Using Objects
Overview
• In this presentation we will discuss:
– Classes and objects
– Methods for objects
– Printing results
Classes and objects
• A class is the type of an object
• Just as a variable classSize may have type
int, Color.red has type Color
• Just as 5 is a literal of type int, "Hello" is a
literal of type String
• There are exactly eight primitive types
• There are thousands of classes, and you can
create more
Declarations
• You declare variables to hold primitive
values like this:
int classSize;
double area;
• You declare variables to hold objects like
this:
Color uglyBrown;
String myName;
Assignment statements
• An assignment statement has the form:
variable = expression ;
• Examples:
classSize = 57;
area = pi * radius * radius;
uglyBrown = new Color(175, 175, 30);
myName = "David Matuszek";
Combining declaration and
assignment
• Declaration and assignment can be combined
into a single statement:
int classSize = 57;
String myName = "David Matuszek";
Color uglyBrown = new Color(175, 175, 30);
• You can only declare a variable once, but you
can assign to it many times in many places
– This rule is “true enough” for now
– Exceptions are complicated and left for later
Methods
• Primitives have operations, classes have
methods
• You cannot define new primitives, but you
can define new classes
• You cannot define new operations, but you
can define new methods
• Here we will talk about using methods
supplied by Java, not defining new ones
Data in classes and objects
• A class is the type of an object
• A class describes:
– How to make a new object of that class
• Example: new Color(175, 175, 30);
– What kind of data is in an object
• Example: a Color object contains three numbers
representing the amount of red, green, and blue
– The methods of an object (the actions it can perform)
• Example: a Color object can tell you how much red it
contains
Sending messages to objects
• We don’t perform operations on objects, we
“talk” to them
– This is called sending a message to the object
• We do it like this:
– object.message(extra information)
• Examples:
g.setColor(Color.pink);
amountOfRed = Color.pink.getRed( );
Messages
• Messages can be used to
– Tell an object some information
– Tell an object to do something
– Ask an object for information (usually about
itself)
– Any and all combinations of the above
Messages to a Graphics
• If you have a Graphics, and its name is g,
here are some things you can do with g:
– Tell it to use a particular color:
g.setColor(Color.orange);
– Ask it what color it is using:
Color currentColor = g.getColor();
– Tell it to draw a line:
g.drawLine(14, 23, 87, 5);
Messages to a Color
• Once you make a Color, you cannot change it;
you can only ask it for information
// Make a new purplish color
Color myColor = new Color(100, 0, 255);
// Ask how much blue is in it
int amountOfBlue = myColor.getBlue();
// Ask the color for a brighter version of itself
Color brightColor = myColor.brighter();
String
• A String is an object, but...
• ...because Strings are used so much, Java
gives them some special syntax
– There are String literals: "This is a String"
• (Almost) no other objects have literals
– There is an operation, concatenation, on Strings:
• "Dave" + "Matuszek" gives "DaveMatuszek"
• In other respects, Strings are just objects
String methods
• A String, like a Color, is immutable: once
you create it, you cannot ever change it
• ...but you can make new Strings and Colors
• If s is the name of the string "Hello", then
– s.length() tells you the number of
chararacters in the string (returns 5)
– s.toUpperCase() returns the new String
"HELLO" (s itself is not changed)
String concatenation
• + usually means “add,” but if either operand
(thing involved in the operation) is a String,
then + means concatenation
• If you concatenate anything with a String,
that thing is first turned into a String
• For example, you can concatenate a String
and a number:
System.out.println("The price is $" + price);
Data in classes
• A class describes objects. It describes:
– How to construct an object of that class,
– the kind of data in an object, and
– the messages that the object can understand
• A class can also contain its own data
– Constants are often provided this way
– Examples:
• class Color contains the constant Color.red
• class Math contains the constant Math.PI
Printing out results, part 1
• In Java, “print” really means “display on the
screen”
– Actually printing on paper is much harder!
• System is one of Java’s built-in classes
• System.out is a data object in the System
class that knows how to “print” to your screen
• We can talk to this mysterious object without
knowing very much about it
Printing out results, part 2
• System.out is a object with useful methods
that will let you print anything:
– print(x) turns x into a String and displays it
– println(x) (pronounced “printline”) turns x into a
String and displays it, then goes to the next line
• Examples:
System.out.print("The sum of x and y is ");
System.out.println(x + y);
New vocabulary
• class: the type, or description, of an object
• object: an instance, or member, of a class
• message: something that you “say” to a
class, either telling it something or asking it
for information
• immutable: cannot be changed after it is
created
• operand: one of the inputs to an operation
The End