Download Objects First With Java

Grouping Objects
Arrays and for loops
Fixed-Size Collections
• Sometimes the maximum collection size
is known.
• Programming languages usually offer a
special fixed-size collection type: the
array.
• Java arrays can store objects or
primitive values.
• Arrays use a special syntax.
Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling
The weblog-analyzer Project
• A web server records details of each
access.
• It supports webmaster tasks:
•
•
•
•
Most popular pages
Busiest periods
How much data is being delivered
Broken references
• It analyses accesses by hour.
Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling
Creating an Array Object
public class LogAnalyzer
{
private int[] hourCounts;
Array variable declaration
private LogfileReader reader;
Array object construction
public LogAnalyzer()
{
hourCounts = new int[24];
reader = new LogfileReader();
}
...
}
Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling
The hourCounts Array
Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling
Using an Array
Square-brackets notation is used to access an
array element - e.g.
hourCounts[hour]
where hour must be an integer variable holding a
valid index to the hourCounts array (0..23).
Array elements are used just like variables. They
can be assigned:
hourCounts[hour] = 42;
Or used in expressions:
adjusted = 2*(hourCounts[hour] - 3);
System.out.println (hourCounts[hour]);
Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling
Using an Array
private int[] hourCounts;
private Person[] students;
declaration
...
hourCounts = new int[24];
students = new Person[100];
construction
...
hourcounts[i] = 0;
students[j].enroll ("Co320-S");
System.out.println (hourcounts[i]);
use
Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling
Special Syntax for Array Literals
declaration and
initialisation
private int[] numbers = {3, 15, 4, 5};
...
System.out.println (numbers[i]);
Array literals can only be used in initialisations
Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling
Special Syntax for Array Literals
declaration and
initialisation
private int[] numbers = {3, 15, 4, 5};
...
no brackets!
int n = numbers.length;
Note:literals
‘length’
a field
of the
array object
Array
can is
only
be used
in initialisations
Unlike ‘length()’, which is a method of String
Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling
Special Syntax for Array Literals
declaration and
initialisation
private int[] numbers = {3, 15, 4, 5};
...
no brackets!
int n = numbers.length;
Note:literals
‘length’
a field
of the
array object
Array
can is
only
be used
in initialisations
Unlike
Unlike‘size()’,
‘length()’,
which
which
is aismethod
a method
of ArrayList
of String
Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling
The for Loop
• There are two variations of the for loop,
for-each (already seen) and for.
• The for loop is often used to iterate a
fixed number of times.
• It is usually used with a control variable
that changes a fixed amount on each
iteration.
Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling
Structure of the for Loop
loop header
for (initialisation; condition; post-body action) {
... loop body
}
Statement(s) to be repeated
Equivalent while-loop form:
initialisation;
while (condition) {
... loop body
... post-body action
}
Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling
An Example
for loop version
for (int hour = 0; hour < hourCounts.length; hour++) {
System.out.println (hour + ": " + hourCounts[hour]);
}
Note: variable hour no longer exists here …
while loop version
int hour = 0;
while (hour < hourCounts.length) {
System.out.println(hour + ": " + hourCounts[hour]);
hour++;
}
Note: variable hour still exists here …
Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling
Practice
Given an array of numbers, print out all the numbers
in the array – one number per line. Use a for loop:
int[] numbers = {4, 1, 22, 9, 14, 3, 9};
for ( ... ) {
...
}
Write this now!
Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling
Practice
Given an array of numbers, print out all the numbers
in the array – one number per line. Use a for loop:
int[] numbers = {4, 1, 22, 9, 14, 3, 9};
for (int
...i )= {0; i < numbers.length; i++) {
...
System.out.println
(numbers[i]);
}
Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling
Practice
Actually,
Given
an arrays
array ofalso
numbers,
count as
print
collections
out all the
– so
numbers
we
in the
can
usearray
a for-each
– one number
loop onper
them:
line. Use a for loop:
int[] numbers = {4, 1, 22, 9, 14, 3, 9};
for (int i = 0; i < numbers.length; i++) {
System.out.println (numbers[i]);
}
for (int n : numbers) {
System.out.println (n);
}
Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling
Warning !!!
Collection (or array) elements accessed through a
for-each loop are copies of the actual elements:
for (int n : numbers) {
System.out.println (n);
}
But don’t try assigning to them:
for (int n : numbers) {
n = 2*n;
}

Changes copies of elements in
numbers – does not change any of the
elements in numbers itself!

Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling
Warning !!!
But don’t try assigning to them:
for (int n : numbers) {
n = 2*n;
}

Changes copies of elements in
numbers – does not change any of the
elements in numbers itself!

Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling
Warning !!!
But don’t try assigning to them:
for (int n : numbers) {
n = 2*n;
}

Changes copies of elements in
numbers – does not change any of the
elements in numbers itself!

To do this, we must use a for loop:
for (int i = 0; i < numbers.length; i++) {
numbers[i] = 2*numbers[i];
}

Changes the elements in
numbers itself!

Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling
Practice
Fill an array with the first 100 numbers of the Fibonacci
sequence.
Fibonacci: 0, 1, 1, 2, 3, 5, 8, 13, 21, 34, ...
(each number is the sum of the previous two numbers)
(start with zero and one)
int[] fib = new int[100];
fib[0] = 0; fib[1] = 1;
for ( ... ) {
...
}
Write this now!
Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling
Practice
Fill an array with the first 100 numbers of the Fibonacci
sequence.
Fibonacci: 0, 1, 1, 2, 3, 5, 8, 13, 21, 34, ...
(each number is the sum of the previous two numbers)
(start with zero and one)
int[] fib = new int[100];
fib[0] = 0; fib[1] = 1;
Cannot be written
with a for-each loop.
for (int
...i )= {2; i < fib.length; i++) {
...
fib[i]
= fib[i-2] + fib[i-1];
}
Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling
for loops can have
any size of increment
// Print multiples of 3 that are below 40.
for (int num = 3; num < 40; num = num + 3) {
System.out.println (num);
}
Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling
while loop without a collection
// Print all even numbers from 0 to 42 inclusive.
int index = 0;
while (index <= 42) {
System.out.println (index);
index = index + 2;
}
// Programming error? This does nothing at all!
int index = 0;
while (index >= 42) {
System.out.println (index);
index = index + 2;
}
// Programming error? This loop never ends!
int index = 0;
while (index <= 42) {
System.out.println (index);
index = index - 2;
}
Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling
The same – but with for loops
// Print all even numbers from 0 to 42 inclusive.
for (int index = 0; index <= 42; index = index + 2) {
System.out.println (index);
}
// Programming error? This does nothing at all!
for (int index = 0; index >= 42; index = index + 2) {
System.out.println (index);
}
// Programming error? This loop never ends!
for (int index = 0; index <= 42; index = index - 2) {
System.out.println (index);
}
Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling
for loops can exit early
/**
* Search an array for a zero.
*
* @return The index of the first zero element
*
(if none found, return -1)
*/
public int findZero (int[] A)
{
for (int i = 0; i < numbers.length; i++) {
if (numbers[i] == 0) {
return i;
}
}
return -1;
}
Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling
Review
• Arrays are appropriate where a fixed-size
collection is required.
• Arrays use special [square brackets] syntax.
• For loops are a better alternative to while loops
when the number of repetitions is known.
• For loops are often used to introduce and control
an index variable when iterating through arrays.
• For loops can exit early (with a return).
Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling