1. More Methods and Arrays
Material from Chapter 7
that we didn’t cover in 1226

2. More from Chapter 7 Objects with Arrays Arrays of Objects
use arrays as instance variables
using partly-filled-in arrays
understand co-reference
Arrays of Objects
creating and filling in an array of objects

3. Using Arrays with Classes
Each student has several grades
array as an instance variable of Student
only some assignments graded so far
Several students in a course
array of Student objects
Make a grade sheet program
list of Students, each with grades
Student.java
ClassList.java
GradeSheet.java

4. Arrays as Instance Variables
Change our Student class so that it has an array of grades rather than a single grade
grades in 8 assignments, say
stu
A_NUMBER
name
asgnGrades A
“Dent, Stu” 81 75
100 98 86 94 96

5. Array IV, Getter, and Setter
Seems like this is the way to do it:
private int[] asgnGrades;
public int[] getAsgnGrades() {
return asgnGrades; }
public void setAsgnGrades(int[] newGrades) {
if (areValidGrades(newGrades)) { // add this method
asgnGrades = newGrades;
But later we’ll see a problem with this!

6. The Constructor
The constructor’s job is to give the first values to each of the instance variables
constructor needs to give a value to asgnGrades
just an array with eight zeroes
should have a constant for the eight values!
public static final int NUM_ASGN = 8;
public Student(String name) { …
this.asgnGrades = new int[Student.NUM_ASGN]; }

7. More Methods Get/set assignment grades individually
use 1-based index in the method
int a1Grade = stu.getGrade(1);
stu.setGrade(2, 100);
stu
A_NUMBER
name
asgnGrades A
“Dent, Stu” 81 75
100 98 86 94 96 81
a1Grade
100

8. getGrade Method A1 stored in asgnGrades[0], and so on
public int getGrade(int asgnNum) {
if (1 <= asgnNum && asgnNum <= NUM_ASGN) {
return asgnGrade[asgnNum – 1]; }
return 0;
should make an isValidAsgnNo method
the setter will use it, too!
there’s an exercise for you!

9. Exercise Write the setGrade method /**
* Set this student’s grade for the given assignment.
asgnNum the assignment number
grade the grade got for that assignment */
public void setGrade(int asgnNum, int grade) { … }
use the isValidAsgnNo method from the previous method

10. More Methods Calculate assignment average grade
double aveGrade = stu.getAverage();
maybe another method that drops low scores?
double aveWithDrops = stu.getAverageDropping(2)
stu
A_NUMBER
name
asgnGrades A
“Dent, Stu” 81 75
100 98 86 94 96
91.25
aveGrade
aveWithDrops

11. Unmarked Assignments? What if only two assignments graded?
still have zeroes on the rest
what does our average calculator do?
what should it do?
stu
A_NUMBER
name
asgnGrades A
“Dent, Stu” 81 75
19.5
aveGrade

12. Partly Full Arrays Sometimes we want space for lots…
need space for all 8 assignment grades
…but using only some of them now
only two have been graded so far
Need to keep track of how many are in use
another variable – instance or class?
average method needs to use it

13. Using Partly Full Arrays
We can make arrays the size user wants…
…but user might not know!
“Lots here, and more coming later!”
We can make a BIG array and use part of it
e.g. up to 150 people in a class
use only as many spaces as actual students

14. Unmarked Assignments? Variable tracks # of assignments graded
update it after entering all students’ grades
method averages only graded assignments
aveGrade = stu.getAverage();
Student
NUM_ASGN
gradedAsgns 8 2
stu
A_NUMBER
name
asgnGrades A
“Dent, Stu” 81 75
78.0
aveGrade

15. Averaging Marked Assignments
Upper bound of the summing loop is the number of graded assignments
instead of NUM_ASGN
public double getAverage() {
int sum = 0;
for (int i = 0; i < gradedAsgns; ++i) {
sum += asgnGrades[i]; }
if (gradedAsgns == 0) { return 0.0; } // don’t / by 0
else { return (double)sum / (double)gradedAsgns; }
or Student.gradedAsgns
or this.asgnGrades[i]

16. Exercise Create an array for up to 100 numbers (int)
create a constant for the maximum
Ask user for how many numbers they have
assume they give a good #
Read that many numbers into the array

17. asgnGrades Getter Revisited
Maybe return only the part of asgnGrades that’s actually in use
that is, just the graded assignments
int[] myGrades = stu.getAsgnGrades();
stu
A_NUMBER
name
asgnGrades A
“Dent, Stu” 81 75
Student
NUM_ASGN
gradedAsgns 8 2
myGrades 81 75

18. Revised getAsgnGrades
Return just the graded assignments
public int[] getAsgnGrades() {
int[] graded = new int[Student.gradedAsgns];
for (int i = 0; i < Student.gradedAsgns; ++i) {
graded[i] = this.asgnGrades[i]; }
return graded;
it’s actually a better way to do getAsgnGrades
compared to just return this.asgnGrades

19. Problems of Arrays as IVs
Remember we make IVs private
so no one else can change them
But making an array private doesn’t do that!
give someone our array  they can change it!
we’ve already seen this!
let’s go back to GiftExchange…
… and its removeName method

20. GiftExchange.removeName
We give this method an array and a name…
…it removes that name from the array
p.s. void removeName(String[] names, String name) {
for (int i = 0; i < names.length; ++i) {
if (names[i].equals(name)) {
names[i] = “”; }
Angela
Barrak
Vladimir
David
Vladimir
Look! Look!
We’re changing an array someone gave us!

21. Arrays: Variables and Objects
Two different variables can refer to the same array
int[] firstVar = new int[]{1, 2, 3}; // a NEW array
System.out.println(Arrays.toString(firstVar));
int[] secondVar = firstVar; // NOT a new array!
secondVar[0] = 5; // same array as firstName
[1, 2, 3]
[5, 2, 3]

22. Co-Reference When two variables point to the same array
Or when they point to the same object
objects work the same way as arrays
Variables can be in the same method
Car myCar = new Car();
Car stevesCar = new Car();
Car disCar = myCar;
disCar.setColor(blue);
(variable)
myCar
(variable)
stevesCar
(variable)
disCar

23. Array Co-Reference Objects (values) get passed, not variables
removeName(getters, givers[i]); …
p.s. void removeName(String[] names, String name)
(variable)
getters
(variable)
givers[i]
(variable)
names
(variable)
name
Angela
Barrak
Vladimir
David
Vladimir

24. Co-Reference Problems
OLD version of getAsgnGrades
int[] stealGrades = stu.getAsgnGrades();
stealGrades[0] = -99;
client can change grades to whatever!
we’re not protecting our information
stu
A_NUMBER
name
asgnGrades A
“Dent, Stu” 81 75
100 98 86 94 96
(variable)
stealGrades
-99
(variable)

25. Co-Reference Problems
NEW version of getAsgnGrades
int[] stealGrades = stu.getAsgnGrades();
stealGrades[0] = -99;
client changes only his COPY!
we’re protected!
stu
A_NUMBER
name
asgnGrades A
“Dent, Stu” 81 75
100 98 86 94 96
(variable)
stealGrades 81 75
100 98 86 94 96
-99
(variable)

26. Another Co-Reference Problem
Setter changes our array to refer to one the client owns – and can modify at will!
int[] stealGrades = new int[]{81,75,100,98,86,94,96,100};
stu.setAsgnGrades(stealGrades);
stealGrades[0] = -99;
stu
(variable)
stealGrades
A_NUMBER
name
asgnGrades A
“Dent, Stu” 81
-99 75
100 98
(variable) 86 94 96
100

27. Make Copies of Arrays To protect array contents, make a copy
public void setAsgnGrades(int[] newGrades) {
if (areValidGrades(newGrades)) {
for (int i = 0; i < newGrades.length; ++i) {
asgnGrades[i] = newGrades[i]; }
stu
(variable)
stealGrades
A_NUMBER
name
asgnGrades A
“Dent, Stu”
-99 81 75
100 98 81 75
100 98
(variable) 86 94 96
100 86 94 96
100

28. areValidGrades New grades must all be correct
and not too many of them!
private static areValidGrades(int[] grades) {
if (grades.length > NUM_GRADES) {
return false; }
for (int i = 0; i < grades.length; ++i) {
if (!isValidGrade(grades[i])) { return false; }
return true;
Too many grades!
One grade (at least) not valid!
Not too many, and all were valid
And the method is static because…?

29. Making Copies of Arrays
Copying arrays needs to be done a lot
so Java provides a tool to do it
Arrays.copyOf(arrayToCopy, sizeOfCopy)
public int[] getAsgnGrades() {
return Arrays.copyOf(asgnGrades, gradedAsgns); }
public void setAsgnGrades(int[] newGrades) {
if (areValidGrades(newGrades)) {
asgnGrades = Arrays.copyOf(newGrades, newGrades.length);

30. new elements are zeroes
Arrays.copyOf
New array can be smaller or larger
int[] original = new int[]{1, 2, 3, 4};
int[] smaller = Arrays.copyOf(original, 2);
int[] larger = Arrays.copyOf(original, 8);
(variable)
original 1 2 3 4
(variable)
smaller 1 2
(variable)
larger 1 2 3 4
extra elements
lopped off
new elements are zeroes

31. Arrays of Objects Generally have many students at once
lots of objects of same type
treated the same
 an array!
Arrays of objects a little bit more complicated than arrays of primitives
but not much!

32. Creating an Array of Students
Create the Student array variable and object
Student[] student = new Student[MAX];
Student[] student creates the array variable
new Student[MAX] creates the array object
Do we now have MAX Student objects?
NO! We just have that many Student variables
An array is a collection of variables
each variable needs to have a Student object created for it!
ClassList.java

33. Creating an Array of Students
Loop reads a name, creates a Student
for (int s = 0; s < student.length; s++) {
S.o.p(“Enter Student’s name: ”);
String name = kbd.nextLine();
student[s] = new Student(name); }
using “s” because it’s a mnemonic for “student”
recall that Student constructor accepts a name, assigns a student number
ClassList.java

34. Arrays of Objects Create the array… …then the array elements student
“Alice”
STUDENT_NUMBER
studentName
& /
student A
“Bill”
STUDENT_NUMBER
studentName
& A
“Carole”
STUDENT_NUMBER
studentName
&

35. Printing Class List
Now we have all the objects created, we can just use them as usual
for (int s = 0; s < student.length; s++) {
S.o.pln(student[s].getANumber() + “ ”
+ student[s].getName()); }
student[s] is a Student
getANumber() the student's # in A format
getName() the student's name
ClassList.java

36. Exercise Create an array of ten Thingamajigs, sized from 1 to 10
Note: new Thingamajig(n) creates a Thingamajig of size n
Write a loop to add up the sizes of the Thingamajig in the array
use the getSize() method!

37. Assigning Grades to a Student
Client assigns grades to a student
create student object
Student stu = new Student(“Student, Lowell E.”);
assign grades
stu.setAsgnGrade(1, 95);
stu.setAsgnGrade(2, 100);
stu.setAsgnGrade(3, 72); …
stu.setAsgnGrade(6, 90);

38. Reading One Student’s Grades
Client can set grade for student
for (a = 1; a <= Student.NUM_ASGN; a++) {
System.out.print("Enter A" + a + " grade: ");
int g = kbd.nextInt(); kbd.nextLine();
stu.setAsgnGrade(a, g); }
loop: for each assignment
prompt for and read a grade
set the student’s grade for that assignment

39. Reading Many Students’ Grades
Enter grades for each assignment
for each assignment, read each student’s grade
for (int a = 1; a <= Student.NUM_ASGN; a++) {
System.out.println("Enter A" + a + " grades: ");
for (int s = 0; s < numStu; s++) {
System.out.print(student[s].getStudentName() + "> ");
int g = kbd.nextInt(); kbd.nextLine();
student[s].setAsgnGrade(a, g); }
GradeSheet.java

40. Calculating the Class Average
Add up averages of all students
ask each student for their average
Divide by number of students
int sum = 0;
for (int s = 0; s < numStu; s++) {
sum += student[s].getPctGrade(); }
double avg = (double)sum / (double)numStu;
GradeSheet.java

41. Exercise
Suppose we have an array of professors and we want to find their average salary
array named professors
numProfs professors in the array
getSalary method returns a prof’s salary
Write the code

42. Big Objects and Little Objects
We have an array of Students
Student[] student = new Student[MAX];
Each element of the array is a Student
Student stu = student[0];
Each Student object has an array of grades
int[] myGrades = student[me].getAsgnGrades();
Each grade is an int value
int myA1Grade = student[me].getGrade(1); // better way!
or myA1Grade = student[me].getAsgnGrades()[0];

43. Composition Joining objects together in bigger objects
Student has two Strings and an int[]
NOTE: has, not is
Can build objects as big as we need them
Section has a Prof and a Student[]
Course has a Section[] and a Course[] (!)
the Course[] is the list of prerequisites!
Department has a Prof[] and a Course[]
University has …

44. Exercise
Get the first letter of the name of the first student in the first section of the first course in the first department in the university
myUniversity, getDepartments(), getCourses(), getSections(), getStudents(), getName(), charAt(#)
University has a Department[] and other stuff…
Department has a Prof[] and a Course[]
Course has a Section[] and a Course[]
Section has a Prof and a Student[]

45. Questions
Next Time
Arrays are nice
ArrayLists are nicer!