F21SF
Software Engineering Foundations
Arrays
The java basic array type
Dept of
Computer
Science
09/04/2015
Monica Farrow EM G30
email : M.Farrow@hw.ac.uk
Material available on Vision
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Topics
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Using the basic java array type to keep lists of integers
and Strings.
With an array
 Adding a value for a specified position
 Traversing the elements in the array, using length
variable
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Returning a String containing details
Returning a total
Returning maximum
Searching, using 2 algorithms
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Arrays
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Arrays are fixed-length structures for storing multiple
values of the same type.
They are useful for keeping lists of items
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All items must be of same type
E.g. int, double, char, String, Name, Car……
In java, an array is a special kind of class, with a unique
syntax
First we will consider arrays of integers, as an example of
arrays of primitive types.
The array can be declared by adding the array symbol [ ]
before or after the variable name

int [] marks;
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String [] prevQuals;
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Instantiating an array - 1
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If the values initially are not known, instantiate the array
using new and the length. This defines the size only, not
the contents. Each element has a default value, which is 0
for integers, null for objects
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final static int NUM_MARKS = 5;
marks= new int [NUM_MARKS];
NUM_MARKS= 5
index
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0
0
0
0
0
0
1
2
3
4
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Instantiating an array - 2
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If the values are known, the array can be declared and
instantiated like this:
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prevQuals = { “MIC”, “ARA”, “FAC”};
(fictitious previous qualifications)
The size of this array is determined by the number of values
supplied within the curly braces
MIC
index
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0
ARA FAC
1
2
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Elements in the array
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Consider the elements in the marks array as 5
separate variables called
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marks[0], marks[1], marks[2], marks[3],
marks[4]
Indexing the items (numbering like Strings)
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The first index is 0
The last index is one less than the number of entries in
the array
The ith element is stored using index i-1
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first element : marks[0], third element : marks[2]
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Accessing individual items
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Storing data into an element of the array
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marks[0] = 2;
Extracting data from an element of the array
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System.out.println(marks[3]);
int lastMark = marks[4];
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(Recap: Accessing instance variables)
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Instance variables describe each particular object
 E.g Car : model, tank size, mpl….
Usually private, so accessed only using a ‘get’ method
myCar.getModel()
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This separates the ‘interface’ of the class (i.e. what methods can you
use) from the implementation (how it is actually done)
IF they were public, they could be accessed using the object name
and a period
myCar.model
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Finding the length of an array
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You can use the array public length instance variable
directly
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i.e. marks.length
NB this is NOT a method, so no brackets
VERY confusing
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Strings have a length method
Arrays have a public length instance variable
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Traversing items in a list
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The index can be calculated at runtime
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Elements can be accessed using an index value which is in
a variable
int index = 4;
int value = marks[index];
The index in the variable can be increased by one each
time, to enable iteration through the array
for ( int index = 0;
index < marks.length;
index++)
{
System.out.println(marks[index]);
}
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Problems : Array Index Out of Bounds
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Your program crashes with an
ArrayOutOfBoundsException if you use an index that
is greater than or equal to the length of the array
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E.g. marks[6] = 10;
The error message tells you the index number and the
program line number that is causing the problem

E.g.
Exception in thread "main"
java.lang.ArrayIndexOutOfBoundsException: 6
at Student.addMark(Student.java:30)
at MainStudent.main(MainStudent.java:12)
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Problems: Assigning memory to an array

When an array is initially declared, a small amount of
space is allocated to it to hold a ‘pointer’. This is true for
all objects in java.
 The pointer is the address in memory of the full
details for the object, and is initially null
marks

Points nowhere
Memory is only allocated once the array is instantiated
(with new or with literal values )
marks
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Points to details
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3
0
9
8
12
Problems : Null pointer exception
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You can get a null pointer exception by not instantiating
the array.
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In a simple example with just a main method, the program
will not compile if you just declare the array without
instantiating it
However, in more complex programs with more classes and
methods, the program may compile. It will then crash with a
null pointer exception if you refer to a specific item in the
un-instantiated array.
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Student example
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In this program, we want to store, for a student, their id,
name, year of study, previous qualifications and marks.
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Previous qualifications is a list of Strings. Each student may
have a different number of qualifications. The number is
fixed for a student – once the qualifications have been
supplied, they won’t change.
A set of 3 marks, initially all 0. The marks can be inserted
during the program run.
We would like to
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Add a mark for a specified assignment (numbered 1, 2 and
3)
Obtain a summary message for each student E.g.
“Helen Scott has an average mark of 6.3, highest mark is 8.
Qualifications: MIC ARA”
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One possible Class Design using CRC cards
MainStudent
Responsibility
Collabor
ators
Creates a couple of
students
Prints student reports to
Standard output
Changes one value
Adds a mark
Student
Student
Responsibility
Holds id, name, year of study
Holds list of previous qualifications
Holds list of marks
Add a mark for a specified assignment
Returns id, name, year (in 3 methods)
Returns a marks average
Returns a mark maximum
Returns a list of qualifications
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Class Diagram
MainStudent
main(arg[] : String) : void
Student
id : String
name : Name
year:int
prevQuals : String []
marks : int[]
Student(id:String, name:Name,
pq:String [], year:int)
getId() :String
getName() : String
addMark(value:int, ass:int) : void
getAveMark() : double
getMaxMark() : int
getQualList() : String
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Student class
public class Student
{
private String id;
//unique id, e.g. 0099
private Name name;
private int year;
//year of study e.g. 1,2,3 etc
private String [] prevQuals;
//previous qualifications
private static final int NUM_MARKS = 3;
private int [] marks;
//marks for the module - 5 assignments
//qual array is provided as a parameter
//marks array is initialised to a fixed size
public Student (String id, Name n, String [] pq, int y)
{
this.id = id;
name = n;
prevQuals = pq;
year = y;
marks = new int [NUM_MARKS];
// initially all 0
}
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Creating a student object in main method
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The qualification array is declared with known values, and
here will have length = 2
The marks array is created in the constructor, of length 3,
and all values start as 0 (the default for integers)
String [] quals1 = {"MIC", "ARA" };
Student s1 = new Student ("0011",
new Name("Helen Scott"), quals1, 1);
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Adding marks
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The method to add marks in the Student class
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Assignments are numbered 1,2,3
Arrays are numbered 0,1,2
public void addMark(int value, int ass ) {
int index = ass-1;
marks[index] = value;
}

Adding marks from main method
s1.addMark(8,1);
s1.addMark(7,2);
s1.addMark(4,3);
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Producing details as a String
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This method produces a String, adding in the location. It
then traverses the array, accessing each element in turn,
and adding the value to the String.
public String getQualDetails() {
String report = "" ;
for (int qualsIndex = 0;
qualsIndex < prevQuals.length; qualsIndex++) {
report += prevQuals[qualsIndex] + "
";
}
return report;
}
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Finding the average
//add up all the marks, then divide by number of marks
//demonstrates forcing double division
//assumes that all marks entered, or, if not, ok to use 0
for calculation
public double getAveMark() {
int total = 0;
for (int marksIndex = 0;
marksIndex < marks.length;
marksIndex++) {
total += marks[marksIndex];
}
return (double) total/marks.length;
}
NB Casting total to a double before dividing by an
integer
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Finding a maximum

Algorithm for finding a maximum
The first element becomes the current maximum
For each element
If the value of this element is greater than the current maximum,
Store this value as the current maximum
public int getMaxMark() {
int max = marks[0];
for (int marksIndex = 1; marksIndex < marks.length;
marksIndex++) {
if (marks[marksIndex] > max) {
max = marks[marksIndex];
}
}
return max;
}
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Testing
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The getMaxMark() method is a good example of how careful you need
to be when testing your program
It is very common to make a mistake in your boundary conditions
e.g. index = 0; index<limit.
 Too big and your program will crash (ok, so you find the error
pretty quickly)
 Too small and your program will work but miss out the last value

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Values like totals and counts and searches will be wrong
So it is a good idea to test with the maximum value
 In the first position
 In the last position
 Somewhere in the middle
And add up totals yourself, to make sure that the value returned is
correct
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Using these methods
double ave = s1.getAveMark();
int max = s1.getMaxMark();
System.out.println(s1.getName().getFullName()
+ " has an average mark of "
+ String.format ("%.1f", s1.getAveMark())
+ " and a highest mark of "
+ s1.getMaxMark() );
System.out.println("Previous qualifications:
" + s1.getQualDetails());
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Searching
Here is a typical way of searching
//returns true if the person has the specified
qualification, false otherwise
//can stop as soon as it is found,
//but might get to end and not find it at all.
//demonstrates searching and returning as soon as
condition is met.
public boolean hasQual(String searchQual) {
for (int qualsIndex = 0;
qualsIndex < prevQuals.length; qualsIndex++) {
String prevQual = prevQuals[qualsIndex];
if (prevQual.equals(searchQual) ) {
return true;
}
}
return false;
}
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Searching with while

This method has the same effect as the previous one
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Uses while condition to stop the loop
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Useful if you don’t want to return from the method at the end of the loop

BUT, if you use the principle ‘one method for one function’, you probably
will always be able to use the first version
public boolean hasQualification(String searchQual) {
int qualsIndex = 0;
boolean found = false;
while (qualsIndex < prevQuals.length && !found) {
String prevQual = prevQuals[qualsIndex];
if (prevQual.equals(searchQual) ) {
found = true;
}
qualsIndex++;
}
return found;
}
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Quiz
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Look at the Student class and the associated main method
What would happen if:
 the index in the loop in the getMaxMarks() method starts at 0
instead of 1?
 the test in that loop was <= instead of <?
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you omitted (double) from the return line in the
getAveMarks() method?
the addMark() method didn’t subtract one from the
assignment number?
you had specified that the addMark() method must provide
an index position not an assignment number?
you added a mark for assignment number 6?
You forgot to use the getFullName() method when printing
the name in the main method – just using ls.getName()
instead?
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The main method
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Now we can understand the parameter in the main
method
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public static void main (String args[])
The parameter is an array of Strings
We probably won’t use the parameter much if at all in this
module
However, it is there, and could be used for providing
runtime data
In the example on the next slide, 2 String parameters are
provided and are referred to as args[0] and args[1]
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A main method with parameters
public class Greetings {
public static void main (String args[])
{
System.out.println("Hello "
+ args[0] + " " + args[1]);
}
}

Running at the command line:
>javac Greetings.java
>java Greetings Sarah Jane
Hello Sarah Jane
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Parameters with an IDE
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You can also enter parameters for the main method when
using an IDE such as eclipse
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You will need to find where this is done
For eclipse, you can set them in the Run Configurations
dialog box, accessible from the Run menu.
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Multiple arrays
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You can have arrays of many dimensions. The
next example shows 2D arrays
Consider the game of ‘noughts and crosses’,
where the aim is to get a row of X’s or O’s.
O
O
X
X
X
O
X
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2 D arrays : notation

Declare a 2D array with the rows first, then the columns
char [][] grid = new char [3][3];
0
1
2
0
1
2
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2D arrays contd
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Each element needs to have the row and the column
specified
grid[2][0] = ‘X’;

Use nested loops to traverse complete array
for(int row=0; row<3; row++)
for (int col =0; col<3; col++)
grid[row][col] = ' ';

Use meaningful indexes e.g. row, col rather than i and
j.
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Arrays so far

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We’ve seen an array of integers and one of Strings
Any object can be held in an array

Pick it out e.g.
String prevQual = prevQuals[qualsIndex];

Use its methods e.g.
prevQual.equals(searchQual)
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Arrays are a fixed size in java
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You couldn’t easily add more qualifications or increase the
number of marks held
Next lecture – the java ArrayList class.
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To Do Now
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Read over lecture and look at code, try running the
program
Do the quiz
You should now be able to add a set of scores to your
competitor class. After the next lecture, you will be able to
add a collection of competitors.
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