1. Object Oriented Systems Lecture 03 Method
Jaeki Song

2. Method
A method is a program module that contains a series of statements that carry out a task
Execute method
Invoke or call from another method

3. Methods
Commonly used to bread a problem down into small manageable pieces.
Divide and conquer
Instead of writing one long method that contains all of the statements necessary to solve a problem, several small methods which each solve a specific part of the problem can be written

4. Methods Java API provides a rich collection of classes and methods
Build-in method
Math class method
User-defined method

5. Built-in Methods Methods already written and provided by Java
Organized as a collection of classes (class libraries)
To use: import package
Package: java.lang
Math class
Method type: data type of value returned by method

6. Commonly Used Math Class Methods
Description
abs (x)
Absolute value of x
exp (x)
Exponential method ex
log (x)
Natural logarithm of x (base e)
max (x, y)
Larger value of x and y
min (x, y)
Smaller value of x and y
pow (x, y)
X raised to power y (xy)
sqrt (x)
Square root of x

7. Method A method must include the following:
A declaration (or header)
An opening curly bracket
A body
A closing curly bracket
The method declaration contains the following:
Optional access modifiers
The return type for the method
The method name
An opening parenthesis
An optional list of method argument
A closing parenthesis

8. Creating Method Structure
modifier returnValueType methodName (list of parameters)
modifiers
Return value Type
Method
header
public static int max (int num1, int num2) {
….. }
parameters
methodName
Method
body

9. Creating Methods Access modifiers
public
private
protected
package
Place entire method within class that will use it
Not within any other method

10. Return Type Void Nonvoid (value-returning method)
does not return a value to the calling program
Nonvoid (value-returning method)
returns a single value to the calling program.

11. Method Name A verb or a verb phrase.
Capitalize the first letter of the second word
Never mention the method name inside of the function definition except if the function is recursive.

12. Parameter Listings
Are variables sent to the method to perform its designated tasks.
Must determine and list the data type of each parameter before listing its name.

13. Method Body Includes the statements that will perform the task
Must be framed with curly braces.
Start by defining variables (only the ones outside of the class and not listed within the parameter listing)

14. Calling a Method A method is invoked by a method call
Specifies the method name and provides information (argument)
Exception: The main method is automatically called when a program starts
Two ways to call a method
Based on whether the method returns a value or not
Return value
int larger = max (3,4);
System.out.println( max (3,4));
Return void
A call to the method must be a statement
System.out.println (“ Welcome”);
Use the method name as a statement

15. Creating Methods: Single Argument
Values that are sent into a method are called arguments
Local variable
Known only within boundaries of method
Each time method executes
Variable redeclared
New memory location large enough to hold type set up and named
e.g: int a = 5
displayValue (a)
public void displayValue(int num) {
System.out.println(num);}
The argument 5 is copied into
The parameter variable num

16. Creating Methods: Multiple Arguments
Method can require more than one argument
List arguments within call to method
Separate with commas
Call a method
Arguments sent to method must match arguments listed in method declaration by:
Number
Type

17. Example public class ComputeCommission {
public static void main (String args[] )
int value = 23000;
double commRate = 0.08;
computeComm(value, commRate) }
public static void computeComm (int v, double cr)
double commission;
commission = v * cr;

18. Creating Methods: Return Values
Return type can be any type used in Java
Primitive types
Class types
void
Returns nothing
Method’s type
return statement
Causes value to be sent from called method back to calling method

19. Exercise
EX 1: Swap
EX 2: Car loan System

20. Overloading Methods
Create another method with the same name, but different parameters
Overloading Rules
Every method must have a different number of formal parameters OR
If the number of formal parameters is the same, then the data type of the formal parameter (in the order listed), must differ in at least one position
Types of parameters determine which method executes
EX 3: Overloading the Max method

21. Output Format Pakage from java.text: import java.text.DecimalFormat;
DecimalFormat precisionTwo = new Decimal Format (“0.00”)
Pakage from javax.swing
Number of columns
import javax.swing.JTextArea;
JTextArea outputTextArea = new JTextArea (11,20)
Number of rows

22. Common Errors Putting semicolon at the end of a method header
Writing modifiers or return types in a method call statement
Forgetting to pass arguments to methods that require them
Attempting to access a parameter variable with code outside the method where the variable is declared

23. Common Errors
Changing the contents of a method’s parameter variable and expecting the argument that was passed into the parameter to change as well
Using a variable to receive a method’s return value when the variable’s data type is incompatible with the data type of the return value
Not writing a return statement in a non-void method