1. Lecture 03 & 04
Method and Arrays
Jaeki Song

2. Outlines Method Array Creating method Method call Passing parameters
Method overloading
Math class
Array
Definition of an Array
How to Declare and Initialize an Array
How to Pass Arrays to Methods

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

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

5. Return Type Void Nonvoid
does not return a value to the calling program
Nonvoid
returns a single value to the calling program.

6. 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.

7. 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.

8. 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)

9. Calling a Method A method is invoked by a method call
Specifies the method name and provides information (argument)
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”);
Example: Max

10. Passing Parameters Pass by value
When you invoke a method with parameters, a copy of the value of the actual parameter is passed to the method
Example: Swap
Formal parameters are changed in the example, but the actual parameter are not affected

11. Overloading Methods
Create another method with the same name, but different parameters
Example
Overloading the Max method

12. 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

13. Example
Car Loan Systems
Shipping Charge

14. Arrays
A list of related values with the same data type that is stored using a single group name
When you need to store multiple values, use an array
It is a static entity, in that it remains the same size throughout program execution

15. Array studentGrade[ ] array The individual variable called
element of the
array
Subscript (or index):
the first element
in every array is the
zeroth element
[0]
[1]
[2]
[3]
[4] .. 87 57 69 78 95
“second element of the array  it has a subscript of 1
“array element two”  has a subscript of 2

16. Declaring Array
Specifies the type of the elements and uses operator new to allocate the number of elements
Arrays are considered to be objects and all objects must be created with new operator

17. Declaring Array General format
DataType ArrayName [ ]; //Declares the array
ArrayName = new DataType [Size]; // allocates the array
DataType ArrayName[ ] = new DataType [ Size]
e.g:
float fltPayment[]; or float[ ] fltPayment;
fltPayment = new float [ 20];
float fltPayment[] = new float [20];

18. Initializing an Array
For primitive data types and Strings, you can give initial values to array element as you declare an array
e.g.
int DepartNumber[ ] = {423, 635, 589};
String DepartName[] = {“MIS”, “Marketing”,
“Finance”};
This does not allow you to specify the size of the array and do not use new keyword

19. Array Length Every array in java knows it own length Example: Grade
fltPayment = new float [ 20];
fltPayment.length
Example: Grade

20. Passing Arrays to Methods
To pass an array argument to a method, specify the name of the array of the array without any brackets
Pass the entire array (call-by-reference)
float score [ ] = new float [20];
grade = calculateAverage (score);
public float calculateAverage (float b [ ])

21. Passing Arrays to Methods
To pass an array element to a method, use the subscripted name of the array element as an argument in the method call
float score [ ] = new float [20];
grade = calculateAverage (score[3] );
public float calculateAverage (float b)

22. Example: Bubble Sort Compare pairs of adjacent memory cells
The small number: “bubble”
The large number: “sink”
Example:
Comparison Results Action
num[0] > num[1] False None
num[1] > num[2] True Swap
num[2] > num[3] True Swap 5 10 8 2
num[0] num[1] num[2] num[3]

23. Example: Bubble Sort for(i=0 ; i<num.length – 1 ; i++) {}
Ascending order
for (i=1; i<num.length; i++) { }
num[1]=10
num[2]=8
>
if (num[j] < num [j-1]) {
temp = num[j]; // one swap
num[j] = num[j-1];
num[j-1] = temp; }
temp=8
num[1]=8
num[2]=10
Example 2

24. Example: Selection Sort
Finds the largest/smallest number in the list and places it last/first.
num[0] num[1] num[2] num[3] 8 9 5 4
num[0] num[1] num[2] num[3] 8 4 5 9
Example 3