Transcript Methods and Intro to OOP

Functions/Methods
1
Function / Method
Definition
collection of statements that may be invoked
by name (from another function or method)
Functions/methods that we have invoked
Input.readInt(), setLayout(), Math.sqrt()
Functions/methods that we have defined
main(), init(), action()
* the term “method” will be used from now on
2
Defining methods that the
program will invoke
Why?
code inside main() (or init() or action()) may
be lengthy
sometimes, the task requires something
redundant (e.g., print two blocks of asterisks,
one of size 5, the other of size 10)
we want to define our own math function
Methods allow us to organize our code
into logical units
3
Defining Methods
Syntax (for applications)
public static void methodName() {
// statements
}
For applets, omit the static keyword
Methods are defined at the level of the
class (same level as main() or init())
4
Parameters and Return
Values
It is possible to pass data to a method
this allows for the possibility of a different
effect based on the value of the parameter
It is possible to return data to the caller
like Input.readInt() or Math.sqrt(), the
method can return a value
Example that uses both features
int factorial(int num) { … }
5
Invoking Methods
Invocation
methodName(expression-list)
Formal versus actual parameters
formal parameter: the variable used to
represent the argument within the method
the expression used during invocation that is
evaluated and then passed as a value to the
method
6
Object-Oriented
Programming:
Classes in Java
7
Object
Definition: a thing that has identity, state,
and behavior
identity: a distinguished instance of a class
state: collection of values for its variables
behavior: capability to execute methods
* variables and methods are defined in a class
8
Class
Definition: a collection of data (variables)
and methods that operate on that data
data/methods define the
contents/capabilities of the instances
(objects) of the class
object creation occurs with the statement
variable = new class(parameters);
classes can be viewed as factories for
objects
9
Class
A class is a template for an object
An object is an instance of a class
10
Class
All data and variables defined within a
class are called instance variables
because each instance of that class (each
object of the class) contains a copy of that
variable
Methods and variables defined within a
class are called members of that class
11
Two Kinds of Variables
in Java
Variables of a primitive type
e.g., int x; char c;
Variables of a reference type (class)
e.g., Button b; String s;
Conventions
Primitive types are reserved words in Java
and are indicated in all-lower-case letters
Class names: first letter usually capitalized
12
Variables and Values
Primitive type variables
int x;
…
x = 5;
X
X
5
13
Variables and References
Reference type variables
X
Button x;
…
x = new Button(“click”);
Button Object
X
“click”
14
The new Keyword
new Button(“click”) creates a Button
object and returns a reference (an
address) to that object that a Button
variable could hold
Button Object
X
1023
1023:
“click”
1023 is some address in memory
15
The Dot (“.”) Operator
Allows access to variables (primitive and
reference types) and methods of
reference type variables.
Ex.
TextField t = new TextField(10);
t.setText(“hi”);
*accessing a method using the dot operator
16
Method Invocation
Syntax for method invocation
object.methodName(arguments)
Method may return a value or simply
produce an effect on the object
To find out what methods are available for
a given class
javap package.name.NameOfClass
ex. Javap java.awt.Button
17
Strings Revisited
Strings are objects as well
String is an existing Java class
s = “Hello” is just a shorthand for
s= new String(“Hello”);
String methods
int length()
String substring(int x,int y);
no “manipulating” methods
18
Variables and Objects
Let Circle be a class with:
variable r that indicates its radius
method area() that computes its area
Declaration:
Circle c;
Instantiation:
c = new Circle();
Usage:
c.r = 5.5;
System.out.println(c.area());
19
The complete Circle class
public class Circle {
public double x,y; // center coordinates
public double r; // radius
// the methods
public double circumference()
{ return 2*3.14*r; }
public double area() { return 3.14*r*r; }
}
20
Using the Circle class
public class TestCircle {
public static void main(String args[]) {
Circle c;
c = new Circle();
c.x = 2.0; c.y = 2.0; c.r = 5.5;
System.out.println(c.area());
}
}
21