyword,
and the value 5 is passed to the constructor.
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44. © Matt B. Pedersen (matt@cs.unlv.edu)
class
object
r = 5
class Circle {
public static final π = ;
public double r;
public double area() { … } public double circumference() { … } }
r = 100
object
// Create a circle with radius 5
Circle c1 = new Circle(5);
// Create a circle with radius 20
Circle c2 = new Circle(20);
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45. © Matt B. Pedersen (matt@cs.unlv.edu)
class
object
r = 5
class Circle {
public static final π = ;
public double r;
public double area() { … } public double circumference() { … } }
r = 100
object
// Create a circle with radius 5
Circle c1 = new Circle(5);
// Create a circle with radius 20
Circle c2 = new Circle(20);
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46. © Matt B. Pedersen (matt@cs.unlv.edu)
class
object
r = 5
class Circle {
public static final π = ;
public double r;
public double area() { … } public double circumference() { … } }
// Create a circle with radius 5
Circle c1 = new Circle(5);
// Create a circle with radius 20
Circle c2 = new Circle(20);
r = 100
object
System.out.println(c1.area());
double cir = c2.circumference();
double pi = Circle.π;
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class
class Circle {
public static final π = ;
public double r;
public double area() { … } public double circumference() { … } }
object
r = 5
public double area() {
return π * r * r; }
c1.area():
Value of r from object
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48. © Matt B. Pedersen (matt@cs.unlv.edu)
class
class Circle {
public static final π = ;
public double r;
public double area() { … } public double circumference() { … } }
r = 100
object
c2.circumference():
public double circumference () {
return 2 * π * r; }
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49. © Matt B. Pedersen (matt@cs.unlv.edu)
class
class Circle {
public static final π = ;
public double r;
public double area() { … } public double circumference() { … } }
double pi = Circle.π;
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50. © Matt B. Pedersen (matt@cs.unlv.edu)
class
class Circle {
public static final π = ;
public double r;
public double area() { … } public double circumference() { … }
public static radToDeg(double rads) {
return rads * 180 / π; }
Let us add a class method (a static method)
radsToDeg cannot reference ANY non-static fields!!
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51. © Matt B. Pedersen (matt@cs.unlv.edu)
class
class Circle {
public static final π = ;
public double r;
public double area() { … } public double circumference() { … }
public static radToDeg(double rads) {
return rads * 180 / π; }
radsToDeg cannot reference ANY non-static fields!!
Circle.radToDeg(35);
There is no object context for radToDeg to access non-static fields in.
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class
object
r = 5
class Circle {
public static final π = ;
public double r;
public double area() { … } public double circumference() { … }
public static radToDeg(double rads) {
return rads * 180 / π; }
r = 100
object
// Create a circle with radius 5
Circle c1 = new Circle(5);
// Create a circle with radius 20
Circle c2 = new Circle(20);
How do I get the radius of c1 or c2?
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class
object
r = 5
class Circle {
public static final π = ;
public double r;
public double area() { … } public double circumference() { … }
public static radToDeg(double rads) {
return rads * 180 / π; }
r = 100
object
// Create a circle with radius 5
Circle c1 = new Circle(5);
// Create a circle with radius 20
Circle c2 = new Circle(20);
raidus in Circle is public, so it can be accessed
Like this: c1.r (= 5) or c2.r (=100) but NOT
Circle.r
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class
object
r = 5
class Circle {
public static final π = ;
public double r;
public double area() { … } public double circumference() { … }
public static radToDeg(double rads) {
return rads * 180 / π; }
BECAUSE: r lives in an object;
it is not static, and only static
fields live in classes and can
be accessed through class.field
r = 100
object
// Create a circle with radius 5
Circle c1 = new Circle(5);
// Create a circle with radius 20
Circle c2 = new Circle(20);
raidus in Circle is public, so it can be accessed
Like this: c1.r (= 5) or c2.r (=100) but NOT
Circle.r
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Non-static methods can reference
Non-static fields
Non-static methods
Static fields
Static methods
Static methods can reference
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The this keyword
A special keyword ‘this’ can be referenced inside objects. It refers to the current context, that is, the object in which the code is located (NOT class, but object)
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The this keyword
public class Circle {
public static final double π = ;
private double radius;
public Circle(double radius) {
// set the radius field equal to the radius parameter }
public double getRadius() { return radius; }
public double area() { return π * radius * radius; }
public double circumference() return 2 * π * radius; }
public static double radToDeg(double rads) {
return rads * 180 / π;
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The this keyword
public class Circle {
public static final double π = ;
private double radius;
public Circle(double radius) {
// set the radius field equal to the radius parameter }
public double getRadius() { return radius; }
public double area() { return π * radius * radius; }
public double circumference() return 2 * π * radius; }
public static double radToDeg(double rads) {
return rads * 180 / π;
Problem: both the field and the
parameter is called radius.
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59. © Matt B. Pedersen (matt@cs.unlv.edu)
The this keyword
public class Circle {
public static final double π = ;
private double radius;
public Circle(double radius) {
// set the radius field equal to the radius parameter }
public double getRadius() { return radius; }
public double area() { return π * radius * radius; }
public double circumference() return 2 * π * radius; }
public static double radToDeg(double rads) {
return rads * 180 / π;
Problem: both the field and the
parameter is called radius.
Solution: The field can be referenced like any
other public field by:
objectRef.fieldName
But we don’t have an object reference do we?
Yes we do, it is called ‘this’
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The this keyword
public class Circle {
public static final double π = ;
private double radius;
public Circle(double radius) {
this.radius = radius; }
public double getRadius() { return radius; }
public double area() { return π * radius * radius; }
public double circumference() return 2 * π * radius; }
public static double radToDeg(double rads) {
return rads * 180 / π;
Problem: both the field and the
parameter is called radius.
Solution: The field can be referenced like any
other public field by:
objectRef.fieldName
But we don’t have an object reference do we?
Yes we do, it is called ‘this’
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Copying an Object
Consider this code:
Circle c1 = new Circle(20); Circle c2 = c1;
c1 and c2 are references to the same object!
There is no built-in way to clone an object, so we need to make one:
public Circle clone() {
return new Circle(radius); }
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62. Multiple Constructors
With the Circle class we must provide an initial radius when instantiating the class: Circle c1 = new Circle(4);
What about Circle c2 = new Circle();
That is illegal, there is no constructor with no arguments.
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63. Multiple Constructors
Lets make one then, but what value should radius have? public Circle() { this.radius = 1; // we pick 1 }
This does the same as calling the other constructor with the value 1! Public Circle() { this(1); }
This is called an explicit constructor invocation. (Must appear on line 1)
CSC 140 Java Programming
© Matt B. Pedersen