1. Defining methods and more arrays
CSC 211 Java I
Defining methods and more arrays

2. Today’s plan
Q&A on the assignment
Defining methods
More about arrays

3. Questions on the assignment
How do you find the maximum element in an array?
What about the index of that element?
How do you exclude the minimum and maximum scores?
Other questions?

4. Today’s plan
Q&A on the assignment
Defining methods
More about arrays

5. Being proactive
In the last homework we had to find the maximum of an array
We might want to do this again
We might need to do it more than once in our program (for example, three times!)
It is a “self contained” operation, easily described:
Input: An array of doubles
Output: A double that is the maximum in the array

6. Being proactive
double myMax = myArray [0]; for (int j = 1; j < myArray.length; j++) { if (myArray[j] > max) myMax = myArray[j]; }
public static
double
findMax
(double[] myArray) {
return myMax; }

7. Methods
A method is a collection of programming statements devoted to performing a clearly specified part of the program’s objective
Ie: a collection of statement that carries out a very specific function
E.g. Given an array, find the highest value

8. Writing a method (sneak preview):
public static double findMax(double[] arr) {
..... }
Begin with a method header that includes
The “visibility” - for now we will use ‘public’
‘static’ -- again, more on this later
The “return type” :
ie the type of data returned by the array such as a double
If the array does not return a value, we say ‘void’
The identifier (name) given to the method
The statements of the method are enclosed by braces { and }

9. Example public class SampleHw6 {
public static void main(String[] args) ……
double max;
max = findMax(scores); }
public static double findMax (double[] myArray)

10. Methods
When a method is invoked (by calling its name) the flow of control (i.e. the order in which the program is executing) is transferred to that group of statements
When the method’s execution is completed, the flow returns to the location from which the method was originally called
A method terminates in one of two ways:
the end of the method is reached i.e. ‘ } ’
by a return statement

11. Example: Tracing the Flow
public class SampleHw6 {
public static void main(String[] args) ……
double max = findMax(scores); }
public static double findMax (double[] myArray)
return myMax; 1 2

12. Methods that return a value
It is common for methods to actually compute something and RETURN the result of that computation
If a method returns a result, the data type returned of that result MUST be specified in the header (the opening line).
Can be any data type such as a int, char, double, boolean, String, Triangle, Square, etc
If the method does not returning anything, the return type is specified as void

13. The return type of a method
public static void main (String[] args)
public static int add ( )
public static double myMethod ( )
public static String[] yourMethod ( )

14. The header of a method public static double findMax (double[] myArray)
VISIBILITY MODIFIERS
The full significance of this modifier is beyond the scope of CSC 211.
The basics: When your program consists of a single class, the main method is declared as public. Methods other than main() may be either public or private

15. The header of a method public static double findMax (double[] myArray)
STATIC MODIFIERS
The full significance of this modifier is beyond the scope of CSC 211.
Essential things to know: When your program consists of a single class, the main method and all the service methods must be declared static

16. The header of a method public static double findMax (double[] myArray)
VISIBILITY AND STATIC MODIFIERS
Magic words for now
RETURN TYPE
Tells us what data-type is returned by this method (if any) after its execution
If the method does not return a value, the return type is called: void
The return type can be any of our familiar primitive data types or an object of a particular class (String, Triangle, Square, etc)

17. The header of a method RETURN TYPE NAME OF METHOD
public static double findMax (double[] scores)
RETURN TYPE
VISIBILITY AND STATIC MODIFIERS
NAME OF METHOD
Identifier chosen by the programmer who wrote the method.
By convention it should start with a lower case.
Initials of words after the first one in the identifier (if any) should be capitalized

18. VISIBILITY AND STATIC MODIFIERS
The header of a method
public static double findMax (double[] myArray)
NAME OF METHOD
VISIBILITY AND STATIC MODIFIERS
RETURN TYPE
PARAMETERS
Data needed (if any) by the method.
Note: Many methods do NOT take any parameters at all.

19. Try It: Write the header described by the comment below.
public class BasicMath { …
//create a method called ‘add’ that
//accepts 2 integers and returns their sum

20. Example: public class BasicMath { …
//create a method called ‘add’ that
//accepts 2 integers and returns their sum
public static int add (int num1, int num2)
int result = num1 + num2;
return result; }

21. System.out.println(sum);
public class BasicMath {
public static void main(String[] args) {
Scanner console = new Scanner (System.in);
int x = console.nextInt();
int y = console.nextInt(); }
A method call with parameters causes IMPLICIT ASSIGNMENT STATEMENTS to be executed, assigning the values of the actual parameters to the formal parameters listed in the method code:
num1 = x;
num2 = y;
int sum = add (x,y);
System.out.println(sum);
public static int add (int num1, int num2) {
int result = num1 + num2;
return result; }

22. System.out.println(sum);
public class BasicMath {
public static void main(String[] args) {
Scanner console = new Scanner (System.in);
int x = console.nextInt();
int y = console.nextInt(); }
With a method that returns a value, (i.e. not a void method),
it is often the case that the method call is part of an ASSIGNMENT STATEMENT as seen here.
Once flow of control returns following execution of the method, that assignment statement will be executed.
The returned value of the method is then used in the assignment statement.
int sum = add (x,y);
result
System.out.println(sum);
public static int add (int num1, int num2) {
int result = num1 + num2;
return result; }

23. Get to work Complete Parts 1.1 – 1.4 on the lab
It involves writing several methods
You must also write a main method to call the methods you have created

24. Sharing variables
In all the examples you have worked out so far no two methods shared any variable
Sometimes you DO want two separate methods to work on the same variables that are used throughout the program
This should rarely be used. Ie: Avoid if at all possible. If they come up, I will tell you the situations where it is acceptable.
We are talking about this now mostly as preparation for CSC 212 …
This is an issue of something called “SCOPE”

25. Scope
The scope of a variable is the area in which that variable can be used (referenced)
A variable can be declared:
Inside a method or a construct
At class level (not inside a method – including main() )

26. Data scope
A variable declared at class level, meaning outside of any method (including main), can be used by every single method in that class
A variable declared inside of a method or construct (loop, if statement, etc), can be only be used inside that method or construct.
These are known as “local variables”
Their existence is LOCAL to the method or construct in which they are declared
Outside of the method/construct, it’s as if the variable does not exist

27. What’s your scope
Complete Part 2.1 and 2.2 in the lab

28. A word on STATIC
If your program consists of a single class, all methods should be declared static
In this situation, every variable declared at class level MUST be declared static
More explanation in 212

29. Today’s plan
Q&A on the assignment
Defining methods
More about arrays

30. Are you with me? Will this code compile? int[] numbers = new int[4];
for (int i = 0; i <= numbers.length; i++) {
System.out.println(numbers[i]); }

31. Are you with me? Will this code run successfully?
int[] numbers = new int[4];
for (int i = 0; i <= numbers.length; i++) {
System.out.println(numbers[i]); }

32. More Arrays: Bounds checking
Once an array is created, it has a fixed size (for example 10).
An index used in an array reference must specify a valid element: the index value must be in bounds (0 to 9)
The Java interpreter will complain if an array index is out of bounds . Notice that the compiler will NOT complain
This is called automatic bounds checking

33. Copying an array
Suppose x is an array of int,that has been instantiated as follows:
int[] x = {1,2,3,4,5};
Suppose we want to make a copy of it
What do we do?
What does “make a copy” really mean?

34. Copy? Deep copy Shallow copy: int[] x = {1,2,3,4,5}; x y y
Deep copy
int[] x = {1,2,3,4,5};
int[] y = new int[5];
for (int i = 0; i < x.length; i++)
{ y[i] = x[i]; }
Shallow copy:
int[] x = {1,2,3,4,5};
int[] y;
y = x;

35. Copying an array Complete Part 3.1 – 3.3 on the lab
You need to write several methods that copy arrays and a main method that tests the methods you have written

36. Swapping values in array
scores[0]
scores[1]
scores[2] 90
______ 30 70
scores[0]
scores[1]
scores[2] 70
______ 30 90
How would you swap the 1st and 3rd element?

37. Swapping values in array
scores[0]
scores[1]
scores[2] 90
______ 30
scores[0]
scores[1]
scores[2] 70
______ 30 90 90 90
scores[0] = scores[2];
scores[2] = scores[0];

38. Swapping values in array
scores[0]
scores[1]
scores[2] 90
______ 30 70
scores[0]
scores[1]
scores[2] 70
______ 30 90 90 70 70
temp = scores[0];
scores[0] = scores[2];
scores[2] = temp;

39. Reversing an array
scores
scores

40.
scores

41. Reversing an array in JEnglish
Initialize some variable (e.g. left ) to the index of the first element in the array: (0);
Initialize another variable (e.g.right)to the index of the last element in the array: (length-1)
while (left < right) {
swap value in pos. left w. value in pos. right
increment left
decrement right }

42. Get your hands dirty Complete Parts 3.4 and 3.5 on the lab
You need to implement and test the algorithm for reversing an array we just discussed

43. Implementation: A method to reverse an array
public static void reverse(int[] anArray) {
int temp;
int left = 0;
int right = anArray.length – 1;
while (left < right)
temp = anArray[left];
anArray[left] = anArray[right];
anArray[right] = temp;
left++;
right--; }
} //end method reverse()

44. Reference
Arrays are objects! *** We pass a reference to the array - NOT THE VALUES of that array. *** In the function we wrote on the previous slide, the parameter, anArray is a reference (i.e. an address) to the array object that was passed when the reverse() method was invoked.
int [] x = {72, 38, 92};
reverse(x); x 90
___ 38 70
anArray x 70
___ 38 90
anArray

45. public static void reverse(int [] anArray){
int temp;
int left = 0;
int right = anArray.length – 1;
while (left < right) {
temp = anArray[left];
anArray[left++] = anArray[right];
anArray[right--} = temp;} } x
Reference
Arrays are objects! We pass a reference, not the value. anArray is a reference to the same object that was passed. x x x x
int [] x = {72, 38, 92};
reverse(x); x x 90
___ 38 70
anArray x 70
___ 38 90
anArray

46. Get your hands dirty Complete Parts 3.6 and 3.7 on the lab
You need to write methods that determines if two arrays have exactly the same values in the same order and a program that tests your methods