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Chapter 5 Methods
Predefined and programmer defined methods
Understand program modules called methods
Use methods
Create methods
passing information to methods arguments
Simulation methods with Random numbers
Understand scope rules
Understand and Create methods that call
themselves
Modularization
What is the value of modularization?
Part of the step-wise refinement
technique we discussed earlier.
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Divide and conquer
Reuse-Don’t reinvent the wheel
Problem simplification
Java’s predefined classes and
methods
The Math class
System.out.print (Math.sqrt(900) )
we pass information into Methods through
the arguments.
for example 900 is a literal argument value.
This method return a number that is a
common way to pass information out of a
method.
Note the “.” member access operator
Calling a method is kind of like
the boss
The boss gives a work assignment to a
worker.
The boss also gives the worker some
information to use in the work assignment.
The arguments (900)
The worker does the job, while the boss waits
idle.
Then the worker passes back the result to the
boss.
Other math class methods
abs(x)
ceil(x)
cos(x)
exp(x)
floor(x)
log(x)
max(x,y)
min(x,y)
pow(x,y)
sin(x)
tan(x)
A = Math.abs(-10)
will store 10 in the
variable A
Static class members
Static methods
Static variables
Compared to instance variables
Note the space invaders anology
Making our own classes and
methods (Programmer Defined)
Use already made classes whenever possible.
Reference the API documentation before
creating a new class
Sometimes easier said then done.
Methods should perform a single well defined
task.
Method names should we actions like “paint”
“draw” “run” “computeTotals”
Should be able to be viewed on 1-2 screens
Method declarations
public float square(float x) {
return x*x;
}
Must be declared inside of a class
using the method in our program
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System.out.println( x + “ squared is “ +
square(x) );
general form of method
declaration
Header or
Interface
Modifiers returntype methodname(parameters 1,2,3…)
{
declarations and statements
Body or
return value; // this is optional
implementation
}
return value type is required but may be void
a return statement must be used unless the return
type is void.
A type is required for each parameter
Arguments are passed to the method and stored in
the method parameters by value.
Arguments are order associated.
Passing variables
The formal argument vs the actual
argument
Primitive data types vs reference data
types (used by objects and arrays)
Argument promotion
the automatic conversion of arguments to
satisfy the declared type requirement of the
method declaration.
For example
square(4);
-orsquare(x);
X or 4 is an integer literal value and will be
promoted to a float when passed to the
square function.
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This is limited by the promotion rules already
discussed. We can promote implicitly when going
to a larger storage class for example float to
double. but not the other way.
Java packages
java.applet
java.awt
java.awt.event
java.io
java.lang
java.net
java.text
java.util
javax.swing
javax.swing.event
Random numbers?
Computer are not random thinkers.
Sometimes we need random thinking for
simulations or games.
So Math.random()
return a pseudorandom number of type
double from 0.0 to < 1.0
we can range this value by multiplying and
adding constants.
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dice role=(int)(Math.random()*6) +1;
Final variables
final double PI = 3.1415;
final float taxrate = 0.06;
These are used to define variables whose
values can not be changed in the program.
Was initialized a final variables value can not
be changed.
Using final variables make programs easier to
maintain and read.
Variable scope
scope of method parameter is limited to the
method in which it is declared
local variable scope is limited to the point in
the code where the variable is defined until
the end of the code block.
the scope of a for loop variable declared in
the loop head is limited to the for loop
the scope of a label is limited to the body of
the label code block.
The scope of a method or class field name is
limited to the entire class.
Method overloading
overloaded methods have the same
name but a different number and type
of parameters
for example
int min(int x, int y)
int min(int x, int y, int z)
double min(double x, double y)
recursion
when a method calls itself this is
recursion.
Hu? is that legal? Yep!!!
Here is the
for example
int factorial(int f) {
if (f<=1) return 1
else return (factorial(f-1)*f);
}
recursive
part!!!
recursion pros and cons
recursion can allow for intuitive solutions to
certain types of problems specifically those
that are based on a recursive definition.
A base case is required as the stopping point
for the recursion to work. Without this out
programs could have serious errors.
Recursion is typically higher overhead then
non-recursive solutions.
An iterative solution can replace any recursive
solution and provide better performance.
Chapter 5 Methods
Summary
Modularization in Java
The Math class and its methods
Method declaration
Argument promotion
Java packages
Random numbers?
Declaration scope
Method overloading
Recursion vs iteration
Homework
Problems 6,10 on page 277-278