1 arrays chapter 8 spring 2006 cs 101 aaron bloomfield

1

Arrays

Chapter 8Spring 2006CS 101Aaron Bloomfield

22

Introduction to arraysIntroduction to arrays

3

Background Programmer often need the ability to represent a group of

values as a list List may be one-dimensional or multidimensional

Java provides arrays and the collection classes The Vector class is an example of a collection class

Consider arrays first

4

Example Definitions

char[] c;int[] value = new int[10];

Causes Array object variable c is un-initialized Array object variable value references a new ten element

list of integers Each of the integers is default initialized to 0

value 0 0 0 0 0

-c

…

5

An array example

int[] v = new int[10];int i = 7;int j = 2;int k = 4;v[0] = 1;v[i] = 5;v[j] = v[i] + 3;v[j+1] = v[i] + v[0];v[v[j]] = 12;System.out.println(v[2]);v[k] = stdin.nextInt();

v 00 0 00 0 00 00

v[2]v[0] v[1] v[5]v[3] v[4] v[7]v[6] v[9]v[8]


v 01 0 00 0 00 00

v[2]v[0] v[1] v[5]v[3] v[4] v[7]v[6] v[9]v[8]


v 01 0 00 0 50 00

v[2]v[0] v[1] v[5]v[3] v[4] v[7]v[6] v[9]v[8]


v 81 0 00 0 50 00

v[2]v[0] v[1] v[5]v[3] v[4] v[7]v[6] v[9]v[8]


v 81 0 06 0 50 00

v[2]v[0] v[1] v[5]v[3] v[4] v[7]v[6] v[9]v[8]


v 81 0 06 0 50 012

v[2]v[0] v[1] v[5]v[3] v[4] v[7]v[6] v[9]v[8]


v 81 0 06 0 50 012

v[2]v[0] v[1] v[5]v[3] v[4] v[7]v[6] v[9]v[8]

8 is displayed


v 81 0 06 3 50 012

v[2]v[0] v[1] v[5]v[3] v[4] v[7]v[6] v[9]v[8]

Suppose 3 is extracted


6

Array variable definition styles Without initialization

Type ofvalues in

list

Name oflist

Bracketsindicate arrayvariable being

defined

ElementType [ ] id;

int [] a;int a[];

7

Array variable definition styles With initialization

ElementType[ ] id = new ElementType [n];

Nonnegative integer expression specifying thenumber of elements in the array

A new array of nelements

8

Where we’ve seen arrays public static void main (String[] args)

Thus, the main() method takes in a String array as the parameter

Note that you can also define it as: public static void main (String args[])

or public static void main (String[] foobar)

9

Basic terminology List is composed of elements

Elements in a list have a common name Example: a[3] = 5; The common name is ‘a’

The list as a whole is referenced through the common name

List elements are of the same type — the base type

Elements of a list are referenced by subscripting (indexing) the common name

10

Java array features Subscripts are denoted as expressions within brackets: [ ]

Base (element) type can be any type

Size of array can be specified at run time This is different that pure C! (for the most part, at least)

Index type is integer and the index range must be 0 ... n-1 Where n is the number of elements Just like Strings indexing!

Automatic bounds checking Ensures any reference to an array element is valid

Data field length specifies the number of elements in the list

Array is an object Has features common to all other objects More on this later…

11

Consider Segment

int[] b = new int[100];b[-1] = 0;b[100] = 0;

Causes Array variable to reference a new list of 100 integers

Each element is initialized to 0 Two exceptions to be thrown

-1 is not a valid index – too small 100 is not a valid index – too large

IndexOutOfBoundsException

12

Consider

Point[] p = new Point[3];p[0] = new Point(0, 0);p[1] = new Point(1, 1);p[2] = new Point(2, 2);p[0].setX(1);p[1].setY(p[2].getY());Point vertex = new Point(4,4);p[1] = p[0];p[2] = vertex;

p

p[0] p[1] p[2]

null null null

Point: (0, 0)

p

p[0] p[1]

Point: (1, 1) Point: (2, 2)

p[2]

Point: (1, 0)

p

p[0] p[1]

Point: (1, 1) Point: (2, 2)

p[2]

Point: (1, 0)

p

p[0] p[1]

Point: (1, 2) Point: (2, 2)

p[2]

Point: (1, 0)

p

p[0] p[1]

Point: (1, 2) Point: (2, 2)

p[2]

vertex

Point: (4, 4)

Point: (1, 0)

p

p[0] p[1]

Point: (2, 2)

p[2]

vertex

Point: (4, 4)

Point: (1, 0)

p

p[0] p[1] p[2]

vertex

Point: (4, 4)

Point[] p = new Point[3];p[0] = new Point(0, 0);p[1] = new Point(1, 1);p[2] = new Point(2, 2);p[0].setX(1);p[1].setY(p[2].getY());Point vertex = new Point(4,4);p[1] = p[0];p[2] = vertex;

1313

New 2005 demotivatiors!New 2005 demotivatiors!

15

Explicit initialization Syntax

ElementType[] id = { exp0 , exp1 , ... expn-1 };

id references an array of n elements. id[0] hasvalue exp0, id[1] has value exp1, and so on.

Each expi is an expression thatevaluates to type ElementType

16

Explicit initialization Example

String[] puppy = { “pika”, “mila”, “arlo”, “nikki” };

int[] unit = { 1 };

Equivalent toString[] puppy = new String[4];puppy[0] = “pika"; puppy[1] = “mila";puppy[2] = “arlo"; puppy[3] = “nikki";

int[] unit = new int[1];unit[0] = 1;

17

Array members Member length

Size of the arrayfor (int i = 0; i < puppy.length; ++i) {

System.out.println(puppy[i]);}

18

Array members Member clone()

Produces a shallow copyPoint[] u = { new Point(0, 0), new Point(1, 1)};Point[] v = u.clone();

v[1] = new Point(4, 30);

Point: (0, 0) Point: (1, 1)

u

u[0] u[1]

Point: (0, 0)

v

v[0] v[1]

Point: (1, 1)

u

u[0] u[1]

Point: (0, 0)

v

v[0] v[1]

Point: (1, 1)

u

u[0] u[1]

Point: (4, 30)

Point[] u = { new Point(0, 0), new Point(1, 1)};Point[] v = u.clone();

v[1] = new Point(4, 30);

19

Member clone() Produces a shallow copy


v[1].setX(10);


v[1].setX(10);

Array members

Point: (0, 0) Point: (1, 1)

u

u[0] u[1]

Point: (0, 0)

v

v[0] v[1]

Point: (1, 1)

u

u[0] u[1]

Point: (0, 0)

v

v[0] v[1]

Point: (10, 1)

u

u[0] u[1]

20

Making a deep copy We want to copy the array and all the objects each element

of the array references This is called a deep copy

ExamplePoint[] w = new Point[u.length];for (int i = 0; i < u.length; ++i) {

w[i] = (Point) u[i].clone();}

21

Making a deep copy

Point: (0, 0)

w

w[0] w[1]

Point: (2, 1) Point: (2, 2)

w[2]

u

u[0] u[1] u[2]

Point: (0, 0) Point: (2, 1) Point: (2, 2)

22

Review of arrays Creating an array:

int[] foo = new int[10];

Accessing an array:foo[3] = 7;System.out.print (foo[1]);

Creating an array:String[] bar = new String[10];

Accessing an array:bar[3] = “qux”;System.out.println (bar[1]);

23

How Java represents arrays Consider

int[] a = { 1, 2, 3, 4, 5 };

a 1 2 3 4 5

+ …

Array

- length = 5

- data = 1 2 3 4 5

24

More about how Java represents Arrays Consider

int[] a;int[] b = null;int[] c = new int[5];int[] d = { 1, 2, 3,

4, 5 };a = c;d = c;

1 2 3 4 5

0 0 0 0 0

a -

b null

c

d

int[] a;int[] b = null;int[] c = new int[5];int[] d = { 1, 2, 3,

4, 5 };a = c;d = c;

2525

What do these pictures What do these pictures mean?mean?

Light beerLight beer Dandy lionsDandy lions Assaulted Assaulted

peanutpeanut EggplantEggplant Dr. PepperDr. Pepper Pool tablePool table Tap dancersTap dancers Card sharkCard shark King of popKing of pop I PodI Pod Gator aideGator aide Knight mareKnight mare Hole milkHole milk

2626

ArrayToolsArrayTools

27

ArrayTools.java We want to create a series of general utility methods to be

used for arrays

We will put these into an ArrayTools class

28

ArrayTools.java – outline

public class ArrayTools {

// class constant private static final int MAX_LIST_SIZE = 1000;

// sequentialSearch(): examine unsorted list for key public static int sequentialSearch(int[] data, int key) { ...

// putList (): prints list to screen public static void putList(int[] data) { ...

// getList(): extract and return up to MAX_LIST_SIZE values public static int[] getList() { ...

// reverse(): reverses the order of the element values public static void reverse(int[] list) { ...

// binarySearch(): examine sorted list for a key public static int binarySearch(char[] data, char key) { ...}

29

ArrayTools.java method putList() To print the array:

public static void putList(int[] data) {for (int i = 0; i < data.length; ++i) {

System.out.println(data[i]);}

}

Considerint[] score = { 6, 9, 82, 11, 29, 85, 11, 28, 91 };putList(score);

30

ArrayTools.java method getList()

public static int[] getList() {Scanner stdin = new Scanner (System.in);int[] buffer = new int[MAX_LIST_SIZE];int listSize = 0;for (int i = 0; (i < MAX_LIST_SIZE) && stdin.hasNext(); ++i) {

buffer[i] = stdin.nextInt();++listSize;

}int[] data = new int[listSize];for (int i = 0; i < listSize; ++i) {

data[i] = buffer[i];}return data;

}

31

ArrayTools.java method reverse()

public static void reverse(int[] data) {int[] clone = data.clone();for ( int i = 0; i < clone.length; ++i ) {

data[i] = clone[clone.length-1-i];}

}

Considerint[] foo = { 1, 2, 3, 4, 5 };reverse (foo);putList (foo);

32

Demo.javapublic class Demo {

// main(): application entry point public static void main(String[] args) {

System.out.println ("");System.out.println ("Enter list of integers:");int[] numbers = ArrayTools.getList ();

System.out.println ("");System.out.println ("Your list");ArrayTools.putList (numbers);

ArrayTools.reverse (numbers);System.out.println ("");System.out.println ("Your list in reverse");ArrayTools.putList (numbers);System.out.println ();

}}

3434

ArrayTools demo…ArrayTools demo…

ArrayDemo.javaArrayDemo.java

3636

… … main (String args[])main (String args[])

37

Consider that main() method again public static void main (String args[])

How does one pass in a parameter to the main method?

public class MainParameters {public static void main (String args[]) {

System.out.println ("Number of paramters to “ + "main(): " + args.length);

if ( args.length > 0 ) {for ( int i = 0; i < args.length; i++ )

System.out.println ("parameter " + i + ": '" + args[i] +

"'");}

}}

3838

Program DemoProgram Demo

MainParameters.javaMainParameters.java Via JCreatorVia JCreator Via the command lineVia the command line

3939

Basic array searchingBasic array searching

40

System.out.println("Enter search value (number): ");int key = stdin.nextInt();

int i;for (i = 0; i < data.length; ++i) {

if (key == data[i]) {break;

}}

if (i != data.length) {System.out.println(key + " is the " + i

+ "-th element");}else {

System.out.println(key + " is not in the list");}

++i

System.out.println("Enter search value (number): ");int key = stdin.nextInt();

int i;

if (key == data[i]) {break;

if (i != data.length) {System.out.println(key + " is the " + i

+ "-th element");}

i < data.length i = 0

Searching for a value

data 54 9

20 1

key 5

i -

data 54 9

20 1

key 5

i 0

data 54 9

20 1

key 5

i 0

data 54 9

20 1

key 5

i 0

data 54 9

20 1

key 5

i 1

data 54 9

20 1

key 5

i 1

data 54 9

20 1

key 5

i 1

data 54 9

20 1

key 5

i 2

data 54 9

20 1

key 5

i 2

data 54 9

20 1

key 5

i 2

data 54 9

20 1

key 5

i 2

data 54 9

20 1

key 5

i 2

41

Searching for the minimum value Segment

int minimumSoFar = sample[0];for (int i = 1; i < sample.length; ++i) {

if (sample[i] < minimumSoFar) {minimumSoFar = sample[i];

}}

42

ArrayTools.java method sequentialSearch()

public static int sequentialSearch(int[] data, int key) {for (int i = 0; i < data.length; ++i) {

if (data[i] == key) {return i;

}}

return -1;}

Considerint[] score = { 6, 9, 82, 11, 29, 85, 11, 28, 91 };int i1 = sequentialSearch(score, 11);int i2 = sequentialSearch(score, 30);

data 826 9 8511 29 2911 91

20 1 53 4 76 8

key 11

4343

Today’s demotivatorsToday’s demotivators

4444

SortingSorting

45

Sorting Problem

Arranging elements so that they are ordered according to some desired scheme Standard is non-decreasing order

Why don't we say increasing order?

Major tasks Comparisons of elements Updates or element movement

46

Selection sorting Algorithm basis

On iteration i, a selection sorting method: Finds the element containing the ith smallest value of

its list v and exchanges that element with v[i]

Example – iteration 0 Swaps smallest element with v[0] This results in smallest element being in the correct place

for a sorted result

v ‘E'‘Q' 'W' 'Y''R' 'T' 'I''U' 'P''O'

20 1 53 4 76 98

v ‘E'‘Q' 'W' 'Y''R' 'T' 'I''U' 'P''O'

20 1 53 4 76 98

v 'Q''E' 'W' 'Y''R' 'T' 'I''U' 'P''O'

20 1 53 4 76 98

47




Example – iteration 1 Swaps second smallest element with v[1] This results in second smallest element being in the

correct place for a sorted result

v 'Q''E' 'W' 'Y''R' 'T' 'I''U' 'P''O'

20 1 53 4 76 98

v 'Q''E' 'I' 'Y''R' 'T' 'W''U' 'P''O'

20 1 53 4 76 98

48

v 'Q''E' 'I' 'Y''R' 'T' 'W''U' 'P''O'

20 1 53 4 76 98




Example – iteration 2 Swaps third smallest element with v[2] This results in third smallest element being in the correct

place for a sorted result

v ‘O''E' 'I' 'Y''R' 'T' 'W''U' 'P'‘Q'

20 1 53 4 76 98

49




Example – iteration 3 Swaps fourth smallest element with v[3] This results in fourth smallest element being in the correct


v ‘O''E' 'I' 'Y''R' 'T' 'W''U' 'P'‘Q'

20 1 53 4 76 98

v ‘O''E' 'I' 'Y'‘P' 'T' 'W''U' ‘R'‘Q'

20 1 53 4 76 98

50




Example – iteration 4 Swaps fifth smallest element with v[4] This results in fifth smallest element being in the correct


v ‘O''E' 'I' 'Y'‘P' 'T' 'W''U' ‘R'‘Q'

20 1 53 4 76 98

v ‘O''E' 'I' 'Y'‘P' ‘Q' 'W''U' ‘R'‘T'

20 1 53 4 76 98

51

ArrayTools.java selection sortingpublic static void selectionSort(int[] v) {

for (int i = 0; i < v.length-1; ++i) {

// find the location of the ith smallest elementint spot = i;for (int j = i+1; j < v.length; ++j) { if (v[j] < v[spot]) { // is current location ok? // update spot to index of smaller element

spot = j; }}

// spot is now correct, so swap elementsint rmbr = v[i];v[i] = v[spot];v[spot] = rmbr;

}}

52

Iteration i

// find the location of the ith smallest element int spot = i;for (int j = i+1; j < v.length; ++j) {

if (v[j] < v[spot]) // is spot ok?// update spot with index of smaller elementspot = j;

}

// spot is now correct, swap elements v[spot] and v[i]

5353

From DubaiFrom Dubai

5454

Binary searchBinary search

55

Binary search

Given a list, find a specific element in the list List MUST be sorted!

Each time it iterates through, it cuts the search space in half

A binary search is MUCH faster than a sequential search

56

Binary search use The ‘BS’ in BSDemo is for Binary Search, mind you

public class BSDemo { public static void main(String[] args) {

int[] numbers = { 9, 3, 1, 8, 4, 6, 10, 2 };System.out.println ("The original list of numbers:");ArrayTools.putList(numbers);System.out.println();

ArrayTools.selectionSort(numbers);System.out.println ("The sorted list of numbers:");ArrayTools.putList(numbers);System.out.println();

System.out.println ("Searching for 0: " + ArrayTools.binarySearch(numbers, 0));System.out.println ("Searching for 1: " + ArrayTools.binarySearch(numbers, 1));System.out.println ("Searching for 4: " + ArrayTools.binarySearch(numbers, 4));System.out.println ("Searching for 5: " + ArrayTools.binarySearch(numbers, 5));System.out.println ("Searching for 6: " + ArrayTools.binarySearch(numbers, 6));System.out.println ("Searching for 10: " + ArrayTools.binarySearch(numbers, 10));System.out.println ("Searching for 11: " + ArrayTools.binarySearch(numbers, 11));

}}

5757

Binary search use demo…Binary search use demo…

BSDemo.javaBSDemo.java

5858

Chapter 2: Computer bugsChapter 2: Computer bugs

59

Binary searchpublic static int binarySearch (int[] data, int key) {

int i = 0; // left endpoint of search interval

int j = data.length-1; // right endpoint of search interval

while ( i < j ) { int m = (i+j)/2; if ( key > data[m] ) {

i = m+1; } else {

j = m; }}if ( key == data[i] ) { return i;} else { return -1;}

}

60

if ( key == data[i] ) {return i;

} else {return -1;

}


} else {return -1;

}

int i = 0;int j = data.length-1;int i = 0;int j = data.length-1;

while ( i < j ) {int m = (i+j)/2;if ( key > data[m] ) { i = m+1;} else { j = m;}

}


}

Binary search, take 1

2 4 6 8 10 12 14 16 18 20

a0 a1 a2 a3 a4 a5 a6 a7 a8 a9

i jm

public static int binarySearch (int[] data, int key) {

0

key 14

945 7 76 656

returns: 6

data

61

Binary search But what if the element is not in the list?

62


} else {return -1;

}


} else {return -1;

}

int i = 0;int j = data.length-1;int i = 0;int j = data.length-1;


}


}

Binary search, take 2

2 4 6 8 10 12 14 16 18 20

a0 a1 a2 a3 a4 a5 a6 a7 a8 a9

i jm

public static int binarySearch (int[] data, int key) {

0

key 15

945 7 76

returns: -1

data

7

63

Binary search A somewhat alternative view of what a binary search does…

64

How long does a binary search take? Given a array of 64 elements

1st iteration cuts the array to 32 2nd iteration cuts the array to 16 3rd to 8 4th to 4 5th to 2 6th to 1

Given a array of 1024 elements 1st iteration cuts the array to 512 ... 10th iteration cuts the list to 1 element

Thus, the binary search takes log2 n iterations! Where n is the size of the array

65

Binary search vs. sequential search Assume the array has n elements

Sequential search takes n iterations to find the element

Binary search takes log2 n iterations to find the element

Consider a list of 1 million elements Binary search takes about 20 iterations Sequential search takes 1,000,000 iterations

Consider a list of 1 trillion elements Binary search takes about 40 iterations Sequential search takes 1,000,000,000,000 iterations

6767

Multi-dimensional arraysMulti-dimensional arrays

68

Multidimensional arrays Many problems require information be organized as a two-

dimensional or multidimensional list

Examples Matrices Graphical animation Economic forecast models Map representation Time studies of population change Microprocessor design

69

Example Segment

int[][] m = new int[3][];m[0] = new int[4];m[1] = new int[4];m[2] = new int[4];

Produces

When an array is created, each

value is initialized!

m

m[0] m[1] m[2]

0 0 0 0 0 0 0 0

00 0 0

m[2][0] m[2][1] m[2][2] m[2][3]

m[0][0] m[0][1] m[0][2] m[0][3] m[1][0] m[1][1] m[1][2] m[1][3]

m

70

Example Alternative

int[][] m = new int[3][4];

Produces

m

m[0] m[1] m[2]

0 0 0 0 0 0 0 0

00 0 0

m[2][0] m[2][1] m[2][2] m[2][3]

m[0][0] m[0][1] m[0][2] m[0][3] m[1][0] m[1][1] m[1][2] m[1][3]

71

Multidimensional array visualization A multi-dimensional array declaration (either one):

int[][] m = new int[3][4];

How we visualize it:

0 0 0

0 0 0

0 0 0

0 0 0

0

0

0

0

0

0

0

0

0

0

0

0

or

72

Example Segment

for (int c = 0; c < m.length; ++c) {for (int r = 0; r < m[c].length; ++r) {

System.out.print("Enter a value: "); m[c][r] = stdin.nextInt();

}}

0

0

0

0

0

0

0

0

0

0

0

0

73

Rows by columns or columns by rows? Consider int[][] m = new int[3][4];

Is that 3 rows by 4 columns or 3 columns by 4 rows?

The answer is that it can be either As long as you are consistent with your column/row

placement

0 0 0

0 0 0

0 0 0

0 0 0

or

0 0 0

0 0 0

0 0 0 0

0

0

74

Rows by columns or columns by rows? This makes it 3 columns by 4 rows:

for (int c = 0; c < m.length; ++c)for (int r = 0; r < m[c].length; ++r) {

System.out.print("Enter a value: "); m[c][r] = stdin.nextInt();

}

This makes it 3 rows by 4 columns:

for (int c = 0; c < m.length; ++c)for (int r = 0; r < m[c].length; ++r) {

System.out.print("Enter a value: "); m[r][c] = stdin.nextInt();

}

75

Example Segment

String[][] s = new String[4][];s[0] = new String[2];s[1] = new String[2];s[2] = new String[4];s[3] = new String[3];

Produces

s

s[0] s[1] s[2]

null null

null null

null null null

s[3][0] s[3][1] s[3][2]

s[1][0] s[1][1]

s[0][0] s[0][1]

null null null null

s[2][0] s[2][1] s[2][2] s[2][3]

s[3]

76

Multidimensional array visualization Segment

String[][] s = new String[4][];s[0] = new String[2];s[1] = new String[2];s[2] = new String[4];s[3] = new String[3];

Produces

Called a “ragged” array

0 0 0

0 0 0

0

0

0

0

0

0

0

0

0

0

0

0

0

or

0

0

0

77

Explicit Initialization Segment

int c[][] = {{1, 2}, {3, 4}, {5, 6}, {7, 8, 9}};

Produces

c

c[0] c[1] c[2]

1 2

3 4

7 8 9

c[3][0] c[3][1] c[3][2]

c[1][0] c[1][1]

c[0][0] c[0][1]

5 6

c[2][0] c[2][1]

c[3]

78

Matrices A two-dimensional array is sometimes known as a matrix

because it resembles that mathematical concept

A matrix a with m rows and n columns is represented mathematically in the following manner

a1 1 a 1 2 a 1 n

a2 1 a 2 2 a 2 n

am 1 a m 2 a m n

79

Matrix addition Definition C = A + B

cij = aij + bij

cij is sum of the elements in the same row and column of A and B

80

Matrix additionpublic static double[][] add(double[][] a, double[][] b) {

// determine number of rows in solutionint m = a.length;

// determine number of columns in solutionint n = a[0].length;

// create the array to hold the sumdouble[][] c = new double[m][n];

// compute the matrix sum row by rowfor (int i = 0; i < m; ++i) {

// produce the current rowfor (int j = 0; j < n; ++j) {

c[i][j] = a[i][j] + b[i][j];}

}

return c;}

81

Homework J8 You will be creating a Universe class

The Universe class contains a 2-D array In each spot will be a Location object

(from a previous HW) Lab 10 is going to be a Map class

Will print out the 2-D Universe via text

8282

Alien SongAlien Song

AlienSong.mpgAlienSong.mpg

8383

Vector classVector class

This is also the review This is also the review

for the third midtermfor the third midterm

84

Limitations of arrays You can’t change their size once created

This can be a big problem!

So we will create a new class that will operate like an array: We can store and get elements by index number It will automatically increase in size as needed And other fancy features…

Let’s call the class Vector As we are basically writing the java.util.Vector class

85

Properties of our Vector class

It needs to have an array to hold the values

As our internal array will often be bigger than the number of elements in the Vector, we need a size as well More on what this means in a slide or two…

Not much else…

86

Methods in our Vector class Insert and remove elements into the Vector Get an element from the Vector Find the length Print it out to the screen

What happens when the array field is full, and we want to add an element? We will need to increase the size of the array So we need a method to do that as well

87

Our first take on our Vector classpublic class Vector { private Object array[]; private int size = 0;

Vector() { array = new Object[100]; }

Vector(int length) { array = new Object[length]; }}

What does this mean? We’ll see that a bit later… But briefly, it means the array can store any object

88

Adding an element to our Vector

public void add (Object o) {array[size++] = o;

}

Pretty easy!

But what if the array is full?

We need a way to increase the capacity of the array

89

Increasing the Vector’s array’s capacity

private void increaseCapacity() {int oldSize = array.length;Object newArray[] = new Object[2*oldSize];for ( int i = 0; i < oldSize; i++ )

newArray[i] = array[i];array = newArray;

}

And our new add() method:

public void add (Object o) {if ( size == array.length )

increaseCapacity();array[size++] = o;

}

90

Methods can be private as well

Notice that the increaseCapacity() method is called only by the add() method when necessary

It’s not ever going to be called by whomever is using our Vector

Thus, we will make it private

That means that only other Vector methods can call it

91

Removing an element from a Vector

public Object remove (int which) {Object ret = array[which];for ( int i = which; i < array.length-1; i+

+ )array[i] = array[i+1];

array[array.length-1] = null;size--;return ret;

}

92

Miscellaneous other methods

public int size() {return size;

}

public Object get (int which) {return array[which];

}

93

Our toString() method

public String toString() {String ret = "[";for ( int i = 0; i < size; i++ ) {

ret += array[i];if ( i != size-1 )

ret += ", ";}ret += "]";return ret;

}

94

Using our Vector This code is in a separate class called VectorUsage

public static void main (String[] args) {Vector v = new Vector();for ( int i = 12; i < 30; i++ ) {

v.add (String.valueOf(i));}System.out.println (v);System.out.println (v.size());String s = (String) v.get(5);System.out.println (s);v.remove (5);System.out.println (v);v.remove (5);System.out.println (v);

}

9595


VectorUsage.javaVectorUsage.java

96

The “real” Vector class Java provides a Vector class

In java.util

It contains all of the methods shown

9797


VectorUsage.javaVectorUsage.java But using java.util.VectorBut using java.util.Vector

98

What about those errors? When compiled with java.util.Vector, we see:

Note: C:\...\VectorUsage.java uses unchecked or unsafe operations.

Note: Recompile with -Xlint:unchecked for details.

You can ignore these They deal with generics, which you will see in future

courses The program was still compiled

99

More on using the Vector class To add a Location object l to the end of a Vector v

v.add(l);

To get the Location object at the end of the Vector v Location l = (Location) v.get(v.size()-1);

To remove a Location object from the end of a Vector v Location l = (Location) v.remove(v.size()-1); This both removes the object from the Vector and stores

the removed value into l

100100

Very unofficial demotivatorsVery unofficial demotivators

102102

Wrapper classesWrapper classes

103

But what about adding variables? The add method takes an Object as a parameter

public void add (Object o) { Although we haven’t seen it yet, this means you can add

any object you want to the vector

Primitive types (i.e. variables) are not objects How can they be added?

The solution: wrapper classes!

104

The Integer wrapper class This is how you add an int variable to a Vector:

int x = 5;Integer i = new Integer(x);vector.add (i);//…Integer j = (Integer) v.get(0);int y = j.intValue();

Pretty annoying syntax – we’ll see how to get around it in a bit…

105

More on wrapper classes

All the primitive types have wrapper classes Usually, the names are just the capitalized version of the

type I.e. Double for double, Byte for byte, etc.

Two exceptions: int and char int has Integer char has Character

106

More on wrapper classes Consider this code:

int x = 5;vector.add (x);//…int y = vector.get(0);

Does this code work? It shouldn’t

As we are adding a variable (not an object) to a vector But it does work!

Why?

107

Auto-boxing Java 1.5 will automatically “wrap” a primitive value into it’s

wrapper class when needed And automatically “unwrap” a wrapper object into the

primitive value

So Java translates the previous code into the following:

int x = 5;vector.add (new Integer(x));//…int y = ((Integer)vector.get(0)).intValue();

This is called autoboxing And auto-unboxing (unauto-boxing?) This does not work in Java 1.4 or before

108

More on auto-boxing Consider the following code:

Double d = 7.5;Double e = 6.5;Double f = d + e;System.println (f);

This is doing a lot of auto-boxing (and auto-unboxing):

Double d = new Double(7.5);Double e = new Double(6.5);Double f = newDouble(d.doubleValue() +

e.doubleValue());System.println (f);

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A solution to commenting your A solution to commenting your codecode

The commentator: The commentator: http://www.cenqua.com/commentatohttp://www.cenqua.com/commentator/r/

1 arrays chapter 8 spring 2006 cs 101 aaron bloomfield

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