1. Arrays and Methods

2. Outline Methods that return arrays Methods that take arrays
(not) returning an array instance variable
java.util.Arrays
Methods that take arrays
adding up the grades
finding the grades to drop

3. Getting All Assignment Grades
Client may want a list of grades
to put up on a Web site, for example
S.o.pln("<h2>Assignment Grades</h2>");
S.o.pln("<table border>");
for (int i = 0; i < Student.NUM_ASGN; ++i) {
S.o.pln(" <tr>");
S.o.pf(" <th>A%02d;</th>%n", (i+1));
S.o.pln(" <td>" + stu.asgnGrades[i] + "</td>");
S.o.pln(" </tr>"); }
S.o.pln("</table>");
Assignment Grades
A01
100
A02
A03 95
A04 80
A05 87
A06 98
A07
A08 96
asgnGrades is private in Student
NOTE: HTML mark-up (the <table>/<th>/<td> stuff) will not be on the test!

4. A Getter for an Array We know how to write a getter
just returns instance/class variable
return type is same as data type of variable
asgnGrades is the variable
it is an int[]
so…
public int[] getAsgnGrades() {
// WARNING: DON'T DO THIS!
return asgnGrades; wrong! wrong! wrong! }
NOTE: Don't return an array instance variable this way!
There's a better way on a later slide.

5. Returning Arrays Methods can return arrays
return type is an array type
private static int[] dice(int howMany) …
private static String[] wordsFrom(String line) …
The array to be returned is (often) new
int[] result = new int[howMany];
for (int d = 0; d < howMany; ++d) {
result[i] = 1 + (int)(6 * Math.random()); }
return result;
ArrayReturn.java

6. Calling an Array-Returning Method
Result of method is (often) stored in a variable
variable must be same type as return type(*)
dice returns an int[], wordsFrom returns a String[]
int[] myDice;
String[] the Words;
myDice = dice(6);
theWords = wordsFrom("These are the words");
methods create and return the arrays
myDice 5 3 2 4 6
theWords
“These”
“are”
“the”
“words”
(*) Except for things like an int return value saved into a double variable….
NOTE: you cannot save an int[] into a double[] variable!

7. Exercise
Make a method that returns a specified number of copies of a String in an array
Call the method to create an array containing three copies of "Hello", and save the result in a local variable named greeting
greeting
Hello
Hello
Hello

8. Variable Names and Arrays
Name of array inside method does not need to match the name of the array in the caller
because they’re two different variables
int[] myDice = dice(6); …
private static int[] dice(int howMany) {
int[] result = new int[howMany];
return result; // array variable result ceases to exist // it's the array object that's returned }
myDice
&
result
& 5 3 2 4 6

9. … then disCar is painted green
Arrays are Objects
Like String, Scanner, Student
variable points to/refers to the object
different variables can refer to the same object
such co-reference can cause problems
Car variables
Car objects
&
myCar:
disCar and myCar are not two different cars. They’re two different names for the same car.
&
disCar:
&
stevesCar:
if I paint myCar green…
… then disCar is painted green

10. Using the Grades Array getAsgnGrades() returns an array object
so when we do…
int[] myCopyOfGrades = stu.getAsgnGrades();
…we’re not actually getting a copy of the array…
…we’re getting the array itself…
&
stu:
&
myCopyOfGrades:
name:
A_NUMBER:
asgnGrades:
Dent, Stu A
100 77 80 85
&

11. Hacking the Grades Array
…and if we have the array itself…
we can change the values inside it
myCopyOfGrades[1] = 800;
myCopyOfGrades[2] = -50;
Student class is no longer protecting its variables!
&
stu:
&
myCopyOfGrades:
name:
A_NUMBER:
asgnGrades:
Dent, Stu A
100 77 80 85
&
800
-50

12. Protecting Array Instance Variables
Protect variables by returning a copy
myCopyOfGrades = stu.getAsgnGrades();
myCopyOfGrades[1] = 800;
myCopyOfGrades[2] = -50;
client has no access to the originals
&
stu:
&
myCopyOfGrades:
name:
A_NUMBER:
asgnGrades:
Dent, Stu A
100 77 80 85
&
100 77 80 85
800
-50

13. Creating a Copy of an Array
What needs to be done:
make a new array of the correct size
copy all the values of the old array into the new
public int[] getAsgnGrades() {
int[] copy = new int[asgnGrades.length];
for (int i = 0; i < copy.length; ++i) {
copy[i] = asgnGrades[i]; }
return copy;
100 77 80 85
&
asgnGrades:
&
copy:
100 77 80 85

14. An Easier Way to Copy Often need to copy an array
Java makes it easier for us
don’t need to write all that code
someone’s already done it for us!
just say what array and how much of it to copy
public int[] getAsgnGrades() {
// THIS is the way to return an array instance variable
return Arrays.copyOf(asgnGrades, NUM_ASGN); }
NOTE: This is the better way I promised you earlier.

15. The Arrays Class Arrays class provides many useful methods
copyOf – copies an array
copyOfRange – copies part of an array
equals – checks if two arrays have same elements
fill – fills an array with a given value
sort – sorts an array
toString – makes a String representing the array
Needs to be imported
import java.util.Arrays;

16. A Convenience Printing an array object gets weird output
int[] arr = new int[]{10, 2, 130, 40};
System.out.println(arr);
Arrays has a method to make it nicer:
System.out.println(Arrays.toString(arr));
[10, 2, 130, 40]

17. Arrays.copyOf Returns a new array of the size requested
extra elements of the copy are zeroed out
int[] longCopy = Arrays.copyOf(asgnGrades, 6);
some elements can be left off
int[] graded = Arrays.copyOf(asgnGrades, numGraded);
100 85
&
asgnGrades: 2
numGraded:
100 85
&
graded:
100 85
&
longCopy:

18. Aside To track the number of graded assignments:
need a private static variable for that value
private static int numAsgnGraded = 0;
want a getter and setter for that variable
setter should only allow valid assignment numbers
(left as exercise)
want a public getter for the graded assignments
public int[] getGradedAsgns() {
return Arrays.copyOf(asgnGrades, numAsgnGraded); }
NOTE: It is possible to create an array with zero elements!

19. Arrays as Arguments to Methods
[Brackets] pick out one element of an array
asgnGrades[0] picks out 1st element of asgnGrades
Don’t use them when sending the whole array!
it’s Arrays.copyOf(asgnGrades, NUM_ASGN);
not Arrays.copyOf(asgnGrades[], NUM_ASGN);
not Arrays.copyOf(asgnGrades[0], NUM_ASGN);
.class expected
cannot find symbol: class asgnGrades
unexpected type: required value found class
no suitable method for method copyOf(int, int)

20. Methods that Take Arrays
Can create our own methods that take arrays
int asgnSum = sumArray(asgnGrades);
just specify correct return/parameter types!
asgnGrades is an int[]; asgnSum is an int
private static int sumArray(int[] arr) {
int sum = 0;
for (int i = 0; i < arr.length; ++i) {
sum += arr[i]; }
return sum;
Remember: array names don’t have to match!
Sadly, java.util.Arrays does not give us this method.

21. Command Line Arguments
Remember how we declare main?
public static void main(String[] args)
never really explained String[] args
it’s an array of Strings...
...passed into your program...
...from the “command line”
prompt] java PrintArgs command line arguments
My 3 command line arguments were:
0: "command"
1: "line"
2: "arguments"

22. Command Line Arguments
Add words after the name of the class
They get passed to the program
(unless they start with < or >, or ...)
Appear in the String[] parameter of main
each word is a separate array element
prompt] java PrintArgs command line
My 2 command line arguments were:
0: "command"
1: "line"

23. NetBeans & the Command Line
File > Project Properties…
or right-click on project name in Projects pane

24. Command Line Arguments
You can use args like any other array
ask it its length
loop thru its elements
ask for some element
System.out.println("My " + args.length + " arguments:");
for (int i = 0; i < args.length; i++) {
System.out.println(i + ":\t\"" + args[i] + "\""); }
My 2 arguments:
0: "command"
1: "line"
args
“command”
“line”

25. args is a String[] OK to have a String[] Even if you type in numbers!
can have an array of anything (pretty much)
Even if you type in numbers!
you can use Integer.parseInt(args[i]) to get the integer value of args[i]
prompt] java PrintArgs 5 10
My 2 command line arguments were:
0: "5"
1: "10"
see AddArgs.java

26. Arrays as Parameters args is a String[] parameter for main
Other methods can have [] parameters, too
just declare the parameter to be an array!
public static int sumArray(int[] arr)
It’s just like any other parameter
gets its value from the method call
It’s just like any other array
it knows how long it is

27. Why a Method to Sum Arrays?
Can use it to add up assignment grades
then divide by NUM_ASGN for getAsgnGrade
Can use it to add up lab grades
then take min with 100 for getLabGrade
Can use it to add up test grades
then divide by NUM_TEST for getTestGrade
It’s something we’ll want to do multiple times
with different arrays

28. Calculating Assignments Grade
Currently average of all assignment grades
Want to drop two lowest
find the lowest grade
int lowestGrade = asgnGrades[0];
for (int i = 1; i < NUM_ASGN; ++i) {
lowestGrade = Math.min(lowestGrade, asgnGrades[i]); }
100 85 78 77
&
asgnGrades:
lowestGrade: 77
100

29. Calculating Assignments Grade
Still need second lowest
lowest greater than lowestGrade?
int secondLow = asgnGrades[0];
for (int i = 1; i < NUM_ASGN; ++i) {
if (asgnGrades[i] > lowestGrade) {
secondLow = Math.min(secondLow, asgnGrades[i]); }
100 85 78 77
&
asgnGrades:
lowestGrade: 77
100
secondLow: 78 85
100
Note: this looks right, but it’s wrong!

30. Calculating Assignments Grade
Still need second lowest
lowest greater than lowestGrade?
int secondLow = asgnGrades[0];
for (int i = 1; i < NUM_ASGN; ++i) {
if (asgnGrades[i] > lowestGrade) {
secondLow = Math.min(secondLow, asgnGrades[i]); }
100 85 77
&
asgnGrades:
lowestGrade:
100 77
secondLow: 85
100
See? Wrong!

31. Find Two Smallest Sort the array, then pick the front two Problem:
java.utils.Array has a sort method
Problem:
sorting the array will break getAsgnGrade(int)
grades won’t be in the proper locations any more
getAsgnGrade(1)  smallest grade on any assignment
but that’s wrong!
Solution:
sort a copy of the grades array

32. Sorting an Array Just give the array to Arrays.sort
It’ll come back sorted
int[] a = new int[]{4, 2, 7, 1, 9, 9, 4};
Arrays.sort(a);
Works for (almost) any kind of array
works for Strings – sort of
String[] words = new String[]{"e", "B", "d","c","a"};
Arrays.sort(words);
capital letters come first! a 1 2 4 7 9 4 2 7 1 9
words B a c d e e B d c a
ArraySorter.java

33. Sorting a String Array Need to give sort a Comparator
an object that says how to sort the Strings
String[] words = new String[]{"e", "B", "d","c","a"};
Arrays.sort(words, String.CASE_INSENSITIVE_ORDER);
now it sorts into alphabetical order!
words e B d c a a B c d e
ArraySorter.java

34. Modifying Arrays Arrays can be modified by methods
int[] a = new int[] {1, 2, 3, 4};
doubleEachElement(a);
Just a normal method
private static void doubleEachElement(int[] arr) { for (int i = 0; i < arr.length; ++i) { arr[i] *= 2; } } a:
& 2 4 6 8 1 2 3 4
arr:
&
ArrayDoubler.java

35. Doesn’t Work for Primitives
Method call passes actual value
parameter has same value as argument
thus method cannot change argument variable
int a = 3;
doubleValue(a); …
private static void doubleValue(int n) { n *= 2; } a 3 n 3 6
ArrayDoubler.java

36. Can’t Change Argument Variable
Method call passes reference to object
parameter points to same object as argument
but it’s NOT the same variable!
int[] a = new int[] {1, 2, 3, 4};
changeTheArray(a); …
public static void changeTheArray(int[] arr) { arr = int[] {5, 6, 7, 8}; }
& a 1 2 3 4 5 6 7 8
&
arr
ArrayDoubler.java

37. Find Two Smallest To get assignments grade:
public int getAsgnGrade() {
int[] copy = getAsgnGrades(); // get a copy of asgnGrades
Arrays.sort(copy); // sort the copy
int sum = sumArray(copy); // add up its elements
sum -= copy[0] + copy[1]; // subtract two smallest
return (int)((double)sum / (NUM_ASGN - 2) + 0.5); }
100 85 77
&
asgnGrades:
100 85 77
&
copy:
sum:
339
185 77 85
100 93

38. Not Always Two Smallest
Might have eleven assignments and drop three
want to make code easy to change to that
create a constant and a method
public int getAsgnGrade() {
// get dropped grades
int[] drops = getSmallest(asgnGrades, NUM_DROPS);
// calculate return value
int sum = sumArray(asgnGrades) - sumArray(drops);
return (int)(0.5+(double)sum/(NUM_ASGN-NUM_DROPS)); }

39. Get N Smallest Same procedure as before…
…but return first few elements as an array
private static int[] getSmallest(int[] arr, int n) {
int[] copy = Arrays.copyOf(arr, arr.length);
Arrays.sort(copy);
return Arrays.copyOf(copy, n); }
100 85 77
&
arr:
100 85 77
&
copy: 77
& n 1 77 85
100
Note: sorting the whole array is not the best way to do it; but it will work….

40. Trouble-Shooting ArrayIndexOutOfBoundsException NullPointerException
tried to access a non-existent array element
int[] arr = new int[5];
S.o.pln(arr[10]); // ArrayIndexOutOfBoundsExc.
NullPointerException
tried to talk to an array element that wasn’t created
Student[] myClass = new Student[10];
myClass[0].setName("First, Ingrid"); // NullPointerExc.
or to an array field that hasn’t been initialized
return labGrade[a – 1];

41. Next Week Tuesday: catch up and review Thursday: midterm test #2
objects and arrays
(you’ll still need to know the earlier stuff, of course)
labs 6 to 9
assignments 5 to 8
Sample test/questions have been posted
sample answers posted later
do the sample test before you look at the answers!