Fixed-sized collections: Introduction to arrays

Object-oriented programming with Java
Fixed-sized collections
Introduction to arrays
Ahmed Al-Ajeli
Lecture 10
2
Main concepts to be covered
•Arrays
•Arrays iteration
•Multi-dimensional arrays
•Miscellaneous topics
Object-oriented programming with Java
3
Fixed-size collections - arrays
•Sometimes the maximum collection size can be pre-determined.
•A special fixed-size collection type is available: an array.
•Unlike the flexible-size ArrayList collections, arrays can store object references or primitive-type values.
•Arrays are more efficient than ArrayList
4
Creating an array object
public class LogAnalyzer
{
private int[] hourCounts;
public LogAnalyzer()
{
hourCounts = new int[24];
}
...
}
Array object creation — specifies size
Array type — does not contain size
Object-oriented programming with Java
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The hourCounts array
6
Using an array
•Square-bracket notation is used to access an array element: hourCounts[...]
•Elements are used like ordinary variables.
•The target of an assignment:
hourCounts[hour] = ...;
•In an expression:
hourCounts[hour]++;
if(hourCounts[hour] > 0) ...
Object-oriented programming with Java
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Standard array use
private int[] hourCounts;
private String[] names;
Private TicketMachine[] machines;
...
hourCounts = new int[24];
machines = new TicketMachine[100];
machines[0] = new TicketMachine (500);
...
hourcounts[i] = 0;
hourcounts[i]++;
System.out.println(hourcounts[i]);
System.out.println(machines[0].getBalance());
declaration
creation
use
8
Array literals
•Array literals in this form can only be used in declarations.
•Related uses require new:
private int[] numbers = { 3, 15, 4, 5 };
declaration, creation and initialization
numbers = new int[] { 3, 15, 4, 5 };
•The size is inferred from the data.
Object-oriented programming with Java
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Array length
•NB: length is a field rather than a method.
•It’s value cannot be changed – ‘fixed size’.
private int[] numbers = { 3, 15, 4, 5 };
int n = numbers.length;
not a method call!
10
Array iteration
for(int hour = 0; hour < hourCounts.length; hour++) {
System.out.println(hour + ": " + hourCounts[hour]);
}
int hour = 0;
while(hour < hourCounts.length) {
System.out.println(hour + ": " + hourCounts[hour]);
hour++;
}
for loop version
while loop version
Object-oriented programming with Java
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Arrays and the for-each loop
int[] hourCounts = {1,3...}
...
for(int value : hourCounts) {
System.out.println(value);
}
12
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 ...
Object-oriented programming with Java
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Practice
•Fill an array with the Fibonacci sequence.
int[] fib = new int[howMany];
fib[0] = 0;
fib[1] = 1;
for(...) ...
0 1 1 2 3 5 8 13 21 34 ...
14
Arrays of more than one dimension
•Array syntax supports multiple dimensions.
–E.g., 2D array to represent a game board, or a grid of cells.
•Can be thought of as an array of arrays.
Object-oriented programming with Java
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Two-dimensional array example
int[][] cells; ... cells = new int[10][20]; ... for(int row = 0; row < 10; row++) { for(int col = 0; col < 20; col++) { cells[row][col] = row + col; } }
16
Alternative iteration
•Works with irregular shape arrays, which are supported in Java.
for(int row = 0; row < cells.length; row++) { for(int col = 0; col < cells[row].length; col++) { cells[row][col] = row + col; } }
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null
•Used with object types.
•Used to indicate, no object .
•We can test if an object variable holds the null value: if(track == null) …
•Used to indicate ‘no track yet’.
18
The conditional operator
•Choose between two values inside expressions:
int b = (a == 1) ? 20: 30; is equivalent to
condition ? value1 : value2
if (a == 1) {
b = 20;
}
else {
b = 30;
}