1. Objects First with Java CITS1001
Arrays II
CITS1001
© David J. Barnes and Michael Kölling

2. Scope of this lecture Arrays of objects Multi-dimensional arrays
The Game of Life (next lecture)

3. Objects First with Java
Arrays of objects
The arrays we have seen so far had elements of a primitive type
int[], double[], boolean[], etc.
But an array can also hold objects
e.g. Term[]
Arrays can also be two-dimensional
e.g. int[][]
Arrays can also be three-dimensional, or more
But we won’t get to that in CITS1001
© David J. Barnes and Michael Kölling

4. Objects First with Java
Arrays of objects
When using an array of primitive type, there are two steps involved
Declare the variable to refer to the array
Create the space for the array elements using new
When using an array of object type, there are three steps involved
Populate the array with objects by repeatedly using new, often in a loop
© David J. Barnes and Michael Kölling

5. A Student class
public class Student {
private String studentNumber;
private int mark;
public Student(String studentNumber, int mark) {
this.studentNumber = studentNumber;
this.mark = mark; }
public String getStudentNumber() {
return studentNumber;
public int getMark() {
return mark;
A skeleton version of a possible Student class in a student records system

6. Creating a unit list Declare Create Populate Student[] unitList;
unitList = new Student[numStudents];
unitList[0] = new Student(“042371X”,64);
unitList[1] = new Student(“ ”,72);
unitList[2] = new Student(“ ”,55); …
unitList[numStudents-1] = new Student(“ ”,85);
Declare
Create
Populate

7. The three steps unit null unit null null null null null unit[0]

8. Using arrays of objects
Using an array of objects is the same as using any array
You just need to remember that the elements are objects!
private Student top(Student[] unitlist) {
Student top = unitlist[0];
for (Student si : unitlist)
if (si.getMark() > top.getMark())
top = si;
return top; }

9. Compare with max private int max(int[] a) { int max = a[0];
for (int i : a)
if (i > max)
max = i;
return max; }

10. Student top(Student[] unitList)
Method signatures
int max(int[] a)
Student top(Student[] unitList)

11. Initialisation
int max = a[0];
Declare a variable to hold the “best so far”, and initialise it to the first element in the array
Student top = unitList[0];

12. Processing
for (int i : a)
if (i > max)
max = i;
Check each element in turn, compare it with the best so far, and update the best if necessary
for (Student si : unitList)
if (si.getMark() > top.getMark())
top = si;

13. Return
return max;
Finally return the extreme element – the highest int or the Student with the best mark
return top;

14. What if we want to find the highest mark?
We could copy-and-paste top, and edit the details to create a new method
private int highestMark(Student[] unitlist) {
int max = unitlist[0].getMark();
for (Student si : unitlist)
if (si.getMark() > max)
max = si.getMark();
return max; }
No!!

15. Much better to use the existing method!
Do not write another looping method!
Use the already-written functionality
private int highestMark(Student[] unitlist) {
return top(unitlist).getMark(); }

16. Objects First with Java
2D arrays
We sometimes need arrays with more than one dimension
int[][] a = new int[4][3];
a[0][0]
a[0][1]
a[0][2]
This creates an array with four “rows” and three “columns”
The “row” index ranges from 0–3 and the “column” index from 0–2
a[1][0]
a[1][1]
a[1][2]
a[2][0]
a[2][1]
a[2][2]
a[3][0]
a[3][1]
a[3][2]
© David J. Barnes and Michael Kölling

17. 2D arrays contd.
This is really an array where each element is itself an array
a is a valid variable, representing the whole thing
a[0] is a valid variable, representing the first “row”
a[0][2] is a valid variable, representing the last element in the first “row”

18. 2D arrays contd. a a[0] a[1] a[2] a[3] a[0][0] a[0][1] a[0][2] a[1][0]
a[0][1]
a[0][2]
a[0]
a[1][0]
a[1][1]
a[1][2]
a[1] a
a[2][0]
a[2][1]
a[2][2]
a[2]
a[3][0]
a[3][1]
a[3][2]
a[3]

19. 2D arrays contd.
Given that an array is an object, this is really just a special case of an array of objects
The single statement int[][] a = new int[4][3]; declares the array variable a, creates the array, and populates it!

20. 2D arrays examples
A 2D array is normally processed with two nested for-loops
private int sum(int[][] a) {
int sum = 0;
for (int i = 0; i < a.length; i++)
for (int j = 0; j < a[i].length; j++)
sum = sum + a[i][j];
return sum; }

21. 2D arrays examples
Suppose we want to return the index of the first row containing any element that is true
private int firsttrue(boolean[][] a) {
for (int i = 0; i < a.length; i++)
for (int j = 0; j < a[i].length; j++)
if (a[i][j])
return i;
return -1; }

22. 2D arrays example
We will illustrate the use of 2D arrays further with a case study of The Game of Life
In the next lecture…
And there’s plenty of practice in the lab too