Transcript Using Objects

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
• A message looks like this:
object.method(extra information)
• The object is the thing we are talking to
• The method is a name of the action we want the object to take
• The extra information is anything required by the method in order
to do its job
• Examples:
g.setColor(Color.pink);
amountOfRed = Color.pink.getRed( );
Messages and methods
• 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
• A method is something inside the object that
responds to your messages
– A message contains commands to do something
– Java contains thousands of classes, each typically
containing dozens of methods
– When you program you use these classes and methods,
and also define your own classes and methods
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();
• The last method doesn’t change the color; it makes a new
color
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, there are no methods to change it
– But you can easily make new Strings:
myName = "Dave";
myName = "Dr. " + myName;
– This is kind of a subtle point; it will be important
later, but you don’t need to understand it right away
• If s is the name of the string "Hello", then
– s.length() tells you the number of characters in
String s (returns 5)
– s.toUpperCase() returns the new String "HELLO"
(s itself is not changed)
• But you can say s = s.toUpperCase();
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);
• + as the concatenation operator is an exception to
the rule: Primitives have operations, Objects have
methods
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, which is the
same for any object of that class
– 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” (send messages 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 “print line”) 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