1. Fixed-sized collections
Objects First with Java
Fixed-sized collections
Introduction to arrays
6.0
© David J. Barnes and Michael Kölling

2. Fixed-size collections
Objects First with Java
Fixed-size collections
An array is a fixed-size collection type:
stores object OR primitive data types
more efficient access than dynamic collections
Special syntax (like other languages)
objects have NO methods
but it may have fields
The maximum collection size may be pre-determined with an upper limit
its size is accessible via a field
Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling
© David J. Barnes and Michael Kölling 2

3. The weblog-analyzer project
Objects First with Java
The weblog-analyzer project
Web server records details of each web page access in a log file
Supports analysis tasks:
Most popular pages
Site referencing page
Busiest periods
How much data is being delivered
Broken references
Analyze accesses by hour to determine:
When to upgrade to more powerful server
When to run maintenance at quietest hour
© 2017 Pearson Education, Inc. Hoboken, NJ. All rights reserved.
© David J. Barnes and Michael Kölling

4. year month day hour minute
Objects First with Java
weblog-analyzer
Our server writes to a text log file named:
weblog.txt
Web server will record a time stamp of June 7, 2011 at 3:45am in the format:
year month day hour minute
Current analyzer is limited to providing a count for the number of accesses in each 1 hour period over the duration covered by the log (0 - 23)
Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling
© David J. Barnes and Michael Kölling 4

5. Declaring array variables
Objects First with Java
Declaring array variables
LogAnalyzer class contains the field:
private int[] hourCounts;
indicates hourCounts is of type integer array
int would be the base type of the array
the array object would store type int values
difference between declarations
int hourCounts; // single int variable
int[] hourCounts; // int-array variable
hourCounts = new int[24];
Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling
© David J. Barnes and Michael Kölling 5

6. Creating an array object
Objects First with Java
Creating an array object
Array variable declaration — does not contain size
public class LogAnalyzer {
private int[] hourCounts;
private LogfileReader reader;
public LogAnalyzer()
hourCounts = new int[24];
reader = new LogfileReader(); }
...
Array object creation — specifies size
Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling
© David J. Barnes and Michael Kölling 6

7. Creating an array object
Objects First with Java
Creating an array object
new type[integer-expression]
new int[24]
creates an object of type integer array
creates array capacity to hold 24 integers
Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling
© David J. Barnes and Michael Kölling 7

8. private int[] hourCounts; hourCounts = new int[24];
Objects First with Java
The hourCounts array
private int[] hourCounts;
hourCounts = new int[24]; =
new int[24];
Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling
© David J. Barnes and Michael Kölling 8

9. Accessing an array array[integer expression]
Objects First with Java
Accessing an array array[integer expression]
Square-bracket notation is used to access an array element by indexing:
labels[6]
machines[0]
people[x y]
Valid indices depend on the length of the array and range from [0 … (length-1)]
Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling
© David J. Barnes and Michael Kölling 9

10. Using an array Elements are used like ordinary variables
Objects First with Java
Using an array
Elements are used like ordinary variables
The target of an assignment:
labels[5] = ... ;
machines[index] = new Machine(10);
In an expression:
double half = readings[0] / 2;
adjusted = hourCounts[hour] – 3;
hourCounts[hour]++;
System.out.print(item[1].getName());
Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling
© David J. Barnes and Michael Kölling 10

11. Objects First with Java
Standard array use
private int[] hourCounts;
private String[] names;
...
hourCounts = new int[24];
names = new String[10];
hourcounts[i] = 0;
hourcounts[i]++;
System.out.println(hourcounts[i]);
declaration
creation
use
© 2017 Pearson Education, Inc. Hoboken, NJ. All rights reserved.
© David J. Barnes and Michael Kölling

12. declaration, creation and initialization
Objects First with Java
Array literals {3, 15, 4, 5, …}
declaration, creation and initialization
The size is inferred from the data:
private int[ ] numbers = { 3, 15, 4, 5 };
Array literals in this form can only be used in declarations and NOT like this:
numbers = { 3, 15, 4, 5 };
Related uses require new:
numbers = new int[ ] { 3, 15, 4, 5 }; XX
Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling
© David J. Barnes and Michael Kölling 12

13. Objects First with Java
Array length
private int[] numbers = { 3, 15, 4, 5 };
int n = numbers.length;
not a method call!
length is a field rather than a method
It is fixed-size and can NOT be changed
© 2017 Pearson Education, Inc. Hoboken, NJ. All rights reserved.
© David J. Barnes and Michael Kölling

14. Objects First with Java
LogAnalyzer example
The hourCounts field is necessary to store the analysis of the access data
hourCounts = new int[24];
The constructor creates an array object of 24 integers for the hourCounts field
hourCounts.length // equals 24
Each of the 24 integer elements represent the number of accesses made within that particular hour (in a 24-hour day)
hourCounts[hour]++;
A larger integer value = more accesses
Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling
© David J. Barnes and Michael Kölling 14

15. Arrays and for-each loops
Objects First with Java
Arrays and for-each loops
Can we use for-each to access EVERY element in the array collection without adding/removing any elements? YES
for-each loops may be used on arrays:
for(int value : hourCounts) {
System.out.println(value); }
However, there is NO index counter to use if location of element is needed
© David J. Barnes and Michael Kölling 15

16. Objects First with Java
The for loop
The two variations of the for loop (use definite iteration):
for-each
for
The for loop is often used:
to iterate a fixed number of times
with a variable that changes a fixed amount on each iteration
The for loop is similar to while loop
Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling
© David J. Barnes and Michael Kölling 16

17. Equivalent in while-loop form
Objects First with Java
for loop pseudo-code
General form of the for loop
for(initialization; condition; post-body action) {
statements to be repeated }
Equivalent in while-loop form
initialization;
while(condition) {
statements to be repeated
post-body action }
Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling
© David J. Barnes and Michael Kölling 17

18. A Java example for loop version
Objects First with Java
A Java example
for loop version
for(int hour = 0; hour < hourCounts.length; hour++) {
System.out.println(hour + ": " + hourCounts[hour]); }
while loop version
int hour = 0;
while(hour < hourCounts.length) {
System.out.println(hour + ": " + hourCounts[hour]);
hour++; }
Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling
© David J. Barnes and Michael Kölling 18

19. Array-related methods
System has static arraycopy
java.util.Arrays contains static utility methods for processing arrays:
copyOf
binarySearch
fill
sort
ArrayList has toArray
© 2017 Pearson Education, Inc. Hoboken, NJ. All rights reserved.

20. Objects First with Java
Practice
Given an array of numbers, print out all the numbers in the array using a for loop:
int[] numbers = { 4, 1, 22, 9, 14, 3, 9};
for ...
Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling
© David J. Barnes and Michael Kölling 20

21. Objects First with Java
Practice
Given an array of numbers, print out all the numbers in the array using a for loop:
int[] numbers = { 4, 1, 22, 9, 14, 3, 9};
for(int num = 0; num < numbers.length; num++) {
System.out.println(numbers[num]); }
Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling
© David J. Barnes and Michael Kölling 21

22. Objects First with Java
Practice
Fill an array with the first 25 numbers in the Fibonacci sequence:
int[] fib = new int[25];
fib[0] = 0;
fib[1] = 1;
for ...
Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling
© David J. Barnes and Michael Kölling 22

23. Objects First with Java
Practice
Fill an array with the first 25 numbers in the Fibonacci sequence:
int[] fib = new int[25];
fib[0] = 0;
fib[1] = 1;
for(int num = 2; num < fib.length; num++) {
fib[num] = fib[num - 1] + fib[num -2]; }
Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling
© David J. Barnes and Michael Kölling 23

24. for loop with flexibility
Objects First with Java
for loop with flexibility
Print multiples of 3 that are below 40
Start at any element
for(int num = 3; num < 40; num = num + 3) {
System.out.println(num); }
End at any element
Bigger steps of larger increments
© David J. Barnes and Michael Kölling 24

25. for loops and iterators
Objects First with Java
for loops and iterators
Can we use a for loop with an Iterator to access EVERY element in a collection and REMOVE selective elements? YES
A special use of for loop may be used:
for(Iterator<Track> it = tracks.iterator();
it.hasNext(); ) {
Track track = it.next();
if(track.getArtist().equals(artist))
it.remove(); }
There is NO post-body action in the loop header, because the increment is being taken care of by it.next in the loop body
(But, the semicolon is still necessary)
No post-body action required.
© David J. Barnes and Michael Kölling 25

26. for PROS CONS may be used on a collection, non-collection or array
flexibility on start/end item and increment amount
ability to add/remove/change the item during the loop
access to loop counter (variable) is provided
increment is completed automatically after each iteration
may even be used with Iterators
CONS
definite iteration so number of elements MUST be known
access to items in sequence [start to end]

27. Review Arrays: for loops:
Objects First with Java
Review
Arrays:
are appropriate where a fixed-size collection is required
use a special syntax (no methods)
for loops:
are used when an index variable is required
offer an alternative to while loops when the number of repetitions is known
Used with a regular step size
Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling
© David J. Barnes and Michael Kölling 27

28. Which loop should I use? for-each: for: while:
Objects First with Java
Which loop should I use?
for-each:
iterate over ALL elements in a collection
no adding or removing of any elements
no loop counter (index) is needed
for:
definite iteration with known start and end
increment amount may be flexible (> 1)
loop counter (index) is needed
while:
indefinite iteration with unknown # of iterations
loop end can be determined by some condition(s)
Non-collections:
use a for or while loop
Removing elements:
(if examining ALL elements) use for with Iterator
(if stopping before the collection ends) use while
© David J. Barnes and Michael Kölling 28

29. The automaton project An array of cells
Each cell maintains a simple state
Usually a small numerical value
e.g. on/off or alive/dead
The states change according to simple rules
Changes affected by neighboring states
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30. Cell states – blank cells are in state 0
A simple automaton
nextState[i] =
(state[i-1] + state[i] + state[i+1]) % 2;
Step
Cell states – blank cells are in state 0 + 1 2 3 4 5 6
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31. The conditional operator
Choose between two values:
condition ? value1 : value2
for(int cellValue : state) {
System.out.print(cellValue == 1 ? '+' : ' '); }
System.out.println();
if(cellValue == 1) {
return ‘+’; }
else {
return ‘ ’;
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32. Further advanced material

33. Arrays of more than one dimension
Array syntax supports multiple dimensions (i.e. 2D array)
representing a game board
a grid of cells
possibly [x, y] coordinates
Can be thought of as an array of arrays
each x row is an array with y columns
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34. Objects First with Java
The brain project
Cell[][] cells;
...
cells = new Cell[numRows][numCols];
for(int row = 0; row < numRows; row++) {
for(int col = 0; col < numCols; col++) {
cells[row][col] = new Cell(); }
© 2017 Pearson Education, Inc. Hoboken, NJ. All rights reserved.
© David J. Barnes and Michael Kölling

35. Alternative iteration
for(int row = 0; row < cells.length; row++) {
Cell[] nextRow = cells[row];
for(int col = 0; col < nextRow.length; col++) {
nextRow[col] = new Cell(); }
Array of array style
Requires no access to numRows and numCols (accesses using length field)
Works with irregular shape arrays, which are supported in Java
© 2017 Pearson Education, Inc. Hoboken, NJ. All rights reserved.

36. Arrays and Streams
java.util.Arrays has several stream() methods that return a Stream based on an array
IntStream is a Stream of int values
Instream.range() creates a sequential IntStream
toArray returns an array from a Stream
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37. toString() Object class Implicit method call Overriding method
System.out.print()
System.out.println()
Overriding method
Determined at runtime
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