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First Data Structure Definition
A data structure is a data type whose
components are smaller data structures
and/or simple data types.
Data Structure Starting Point
Any data type that can store more than
one value is a data structure.
First Array Definition
An array is a data structure with one, or
more, elements of the same type.
A one-dimensional
array is frequently
also called a vector.
A two-dimensional
array is frequently
also called a matrix.
First Record Definition
A record is a data structure with one,
or more, elements, called fields, of the
same or different data types.
File Definition
A file is an internal data structure - with an
unspecified number of elements of the same
type - assigned to an external file name.
The file data structure
allows transfer of data
between internal and
external storage.
Stack Definition
A stack is a data structure
with elements of the same
type.
Data elements of the stack
data structure can only be
accessed (stored or
retrieved) at one end of the
stack in a LIFO (Last In,
First Out) manner.
Improved Data Structure Definition
A data structure is a data type whose
components are smaller data structures
and/or simple data types.
The storing and retrieval of the data elements
is performed by accessing methods that
characterize the data structure.
Improved Array Definition
An array is a data structure with a fixed
number of elements of the same type.
Every element of the array can be
accessed directly.
What is an Array
• An array is a block of consecutive memory
locations that hold values of the same data
type.
• Individual locations are called array’s
elements.
• When we say “element” we often mean
the value stored in that element.
1.39
1.69
1.74
0.0
An array of
doubles
What is an Array (cont’d)
• Rather than treating each element as a
separate named variable, the whole array
gets one name.
• Specific array elements are referred to by
using array’s name and the element’s
number, called index or subscript.
1.39
1.69
1.74
0.0
c[0]
c[1]
c[2]
c[3]
c is array’s
name
Array Example
[16]
Ingrid
[17]
Darlene
[18]
Gene
[19]
Sean
[20]
Stephanie
[11]
Holly
[12]
Blake
[13]
Michelle
[14]
Remy
[15]
Haley
[06]
Diana
[07]
Jessica
[08]
David
[09]
Anthony
[10]
Alec
[01]
Isolde
[02]
John
[03]
Greg
[04]
Maria
[05]
Heidi
Defining Static Arrays
int list[ ];
list = new int[10];
// declares the array list identifier
// allocates memory for 10 integers
char names[ ];
names = new char[25];
// declares the names array identifier
// allocates memory for 25 characters
double grades[ ];
// declares the grades array identifier
grades = new double[50]; // allocates memory for 50 doubles
Defining Static Arrays
Preferred Method
int list[ ] = new int[10];
char names[ ] = new char[25];
double grades[ ] = new double[50];
// Java1201.java
// This program demonstrates how to declare an array of integers.
// Note how each element of the array defaults to zero.
// Java allows the display of uninitialized objects because object
// elements get assigned values from the default constructor.
public class Java1201
{
public static void main(String args[])
{
int list[];
// declares the array object identifier
list = new int[10]; // allocates memory for 10 array elements
for (int k = 0; k < 10; k++)
System.out.println("list[" + k + "] = " + list[k]);
System.out.println();
0
1
2
3
4
5
6
7
8
9
}
0 0 0 0 0 0 0 0 0 0
}
Array Index Note
Java arrays indicate individual elements with
an index inside two brackets, following the
array identifier, like list[k]
The array index is always an integer and
starts at 0.
In an array of N elements, the largest index is
N-1.
Indices (Subscripts)
• In Java, an index is written within square
brackets following array’s name (for
example, a[k]).
• Indices start from 0; the first element of
an array a is referred to as a[0] and the
n-th element as a[n-1].
• An index can have any int value from 0
to array’s length - 1.
Indices (cont’d)
• We can use as an index an int variable or
any expression that evaluates to an int
value. For example:
a [3]
a [k]
a [k - 2]
a [ (int) (6 * Math.random()) ]
Indices (cont’d)
• In Java, an array is declared with
fixed length that cannot be changed.
• Java interpreter checks the values of
indices at run time and throws
ArrayIndexOutOfBoundsException if
an index is negative or if it is greater
than the length of the array - 1.
// Java1202.java
// This program is almost identical to Java1201. The difference
// is that the array declaration and array definition to allocate
// memory is done in one program statement.
public class Java1202
{
public static void main(String args[])
{
int list[] = new int[10];
// combined array declaration and definition
for (int k = 0; k < 10; k++)
System.out.println("list[" + k + "] = " + list[k]);
System.out.println();
}
}
0
1
2
3
4
5
6
7
8
9
0
0
0
0
0
0
0
0
0
0
// Java1203.java
// This program demonstrates how to initialize array elements.
// The <new> operator is not necessary in this case.
public class Java1203
{
public static void main(String args[])
{
int list[] = {11,22,33,44,55,66,77,88,99};
for (int k = 0; k < 9; k++)
System.out.println("list[" + k + "] = " + list[k]);
System.out.println();
}
}
0
1
2
3
4
5
6
7
8
11 22 33 44 55 66 77 88 99
// Java1204.java
// This program demonstrates how to declare an array of characters.
// Are the array elements initialized to any kind of value?
public class Java1204
{
public static void main(String args[])
{
int k;
char list1[] = new char[5];
for (k = 0; k < 5; k++)
System.out.println("list1[" + k + "] = " + list1[k]);
System.out.println();
char list2[] = {'c','h','a','r'};
for (k = 0; k < 4; k++)
System.out.println("list2[" + k + "] = " + list2[k]);
System.out.println();
0 1 2 3 4
0 1 2 3
list1
list2
}
? ? ? ? ?
c h a r
}
// Java1205.java
// The purpose of this program is to investigate the type of character
// that is used to initialize a character array.
// Is it no-character-at-all or a blank space?
public class Java1205
{
public static void main(String args[])
{
char List1[] = new char[10];
System.out.print("Start");
for (int K = 0; K < 10; K++)
System.out.print(List1[K]);
System.out.println("End");
System.out.println();
} 1
0
2
3
4
5
6
7
8
9
}<sp> <sp> <sp> <sp> <sp> <sp> <sp> <sp> <sp> <sp>
// Java1206.java
// This program demonstrates how String objects are initialized,
// both without and with specified array values.
public class Java1206
{
public static void main(String args[])
{
String list1[] = new String[5];
for (int k = 0; k < 5; k++)
System.out.println("list[" + k + "] = " + list1[k]);
System.out.println();
String list2[] = {"AAA","BBB","CCC","DDD","EEE"};
for (int k = 0; k < 5; k++)
System.out.println("list2[" + k + "] = " + list2[k]);
0
1
2
3
4
0 System.out.println();
1
2
3
4
list1
} null null null null null list2 AAA BBB CCC DDD EEE
}
// Java1207.java
// This program fills an integer array with a random set of numbers.
import java.util.Random;
public class Java1207
{
public static void main(String args[])
{
int list[] = new int[20];
Random random = new Random(12345);
for (int k = 0; k < 20; k++)
list[k] = random.nextInt(900) + 100;
for (int k = 0; k < 20; k++)
System.out.println("list[" + k + "] = " + list[k]);
System.out.println();
0
1}
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
} 680 241 928 455 284 575 802 701 889 717 142 890 206 312 584 687 803
851
18
19
432 775
// Java1208.java
// This program introduces the length field to determine the
// number of elements in the array. Remove the comments from line
// 16 to observe what happens when the length field is altered.
public class Java1208
{
public static void main(String args[])
{
String names[] = {"Joe","Tom","Sue","Meg"};
int n = names.length;
// data field access; not a method call
System.out.println("There are " + n + " array elements.");
for(int k = 0; k < n; k++)
System.out.println("names[" + k + "] = " + names[k]);
// names.length = 10; // Line 16
System.out.println();
}
}
0
1
2
3
Joe
Tom
Sue
Meg
// Java1209.java
// This program demonstrates how to create an array of random strings.
import java.util.Random;
public class Java1209
{
public static void main(String args[])
{
Random random = new Random(12345);
int rndInt;
String names[] = {"AA","BB","CC","DD","EE","FF","GG","HH","II","JJ"};
for(int k = 1; k < 15; k++)
{
rndInt = random.nextInt(names.length);
System.out.println("names[" + rndInt + "] = " + names[rndInt]);
}
System.out.println();
0
}AA
}
1
2
3
4
5
6
7
8
9
BB
CC
DD
EE
FF
GG
HH
II
JJ
Why Do We Need Arrays?
• The power of arrays comes from the fact
that the value of a subscript can be
computed and updated at run time.
No arrays:
1000
times!
int sum = 0;
sum += score0;
sum += score1;
…
sum += score999;
With arrays:
int n = 1000;
int sum = 0, k;
for (k = 0; k < n; k++)
sum += scores[k];
Why Arrays? (cont’d)
• Arrays give direct access to any element
— no need to scan the array.
1000
times!
No arrays:
With arrays:
if (k == 0)
display (score0);
else if (k == 1)
display (score1);
else
… // etc.
display (scores[k]);
Arrays as Objects
• In Java, an array is an object. If the type
of its elements is anyType, the type of the
array object is anyType[ ].
• Array declaration:
anyType [ ] arrName;
Arrays as Objects (cont’d)
• As with other objects, the declaration
creates only a reference, initially set to
null. An array must be created before it
can be used.
• OnearrName
way to=create
an array:
new anyType
[length] ;
Brackets,
not parens!
Declaration and Initialization
• When an array is created, space is
allocated to hold its elements. If a list of
values is not given, the elements get the
length 10,
default values. For example:
scores = new int [10] ;
words = new String [10000];
all values
set to 0
length 10000,
all values set to
null
Initialization (cont’d)
• An array can be declared an initialized in
one statement. For example:
int [ ] scores = new int [10] ;
private double [ ] gasPrices = { 3.05, 3.17, 3.59 };
String [ ] words = new String [10000];
String [ ] cities = {"Atlanta", "Boston", "Cincinnati" };
Initialization (cont’d)
• Otherwise, initialization can be postponed
until later. For example:
String [ ] words;
Not yet
initialized
...
words = new String [ console.readInt() ];
Not yet
private double[ ] gasPrices;
initialized
…
gasPrices = new double[ ] { 3.05, 3.17, 3.59 };
Array’s Length
• The length of an array is determined when
that array is created.
• The length is either given explicitly or
comes from the length of the {…}
initialization list.
• The length of an array arrName is referred
to in the code as arrName.length.
• length is like a public field (not a method)
in an array object.
Initializing Elements
• Unless specific values are given in a {…}
list, all the elements are initialized to the
default value: 0 for numbers, false for
booleans, null for objects.
• If its elements are objects, the array holds
references to objects, which are initially
set to null.
• Each object-type element must be
initialized before it is used.
Initializing Elements (cont’d)
• Example:
Array not
created yet
Color[ ] pens;
Array is created;
...
all three elements
pens = new Color [ 3 ];
are set to null
...
pens [0] = Color.BLUE;
Now all three
pens [1] = new Color (15, 255, 255);
elements are
pens [2] = g.getColor();
initialized
Passing Arrays to Methods
• As other objects, an array is passed to a
method as a reference.
• The elements of the original array are not
copied and are accessible in the method’s
code.
// Swaps a [ i ] and a [ j ]
public void swap (int [ ] a, int i, int j)
{
int temp = a [ i ];
a [ i ] = a [ j ];
a [ j ] = temp;
}
Returning Arrays from Methods
• As any object, an array can be returned
from a method.
• The returned array is usually constructed
within the method or obtained from calls to
other methods.
• The return type of a method that returns
an array with someType elements is
designated as someType[ ].
Returning Arrays from Methods
(cont’d)
public double[ ] solveQuadratic
(double a, double b, double c)
{
double d = b * b - 4 * a * c;
if (d < 0) return null;
d = Math.sqrt(d);
double[ ] roots = new double[2];
roots[0] = (-b - d) / (2*a);
roots[1] = (-b + d) / (2*a);
return roots;
}
Or simply:
return new double[ ]
{ (-b - d) / (2*a),
(-b + d) / (2*a) };
AP Exam Alert
One-dimensional arrays will be tested
for the "A-level" and "AB-level" APCS
examination.
Two-dimensional
arrays will only be
tested for the "AB-level"
APCS examination.
java.util.ArrayList<E>
• Implements a list using an array.
• Can only hold objects (of a specified type),
not elements of primitive data types.
• Keeps track of the list capacity (the length
of the allocated array) and list size (the
number of elementscapacity
currently in the list)
size
"Cat"
"Hat"
"Bat"
...
ArrayList  Generics
• Starting with Java 5, ArrayList and other
collection classes hold objects of a
specified data type.
• The elements’ data type is shown in angle
brackets and becomes part of the
ArrayList type. For example:
ArrayList<String> words = new ArrayList<String>();
ArrayList<Integer> nums = new ArrayList<Integer>();
ArrayList<E> Constructors
Java docs use the letter E as
the type parameter for elements
in generic collections
ArrayList<E> ( )
ArrayList<E> (int capacity)
Creates an empty
ArrayList<E> of the
specified capacity
Creates an empty
ArrayList<E> of
default capacity (ten)
// Java2004.java
// This program introduces the <ArrayList> class with the <add> method
// to add additional elements to the end of the list.
// Note, that it is possible to display ArrayList elements directly.
import java.util.ArrayList;
public class Java2004
{
public static void main(String args[])
{
System.out.println();
System.out.println("Java2004.java\n");
ArrayList names = new ArrayList();
names.add("Isolde");
names.add("John");
names.add("Greg");
names.add("Maria");
names.add(new String("Heidi"));
System.out.println("names contains " + names);
System.out.println();
}
}
ArrayList Access
ArrayList objects cannot be accessed with an
index [ ] operator, like a Java static array.
All access is performed with ArrayList methods.
ACCESS DENIED!
ArrayList<E> Methods
(a Subset)
int size()
boolean isEmpty ()
boolean add (E obj)
returns true
void add (int i, E obj)
inserts obj as the
i-th value; i must
be from 0 to size()
E set(int i, E obj)
E get(int i)
E remove(int i)
boolean contains(E obj)
int indexOf(E obj)
i must be from 0 to
size() -1
use equals to
compare objects
ArrayList Example
ArrayList<String> names =
new ArrayList<String>( );
names.add("Ben");
names.add("Cat");
names.add(0, "Amy");
System.out.println(names);
Output
[Amy, Ben, Cat]
ArrayList’s toString
method returns a string of
all the elements, separated
by commas, within [ ].
ArrayList Method add
names.add("Tom");
The add method allocates space for the newly
enlarged array and then stores the new array
element at the end of the ArrayList object.
Displaying ArrayList Elements
ArrayList elements can be accessed with
various methods.
It is possible to display all the elements inside
square brackets, separated by commas by using
the print method.
System.out.println(names);
[Isolde, John, Greg, Maria, Heidi]
// Java2005.java
// This program uses the <size>method to determine the number of elements
// in an <ArrayList object.
import java.util.ArrayList;
public class Java2005
{
public static void main(String args[])
{
System.out.println();
System.out.println("Java2005.java\n");
ArrayList names = new ArrayList();
names.add("Isolde");
names.add("John");
names.add("Greg");
names.add("Maria");
names.add(new String("Heidi"));
System.out.println("names contains " + names);
System.out.println();
System.out.println("There are " + names.size() + " elements in the names object.");
System.out.println();
}
}
ArrayList Method size
int count = names.size();
The size method returns the number of
elements of the ArrayList object names.
Remember Strings and Java static
arrays use length instead of size.
// Java2006.java
// This program shows how to access specified elements in an <ArrayList> object
// with the <get> method.
import java.util.ArrayList;
public class Java2006
{
public static void main(String args[])
{
System.out.println();
System.out.println("Java2006.java\n");
ArrayList names = new ArrayList();
names.add("Isolde");
names.add("John");
names.add("Greg");
names.add("Maria");
names.add(new String("Heidi"));
System.out.println();
for (int k = 0; k < names.size(); k++)
System.out.println(names.get(k));
System.out.println();
for (int k = names.size()-1; k >= 0; k--)
System.out.println(names.get(k));
}
}
ArrayList Method get
System.out.println(names.get(3));
The get method accesses a specified array
element.
The parameter of the get method is the index
of the ArrayList object and starts at 0.
Any attempt to access an element at a
non-existing index results in an
IndexOutOfBoundsException error.
// Java2007.java
// This program demonstrates the <set> method of the <ArrayList> class, which
// replaces existing elements with a new object.
import java.util.ArrayList;
public class Java2007
{
public static void main(String args[])
{
System.out.println("Java2007.java\n");
ArrayList names = new ArrayList();
names.add("Isolde");
names.add("John");
names.add("Greg");
names.add("Maria");
names.add("Heidi");
System.out.println("names contains " + names);
System.out.println();
names.set(1,"Jessica");
names.set(2,"Anthony");
names.set(3,"Haley");
names.set(4,"Alec");
System.out.println("names contains " + names);
}
}
ArrayList Method set
names.set(4,"Tom");
The set method uses the first parameter as the
index location to find an array element and then
replaces it with the value of the second set
parameter.
You will get an error if you try to access an index
location, which has not been allocated yet.
// Java2008.java
// This program demonstrates the <remove> method of the <ArrayList> class to
// delete a specified list element.
import java.util.ArrayList;
public class Java2008
{
public static void main(String args[])
{
System.out.println();
System.out.println("Java2008.java\n");
ArrayList names = new ArrayList();
names.add("Isolde");
names.add("John");
names.add("Greg");
names.add("Maria");
names.add("Heidi");
System.out.println("names contains " + names);
System.out.println();
names.remove(2);
System.out.println("names contains " + names);
System.out.println();
names.remove(3);
System.out.println("names contains " + names);
System.out.println();
}
}
ArrayList Method remove
names.remove(3);
The remove method removes the array element
at the index location of its parameter and
decreases the object size by one array element.
You will get an error if you try to access an index
location, which does not exist.
// Java2009.java
// This program demonstrates how to use the <add> method of the <ArrayList> class to
// insert new elements at a specified location.
import java.util.ArrayList;
public class Java2009
{
public static void main(String args[])
{
System.out.println("Java2009.java\n");
ArrayList names = new ArrayList();
names.add("Isolde");
names.add("John");
names.add("Greg");
names.add("Maria");
names.add("Heidi");
System.out.println("names contains " + names);
System.out.println();
names.add(2,"Jessica");
System.out.println("names contains " + names);
System.out.println();
names.add(3,"Anthony");
System.out.println("names contains " + names);
}
}
ArrayList Method add
nd
(2 Overloaded method)
names.add(3,"Kathy");
The overloaded add(3,"Kathy") method adds or
rather inserts a new array element at the
indicated index.
As always, you will get an error if you try to
access an index location, which does not exist.
// Java2010.java
// This program demonstrates how to use the <clear> method to remove all the
// array elements and the <isEmpty> method to check if emptiness is true.
import java.util.ArrayList;
public class Java2010
{
public static void main(String args[])
{
System.out.println();
System.out.println("Java2010.java\n");
ArrayList names = new ArrayList();
names.add("Isolde");
names.add("John");
names.add("Greg");
names.add("Maria");
names.add("Heidi");
System.out.println("names contains " + names);
System.out.println("Empty list is " + names.isEmpty());
names.clear();
System.out.println();
System.out.println("names contains " + names);
System.out.println("Empty list is " + names.isEmpty());
System.out.println();
}
}
ArrayList<E> Details
• Automatically increases (doubles) the capacity
when the list runs out of space (allocates a
bigger array and copies all the values into it).
• get(i) and set(i, obj) are efficient because an
array provides random access to its elements.
• Throws IndexOutOfBoundsException when
i < 0 or i  size()
(or i > size() in add (i, obj) )
ArrayList<E> Autoboxing
• If you need to put ints or doubles into a
list, use a standard Java array or
convert them into Integer or Double
objects
• In Java 5, conversion from int to Integer
and from double to Double is, in most
cases, automatic (a feature known as
autoboxing or autowrapping); the
reverse conversion (called
autounboxing) is also automatic.
ArrayList<E> Autoboxing
Example
ArrayList<Integer> counts =
new ArrayList<Integer>( );
counts.add(17);
...
int count = counts.get(0);
Autounboxing: count
gets the value 17
Autoboxing: compiled as
counts.add(new Integer(17));
ArrayList Pitfalls
// Remove all occurences
// of "like" from words:
int i = 0;
Caution: when you remove
elements, a simple for loop
doesn’t work:
while (i < words.size())
for (int i = 0; i < words.size();
{
i++)
if ("like".equals(words.get(i))
{
words.remove(i);
if ("like".equals(words.get(i))
else
words.remove(i);
i++;
}
}
Shifts all the elements
after the i-th to the left
and decrements the
size
“For Each” Loop
• Introduced in Java 5
• Works both with standard arrays and
ArrayLists
• Convenient for traversing
• Replaces iterators for collections
(Chapter 19)
“For Each” Loop: Example 1
int [ ] scores = { ... };
...
int sum = 0;
for (int s : scores)
{
sum += s;
Basically the same as:
}
...
for (int i = 0;
i < scores.length; i++)
{
int s = scores[i];
sum += s;
}
“For Each” Loop: Example 2
ArrayList<String> words = new ArrayList<String>();
...
for (String str : words)
{
System.out.println(str); // process str
}
Basically the same as:
for (int i = 0;
i < words.size(); i++)
{
String str = words.get(i);
System.out.println(str);
}
“For Each” Loop (cont’d)
• You cannot add or remove elements within
a “for each” loop.
• You cannot change elements of primitive
data types or references to objects within
a “for each” loop.
// Java1213.java
// This program introduces the Java5.0 enhanced <for> loop
// with an <int> array.
public class Java1213
{
public static void main(String args[])
{
System.out.println("Java1213\n");
int list[] = {11,22,33,44,55,66,77,88,99};
for (int k = 0; k < 9; k++)
// Old <for> loop syntax
System.out.print(list[k] + " ");
System.out.println("\n\n");
for (int item: list)
// New <for> loop syntax
System.out.print(item + " ");
System.out.println("\n\n");
}
}
// Java1214.java
// This program uses the Java 5.0 <for> loop with a <String> array.
public class Java1214
{
public static void main(String args[])
{
System.out.println("Java1214\n");
String names[] = {"Tom","Sue","Joe","Jan","Bob","Lee","Ann","Meg"};
for (int k = 0; k < 8; k++)
System.out.print(names[k] + " ");
System.out.println("\n\n");
// Old <for> loop syntax
for (String name: names)
// New <for> loop syntax
System.out.print(name + " ");
System.out.println("\n\n");
}
}
// Java1215.java
// This program demonstrates a very generalized <for..each> loop usage
// with the <Object> class.
public class Java1215
{
public static void main(String args[])
{
System.out.println("Java1215\n");
String names[] = {"Tom","Sue","Joe","Jan","Bob","Lee","Ann","Meg"};
for (int k = 0; k < 8; k++)
System.out.print(names[k] + " ");
System.out.println("\n\n");
for (Object obj: names)
System.out.print(obj + " ");
System.out.println("\n\n");
}
}
Enhancing the for Loop
The enhanced for loop is called the for .. each loop.
This loop structure is available in Java 5.0
The new loop structure does not replace the older for
loop, because it is not possible to access specific array
elements.
int numbers[ ] = {1,2,3,4,5,6,7,8,9};
for (int number: numbers)
System.out.print(number + "
");
Inserting a Value into
a Sorted Array
• Given: an array, sorted in ascending order.
The number of values stored in the array
is smaller than array’s length: there are
some unused elements at the end.
• Task: insert a value while preserving the
order.
Inserting a Value (cont’d)
1. Find the right place to insert:
5
1 1 2 3 8 13
2. Shift elements to the right,
starting from the5 last one:
1 1 2 3 8
13
3. Insert
1 1the 2value
3 in5its proper
8 13 place:
Can be combined
together in one
loop: look for the
place to insert
while shifting.
Inserting a Value (cont’d)
// Returns true if inserted successfully, false otherwise
public boolean insert(double[ ] arr, int count, double value)
{
if (count >= arr.length)
return false;
int k = count - 1;
while ( k >= 0 && arr [ k ] > value )
{
arr [ k + 1 ] = arr [ k ];
k--;
}
arr [ k + 1] = value;
return true;
}
Two-Dimensional Arrays
• 2-D arrays are used to represent tables,
matrices, game boards, images, etc.
• An element of a 2-D array is addressed
using a pair of indices, “row” and “column.”
For example:
board [ r ] [ c ] = 'x';
2-D Arrays: Declaration
// 2-D array of char with 5 rows, 7 cols:
char[ ] [ ] letterGrid = new char [5][7];
// 2-D array of Color with 1024 rows, 768 cols:
Color[ ] [ ] image = new Color [1024][768];
// 2-D array of double with 2 rows and 3 cols:
double [ ] [ ] sample =
{ { 0.0, 0.1, 0.2 },
{ 1.0, 1.1, 1.2 } };
2-D Arrays: Dimensions
• In Java, a 2-D array is basically a 1-D
array of 1-D arrays, its rows. Each row is
stored in a separate block of consecutive
memory locations.
• If m is a 2-D array, then m[k] is a 1-D
array, the k-th row.
• m.length is the number of rows.
• m[k].length is the length of the k-th row.
Dimensions (cont’d)
• Java allows “ragged” arrays, in which
different rows have different lengths.
• In a rectangular array, m[0].length can be
used to represent the number of columns.
“Ragged” array:
Rectangular array:
m.length
m[3].length
m.length
m[0].length
2-D Arrays and Nested Loops
• A 2-D array can be traversed using
nested loops:
for (int r = 0; r < m.length; r++)
{
for (int c = 0; c < m[0].length; c++)
{
... // process m[ r ][ c ]
}
}
“Triangular” Loops
• “Transpose a matrix” idiom:
int n = m.length;
for (int r = 1; r < n; r++)
{
for (int c = 0; c < r; c++)
{
double temp = m [ r ][ c ];
m [ r ][ c ] = m [ c ][ r ];
m [ c ][ r ] = temp;
}
}
The total number of
iterations through the
inner loop is:
1 + 2 + 3 + ... + n-1 =
n (n - 1) / 2
AP Exam Alert
The Arrays class is not part of the AP Java Subset
and will not be tested.
Students are expected to know how to use and
implement array algorithms, such as traversing,
element access, sorting and searching.
In this chapter these array algorithms are used with
the provided Arrays class methods.
In a future Algorithm chapter you will learn how to
implement these methods yourself.
// Java1216.java
// This program introduces the static <Arrays> class.
// In this program the <toString> method is used to display the array elements.
import java.util.Arrays;
// necessary to use the <Arrays> class
public class Java1216
{
public static void main (String args[])
{
System.out.println("Java1216\n");
int list1[] = {11,22,33,44,55,66,77,88,99};
double list2[] = {1.1,2.2,3.3,4.4,5.5,6.6,7.7,8.8,9.9};
char list3[] = {'A','B','C','D','E','F','G','H','I'};
String list4[] = {"AA","BB","CC","DD","EE","FF","GG","HH","II"};
System.out.println(Arrays.toString(list1));
System.out.println(Arrays.toString(list2));
System.out.println(Arrays.toString(list3));
System.out.println(Arrays.toString(list4));
System.out.println("\n\n");
}
}
Class Arrays
Method toString
Method toString displays the elements of
an array object separated by commas and
bounded by square brackets. [ ]
int list[] = {11,22,33,44,55,66,77,88,99};
System.out.println(Arrays.toString(list));
// Java1217.java
// This program demonstrates the <fill> method of the <Arrays> class.
// The <fill> method assigns the same value to every array element.
import java.util.Arrays;
// necessary to use the <Arrays> class
public class Java1217
{
public static void main (String args[])
{
System.out.println("Java1217\n");
int list1[] = new int[10];
double list2[] = new double[10];
char list3[] = new char[10];
String list4[] = new String[10];
Arrays.fill(list1,123);
Arrays.fill(list2,1.23);
Arrays.fill(list3,'Q');
Arrays.fill(list4,"USA");
System.out.println(Arrays.toString(list1));
System.out.println(Arrays.toString(list2));
System.out.println(Arrays.toString(list3));
System.out.println(Arrays.toString(list4));
System.out.println("\n\n");
}
}
Class Arrays
Method fill
Method fill assigns the same
value to every array element.
int list1[] = new int[10];
double list2[] = new double[10];
char list3[] = new char[10];
String list4[] = new String[10];
Arrays.fill(list1,123);
Arrays.fill(list2,1.23);
Arrays.fill(list3,'Q');
Arrays.fill(list4,"USA");
// Java1218.java
// This program demonstrates the <sort> method of the <Arrays> class.
// Method <sort> arranges array elements in ascending order.
import java.util.Arrays;
// necessary to use the <Arrays> class
public class Java1218
{
public static void main (String args[])
{
System.out.println("Java1218\n");
int list1[] = {11,99,22,88,33,77,44,66,55};
double list2[] = {1.1,2.2,3.3,4.4,5.5,6.6,7.7,8.8,9.9};
char list3[] = {'A','I','B','H','C','G','D','F','E'};
String list4[] = {"AA","II","BB","HH","CC","GG","DD","FF","EE"};
String list5[] =
{"aardvark","bobcat","cougar","dog","ELEFANT","FOX","GORILLA","HARE"};
Arrays.sort(list1);
Capital Letters have numeric
Arrays.sort(list2);
Arrays.sort(list3);
code values from 65-90.
Arrays.sort(list4);
Arrays.sort(list5);
System.out.println(Arrays.toString(list1));
System.out.println(Arrays.toString(list2));
System.out.println(Arrays.toString(list3));
System.out.println(Arrays.toString(list4));
System.out.println(Arrays.toString(list5));
System.out.println("\n\n");
}
}
Lowercase letters have numeric
code values from 97-112.
If lowercase letters are sorted
together with capital letters, the
capitals will come first because
of the smaller code values.
Class Arrays
Method sort
Method sort arranges the array elements in ascending
order. String and character array elements are sorted
in ascending order of the numerical code values.
Incorrect processing may occur if string values are
mixed upper-case and lower-case.
int list1[] = {11,99,22,88,33,77,44,66,55};
double list2[] = {1.1,2.2,3.3,4.4,5.5,6.6,7.7,8.8,9.9};
char list3[] = {'A','I','B','H','C','G','D','F','E'};
Arrays.sort(list1);
Arrays.sort(list2);
Arrays.sort(list3);
// Java1219.java
// This program demonstrates the <binarySearch> method of the <Arrays> class.
// Method <binarySearch> returns the index of a search element if it exists,
// and returns a negative index value otherwise.
import java.util.Arrays;
// necessary to use the <Arrays> class
public class Java1219
{
public static void main (String args[])
{
System.out.println("Java1219\n");
int list[] = {11,99,22,88,33,77,44,66,55};
System.out.println(Arrays.toString(list));
Arrays.sort(list);
System.out.println(Arrays.toString(list));
System.out.println();
System.out.println("Index of 33 is " + Arrays.binarySearch(list,33));
System.out.println("Index of 11 is " + Arrays.binarySearch(list,11));
System.out.println("Index of 99 is " + Arrays.binarySearch(list,99));
System.out.println("Index of 10 is " + Arrays.binarySearch(list,10));
System.out.println("\n\n");
}
}
// Java1220.java
// This program demonstrates that an array must be sorted before
// the <binarySearch> method is called.
// Erroneous indexes are returned if the list is
import java.util.Arrays;
not sorted!!!
// necessary to use the <Arrays> class
public class Java1220
{
public static void main (String args[])
{
System.out.println("Java1220\n");
int list[] = {11,99,22,88,33,77,44,66,55};
System.out.println(Arrays.toString(list));
System.out.println();
System.out.println("Index of 33 is " + Arrays.binarySearch(list,33));
System.out.println("Index of 11 is " + Arrays.binarySearch(list,11));
System.out.println("Index of 99 is " + Arrays.binarySearch(list,99));
System.out.println("Index of 10 is " + Arrays.binarySearch(list,10));
System.out.println("\n\n");
}
}
Class Arrays
Method binarySearch
Method binarySearch searches the elements of an array
object for a specified value.
The index of the array element is returned, if the element is
found and a negative index is returned otherwise.
Array elements must be sorted, otherwise the
binarySearch method returns incorrect information.
int list[] = {11,99,22,88,33,77,44,66,55};
System.out.println("Index of 33 is " + Arrays.binarySearch(list,33));
// Java1225.java
// STATIC IMPORTS #1
// This program introduces static imports.
// With Java 5.0 it is now possible to use class methods without the class identifier.
// In this example the <System> class name is not necessary.
import static java.lang.System.*; // necessary to eliminate the <System> identifier
public class Java1225
{
public static void main (String args[])
{
System.out.println("Java1225\n");
///// JAVA 1.4 /////
System.out.println("This demonstrates Java 1.4");
System.out.println();
System.out.println("The static class identifier <System> must be used.\n\n");
///// JAVA 5.0 /////
out.println("This demonstrates Java 5.0");
out.println();
out.println("It is not necessary to use the <System> class identifier.\n\n");
}
}
// Java1226.java
// STATIC IMPORTS #2
// This program introduces more static imports.
// In this example the <Math> and <System class names are not necessary.
import static java.lang.System.*;
// necessary to eliminate the <System> identifier
import static java.lang.Math.*; // necessary to eliminate the <Math> identifier
public class Java1226
{
public static void main (String args[])
{
System.out.println("Java1226\n");
///// JAVA 1.4 /////
System.out.println("Math class methods are a good example of working with static imports.");
System.out.println();
System.out.println("The square root of 100 is " + Math.sqrt(100));
System.out.println("13 to the 4th power is
" + Math.pow(13,4));
System.out.println("\n\n");
///// JAVA 5.0 /////
out.println("Math class methods are a good example of working with static imports.");
out.println();
out.println("The square root of 100 is " + sqrt(100));
out.println("13 to the 4th power is
" + pow(13,4));
out.println("\n\n");
}
}
// Java1227.java
// STATIC IMPORTS #3
// This program introduces more static imports.
// In this example the <Arrays> and <System> class names are "sometimes" not necessary.
import java.util.Arrays;
// necessary to use the <Arrays> class
import static java.lang.System.*;
// necessary to eliminate the <System> identifier
import static java.util.Arrays.*; // necessary to eliminate the <Arrays> identifier
public class Java1227
{
public static void main (String args[])
{
System.out.println("Java1227\n");
int list[] = {12,56,34,91,65,27,45,70,85};
///// JAVA 1.4 /////
System.out.println(Arrays.toString(list));
Arrays.sort(list);
System.out.println(Arrays.toString(list));
System.out.println("\n\n");
///// JAVA 5.0 /////
out.println(Arrays.toString(list));
sort(list);
out.println(Arrays.toString(list));
out.println("\n\n");
}
}
// this is intentional; check next program
// this is intentional; check next program
// Java1228.java
STATIC IMPORTS #4
// This program demonstrates a potential problem with static imports.
// The <toString> method without the <Arrays> identifier confuses the Java compiler.
// The <Object> class has a <toString> method, as do many other classes.
// This program will not compile.
import java.util.Arrays;
// necessary to use the <Arrays> class
import static java.lang.System.*;
// necessary to eliminate the <System> identifier
import static java.util.Arrays.*;
// necessary to eliminate the <Arrays> identifier
public class Java1228
{
public static void main (String args[])
{
System.out.println("Java1228\n");
int list[] = {12,56,34,91,65,27,45,70,85};
///// JAVA 1.4 /////
System.out.println(Arrays.toString(list));
Arrays.sort(list);
System.out.println(Arrays.toString(list));
System.out.println("\n\n");
///// JAVA 5.0 /////
out.println(toString(list));
sort(list);
out.println(toString(list));
out.println("\n\n");
}
}
Review:
• Why are arrays useful?
• What types of elements can an array
have?
• How do we refer to an array’s element in
Java?
• What happens if an index has an invalid
value?
• How do we refer to the length of an array?
Review (cont’d):
• Can we resize an array after it has been
created?
• Are arrays in Java treated as primitive
data types or as objects?
• What values do an array’s elements get
when the array is created?
• Are the array’s elements copied when an
array is passed to a method?
• Can a method return an array?
Review (cont’d):
• When is an ArrayList more convenient
than an array?
• Explain the difference between the
capacity and size in an ArrayList?
• What method returns the number of
elements currently stored in an ArrayList?
• What method is used to insert an element
into an ArrayList?
Review (cont’d):
• What is autoboxing?
• Can a double value be stored in an
ArrayList<Double>?
• Can a “for each” loop be used with
ArrayLists? Standard arrays?
• Describe an algorithm for inserting a value
into a sorted array?
• Can a class extend ArrayList<String>?
Review (cont’d):
• Name a few applications of twodimensional arrays.
• If m is a 2-D array of ints, what is the type
of m[0]?
• How do we get the numbers of rows and
cols in a 2-D array?
// Java2011.java
// This program tries to make a copy of the names1 <ArrayList> but does it improperly.
// Instead of making a "deep copy" of the contents of names1, it only makes a "shallow copy" of the
// memory address of names1. The result is aliasing. names1 and names2 are the exact same array
// and when one <ArrayList> object is changed, the other is also changed.
import java.util.ArrayList;
public class Java2011
{
public static void main(String args[])
{
System.out.println();
System.out.println("Java2011.java\n");
ArrayList names1 = new ArrayList();
names1.add("Isolde");
names1.add("John");
names1.add("Greg");
names1.add("Maria");
names1.add("Heidi");
System.out.println("names1 contains " + names1);
ArrayList names2 = names1;
// This causes aliasing.
System.out.println();
System.out.println("names2 contains " + names2);
System.out.println();
names2.remove(0);
// Removes the 1st name from the copied array
System.out.println();
System.out.println("names1 contains " + names1);
System.out.println();
System.out.println("names2 contains " + names2);
System.out.println();
}
}
// Java2012.java
// This program uses the <ArrayList> copy constructor to properly make a deep copy
// of the contents of names1. Now when we remove the 1st name from names2 (the copy),
// it does not have the aliasing side effect of altering names1 (the original).
import java.util.ArrayList;
public class Java2012
{
public static void main(String args[])
{
System.out.println();
System.out.println("Java2012.java\n");
ArrayList names1 = new ArrayList();
names1.add("Isolde");
names1.add("John");
names1.add("Greg");
names1.add("Maria");
names1.add("Heidi");
System.out.println("names1 contains " + names1);
ArrayList names2 = new ArrayList(names1);
System.out.println();
System.out.println("names2 contains " + names2);
System.out.println();
names2.remove(0);
System.out.println();
// Removes the 1st name from the copied array
System.out.println("names1 contains " + names1);
System.out.println();
System.out.println("names2 contains " + names2);
System.out.println();
}
}
// Uses copy constructor
// Java2013.java
// This program shows another way to properly make a deep copy of an <ArrayList> object with
// the <clone> method.
import java.util.ArrayList;
public class Java2013
{
public static void main(String args[])
{
System.out.println();
System.out.println("Java2013.java\n");
ArrayList names1 = new ArrayList();
names1.add("Isolde");
names1.add("John");
names1.add("Greg");
names1.add("Maria");
names1.add("Heidi");
System.out.println("names1 contains " + names1);
ArrayList names2 = (ArrayList) names1.clone();
System.out.println();
System.out.println("names2 contains " + names2);
System.out.println();
names2.remove(0);
System.out.println();
// Removes the 1st name from the copied array
System.out.println("names1 contains " + names1);
System.out.println();
System.out.println("names2 contains " + names2);
System.out.println();
}
}
Shallow Copy
ArrayList names2 = names1;
A shallow copy of an object only copies
the memory address of the object and not
the actual information stored.
This results in aliasing and can cause
side effects.
names1
Isolde
&
names2
John
Greg
Maria
Heidi
Deep Copy
ArrayList names2 = new ArrayList (names1);
ArrayList names2 = (ArrayList) names1.clone();
A deep copy of an object creates an entirely new object and
then copies the contents from one object to the other.
This is the preferred way to copy objects and avoids aliasing.
For ArrayList, this can be done with its copy constructor, or
the clone method.
When using the clone method, typecasting is necessary
because the clone method returns a generic object.
names1 Isolde
John
Greg
Maria
Heidi
names2 Isolde
John
Greg
Maria
Heidi
// Java2014.java
// This program uses the <contains> method to determine if some object is an element
// in an <ArrayList> object. It also uses the <indexOf> method to return the index of
// a found element.
import java.util.ArrayList;
public class Java2014
{
public static void main(String args[])
{
System.out.println("Java2014.java\n");
ArrayList names = new ArrayList();
names.add("Isolde");
names.add("John");
names.add("Greg");
names.add("Maria");
names.add("Heidi");
System.out.println("names contains " + names);
System.out.println();
System.out.println("Greg is a member of names: " + names.contains("Greg"));
System.out.println("Suzy is a member of names: " + names.contains("Suzy"));
System.out.println();
System.out.println("Greg is located at index: " + names.indexOf("Greg"));
System.out.println("Suzy is located at index: " + names.indexOf("Suzy"));
}
}
ArrayList Methods
contains & indexOf
System.out.println(names.contains("Greg"));
System.out.println(names.indexOf("Greg"));
The contains method returns true if the
parameter value is an element of the names
array object and false otherwise.
The indexOf method returns the index of the
parameter if the parameter is an element of the
names object and -1 otherwise.
Primitive Type Warning
Java primitive types like
int, double, char and boolean
must be converted to
object before they can
be added to an
ArrayList object.
// Java2015.java
// This program demonstrates that it is possible to store integers in an <ArrayList> object
// with a "wrapper" class. It is easier to use a regular array to store primitive data elements.
import java.util.ArrayList;
public class Java2015
{
public static void main(String args[])
{
System.out.println();
System.out.println("Java2015.java\n");
ArrayList numbers = new ArrayList();
numbers.add(new Integer(12345));
numbers.add(new Integer(23451));
numbers.add(new Integer(34512));
numbers.add(new Integer(45123));
numbers.add(new Integer(51234));
for (int k = 0; k < numbers.size(); k++)
System.out.println((Integer) numbers.get(k));
System.out.println();
}
}
// Java2016.java
// This program demonstrates that integer stored into an <ArrayList> object cannot
// be added with direct access, like static Java array elements.
import java.util.ArrayList;
import java.util.Random;
public class Java2016
{
public static void main(String args[])
{
System.out.println("Java2016.java\n");
Random rand = new Random(12345);
ArrayList numbers = new ArrayList();
for (int k = 1; k <= 48; k++)
{
int rndInt = (rand.nextInt(900) + 100);
numbers.add(new Integer(rndInt));
}
int sum = 0;
for (int k = 0; k < numbers.size(); k++)
sum += numbers.get(k);
System.out.println("Sum: " + sum);
}
}
// Java2017.java
// This program demonstrates how to access the "int" value within an "integer" object.
import java.util.ArrayList;
import java.util.Random;
public class Java2017
{
public static void main(String args[])
{
System.out.println("Java2017.java\n");
Random rand = new Random(12345);
ArrayList numbers = new ArrayList();
for (int k = 1; k <= 48; k++)
{
int rndInt = (rand.nextInt(900) + 100);
numbers.add(new Integer(rndInt));
}
int sum = 0;
for (int k = 0; k < numbers.size(); k++)
{
Integer temp = (Integer) numbers.get(k);
sum += temp.intValue();
}
System.out.println("Sum: " + sum);
System.out.println();
}
}
// Java2018.java
// This program takes an earlier program from the Algorithm chapter, implemented
// with a <Person> array and converts it to an ArrayList implementation.
import java.io.*;
import java.util.*;
public class Java2018
{
public static void main(String args[]) throws IOException
{
System.out.println("Java2018.java\n");
List list = new List();
list.getList();
list.pause();
list.display();
list.pause();
list.bubbleSort();
list.display();
System.out.println();
}
}
class Person
{
public String name;
public int age;
public double gpa;
Person(String n, int a,double g) { name = n; age = a; gpa = g; }
}
class List
{
private ArrayList students;
public List() { students = new ArrayList(); }
public void getList() throws IOException
{
System.out.println("\nRetrieving Students.dat\n");
FileReader inFile = new FileReader("Students.dat");
BufferedReader inStream = new BufferedReader(inFile);
String s1,s2,s3;
while( ((s1 = inStream.readLine()) != null) &&
((s2 = inStream.readLine()) != null) &&
((s3 = inStream.readLine()) != null) )
{
String name = s1;
int age = Integer.parseInt(s2);
double gpa = Double.parseDouble(s3);
students.add(new Person(name,age,gpa));
}
inStream.close();
}
public void display()
{
System.out.println("\nDISPLAYING LIST ELEMENTS");
for (int k = 0; k < students.size(); k++)
{
Person person = (Person) students.get(k);
System.out.println(person.name + "\t\t" + person.age + "\t\t"
+ person.gpa);
}
}
public void pause()
{
Scanner input = new Scanner(System.in);
String dummy;
System.out.print("\nPress <Enter> to continue ===>> ");
dummy = input.nextLine();
}
private void swap(int x, int y)
{
Person temp = (Person) students.get(x);
students.set(x,students.get(y));
students.set(y,temp);
}
public void bubbleSort()
{
Person p, q;
for (int s = 1; s < students.size(); s++)
for (int t = 0; t < students.size()-s; t++)
{
p = (Person) students.get(t);
q = (Person) students.get(t+1);
if (p.gpa < q.gpa)
swap(t,t+1);
}
}
}