1. Arrays

2. Java Collections Uncovered
Collection classes (particularly ArrayList) are implemented using arrays
Arrays are structures that are part of the Java language
If you need a collection of primitive types (e.g., ints or doubles), arrays are arguably more convenient to use than ArrayLists
Wrapper classes can be avoided
Copyright © 2003, by the authors of these slides, and Ateneo de Manila University. All rights reserved
L10: Arrays
Slide 2

3. Arrays Definition In Java,
pronounced “ar-ray”, w/ stress on 2nd syllable
Definition
sequence of elements of the same type
each element is accessed through an index
In Java,
declaration: double[] nums;
creation: nums = new double[8];
use: nums[3] = 6.6;
Note: starting index is 0 (0 to 7, above)
double nums[] is also legal, but double[] nums is preferred, since it emphasizes that the type is double[]
(“double array” or “array of doubles”)
Copyright © 2003, by the authors of these slides, and Ateneo de Manila University. All rights reserved
L10: Arrays
Slide 3

4. Visualizing an Array nums null Declare: double[] nums;
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L10: Arrays
Slide 4

5. Visualizing an Array nums null Declare: double[] nums;
0.0 1 2 3 4 5 6 7
null
Declare: double[] nums;
Create: nums = new double[8];
Copyright © 2003, by the authors of these slides, and Ateneo de Manila University. All rights reserved
L10: Arrays
Slide 5

6. Visualizing an Array nums 6.6 Declare: double[] nums;
0.0 1 2 3 4 5 6 7
6.6
Declare: double[] nums;
Create: nums = new double[8];
Use: nums[3] = 6.6;
Copyright © 2003, by the authors of these slides, and Ateneo de Manila University. All rights reserved
L10: Arrays
Slide 6

7. Array of Objects Declaration Creation of the Array Creation of Objects
Circle[] circles;
Creation of the Array
circles = new Circle[8];
creates an array of references to Circles
but no actual Circles yet
Creation of Objects
for ( i = 0; i < 8; i++ ) { circles[i] = new Circle( 10 + i * 10, 10, 5 ); }
creates the Circles themselves and assigns them to the references
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L10: Arrays
Slide 7

8. Visualizing an Array of Objects
circles
null
Declare: Circle[] circles;
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L10: Arrays
Slide 8

9. Visualizing an Array of Objects1 2 3 4
null
Circle-type references
circles
null
Declare: Circle[] circles;
Create array: circles = new Circle[5];
Copyright © 2003, by the authors of these slides, and Ateneo de Manila University. All rights reserved
L10: Arrays
Slide 9

10. Visualizing an Array of Objects
Circle-type references
Circle
x y radius
circles
null
Circle
x y radius
null 1
null
Circle
x y radius 2
null
Circle
x y radius
null 3
Declare: Circle[] circles; 4
Circle
x y radius
null
Create array: circles = new Circle[5];
Create objects:
for ( i = 0; i < 5; i++ ) { circles[i] = new Circle( 10 + i * 10, 10, 5 ); }
Copyright © 2003, by the authors of these slides, and Ateneo de Manila University. All rights reserved
L10: Arrays
Slide 10

11. Visualizing an Array of Objects
Circle-type references
Circle
x y radius
circles
null
Circle
x y radius
null 1
null
Circle
x y radius 2
null
null 3
Declare: Circle[] circles; 4
Circle
x y radius
null
Create array: circles = new Circle[5];
Circle
x y radius
Circle
x y radius
Create objects:
for ( i = 0; i < 5; i++ ) { circles[i] = new Circle( 10 + i * 10, 10, 5 ); }
Use objects: e.g., circles[3].getX();
(returns 40)
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L10: Arrays
Slide 11

12. More About Arrays Arrays are objects Special features
the array variable is just a reference to the actual array that contains the values
need to use “new” after declaring
passed as a reference when used as method parameter
Special features
a public final int length field returns the array size
in recent example, circles.length would return 5
[] operator only work with arrays
ArrayIndexOutOfBounds exception
valid indices for array of size n: 0 to n-1
any acces to other indices causes an error
Array size can’t be changed after array is created
To expand array, we need to create a new array, copy old array contents, then point array variable to new array
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L10: Arrays
Slide 12

13. Arrays versus ArrayList
Maximum size
For arrays, need to establish establish this quantity upon creation
For ArrayLists, underlying array grows and shrinks as needed
Types and casting
Arrays are “typed”; i.e., programmer indicates what kind of elements are allowed in the collection
ArrayLists take any Object. Need to cast upon invoking the get() method. Need to use a wrapper class when storing primitives
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L10: Arrays
Slide 13

14. Multi-dimensional Arrays
A natural extension of simple (1D) arrays
2D declaration: char[][] grid;
think “array of arrays”
Array creation
grid = new char[10][20]; // 10 rows, 20 columns
Another way
grid = new char[10][]; // creates array of 10 char[]’s
for (i = 0; i < 10; i++)
{ grid[i] = new char[20]; // creates a size-20 array }
This way allows for varying row sizes
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L10: Arrays
Slide 14

15. Visualizing 2D Arrays char[][] null C Declare: char[][] grid;1 2 3 4
char[]-type references
char[][]
null C
Declare: char[][] grid;
Create array of rows: grid = new char[5][];
Create rows:
for ( i = 0; i < 5; i++ ) { grid[i] = new char[3]; }
Use objects: e.g., grid[3][2] = ‘C’
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L10: Arrays
Slide 15

16. Using 2D Arrays To refer to individual element, use two indices
e.g., grid[5][18] = ‘X’;
Using only one index refers to a single dimensional array
e.g., grid[5] refers to row 5
grid[5].length is the length of row 5 (in this case, it’s 20)
The array variable by itself refers to the top-level array (i.e., the array of rows)
grid.length is the length of the array of rows (i.e., it’s the number of rows)
Copyright © 2003, by the authors of these slides, and Ateneo de Manila University. All rights reserved
L10: Arrays
Slide 16

17. Sorting
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L10: Arrays
Slide 17

18. Sorting Goal: reorder array elements to be in a desired order
Comparing Strings
s1.compareTo( s2 ) – returns an int:
0 if s1 and s2 have same content
> 0 if s1 > s2 (e.g., “Homer” > “Bart”)
< 0 if s1 < s2 (e.g., “Lisa” < “Maggie” )
Note: Uses Unicode values, so all Capital letters are considered less than lowercase letters (e.g., “G” < “a”)
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L10: Arrays
Slide 18

19. Bubble Sort
Starting from beginning, swap adjacent pairs, putting greater value to right
At the end of one pass, greatest value will “bubble” to rightmost position
Repeat for next-to-greatest value, etc. Q W E R T Q W E R T Q E W R T Q E R W T Q E R T W
Greatest Value is now at rightmost position
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L10: Arrays
Slide 19

20. Bubble Sort end-1 end 1 2 3 4 Q W E R T First loop goes from i=0 to 3,1 2 3 4
public void sort( String[] a ) {
for ( int end = a.length-1; end > 0; end--)
for ( int i = 0; i < end; i++ )
if ( a[i].compareTo( a[i+1] ) > 0 )
{ // swap
String temp = a[i];
a[i] = a[i+1];
a[i+1] = temp; } Q W E R T
First loop goes from i=0 to 3,
repeat for i=0 to 2, i=0 to 1, i=0
Copyright © 2003, by the authors of these slides, and Ateneo de Manila University. All rights reserved
L10: Arrays
Slide 20

21. Sorting Objects assuming GradeRecord has
public void sort( GradeRecord[] a ) {
for ( int end = a.length-1; end > 0; end--)
for (int i = 0; i < end; i++)
if ( a[i].getName().compareTo( a[i+1].getName() ) > 0 )
{ // swap the whole object (not just the Strings)
GradeRecord temp = a[i];
a[i] = a[i+1];
a[i+1] = temp; }
assuming GradeRecord has
a getName() method that returns a String
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L10: Arrays
Slide 21

22. Performance of Sorting
How long does BubbleSort take?
N-1 turns + N-2 + N-3, etc.
Roughly N2 for N items
We say O(N2) time (“order N2” time)
Not very efficient for large N
There are faster sort algorithms
Fastest comparison-based sort is O(N log N)
Copyright © 2003, by the authors of these slides, and Ateneo de Manila University. All rights reserved
L10: Arrays
Slide 22