• An array is a data structure used to process a collection of data that is all of the same type

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Chapter 6

Arrays

Introduction to Arrays Introduction to Arrays

A i d t t t d t

• An array is a data structure used to process a collection of data that is all of the same type

– An array behaves like a numbered list of variables with aAn array behaves like a numbered list of variables with a uniform naming mechanism

– It has a part that does not change: the name of the array – It has a part that can change: an integer in square brackets – For example, given five scores:

score[0] score[1] score[2] score[3] score[4]

score[0], score[1], score[2], score[3], score[4]

Creating and Accessing Arrays Creating and Accessing Arrays

A h b h lik hi ll i f i bl ll f

• An array that behaves like this collection of variables, all of type double, can be created using one statement as follows:

double[] score = new double[5];[] [ ];

• Or using two statements:

double[] score;

score = new double[5];

– The first statement declares the variable scoreto be of the array type double[]yp []

– The second statement creates an array with five numbered variables of type doubleand makes the variable scorea name for the array

Creating and Accessing Arrays Creating and Accessing Arrays

• The individual variables that together make up the array are called indexed variables

– They can also be called subscripted variables or elements of the array

– The number in square brackets is called an index or subscript

I J i di t b b d t ti ith0 d

– In Java, indices must be numbered starting with 0, and nothing else

score[0] score[1] score[2] score[3] score[4]

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Creating and Accessing Arrays Creating and Accessing Arrays

Th b f i d d i bl i i ll d

• The number of indexed variables in an array is called the length or size of the array

Wh i d h l h f h i

• When an array is created, the length of the array is given in square brackets after the array type

Th i d d i bl h b d i

• The indexed variables are then numbered starting with 0, and ending with the integer that is one less than the length of the array

than the length of the array

Creating and Accessing Arrays Creating and Accessing Arrays

do ble[] score ne do ble[5]

double[] score = new double[5];

• A variable may be used in place of the integer (i.e., in place of the integer ⁵above)

– The value of this variable can then be read from the keyboard

– This enables the size of the array to be determined when the program is run

double[] score = new double[count];

• An array can have indexed variables of any type, including any class type

class type

• All of the indexed variables in a single array must be of the same type, called the base type of the array

Declaring and Creating an Array Declaring and Creating an Array

• An array is declared and created in almost the same way that objects are declared and created:

BaseType[] ArrayName = new BaseType[size];

– The size may be given as an expression that evaluates

t ti i t f l i t i bl

to a nonnegative integer, for example, an intvariable

char[] line = new char[80];

double[] reading = new double[count];

double[] reading new double[count];

Person[] specimen = new Person[100];

Referring to Arrays and Array Elements Referring to Arrays and Array Elements

E h l b d j lik h i l

• Each array element can be used just like any other single variable by referring to it using an indexed expression:

score[0][ ]

• The array itself (i.e., the entire collection of indexed variables) can be referred to using the array name (without any square

b k )

brackets): score

• An array index can be computed when a program is run

It may be represented by a variable: score[index]

– It may be represented by a variable: score[index]

– It may be represented by an expression that evaluates to a suitable integer: score[next + 1]

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Using the score Array in a Program Using the score Array in a Program

• The for loop is ideally suited for performing array manipulations:

for (index = 0; index < 5; index++) System.out.println(score[index] +

" differs from max by " +

" differs from max by " + (max-score[index]) );

Three Ways to Use Square Brackets [] with an Array Name

S b k t b d t t t

• Square brackets can be used to create a type name:

double[] score;

• Square brackets can be used with an integer value as

• Square brackets can be used with an integer value as part of the special syntax Java uses to create a new array: y

score = new double[5];

• Square brackets can be used to name an indexed i bl f

variable of an array:

max = score[0];

The length Instance Variable The length Instance Variable

• An array is considered to be an object

• Since other objects can have instance variables, so can arrays

arrays

• Every array has exactly one instance variable named lengthe gt

– When an array is created, the instance variable lengthis automatically set equal to its size

The value oflengthcannot be changed (other than by – The value of lengthcannot be changed (other than by

creating an entirely new array with new) double[] score = new double[5];

– Given scoreabove, score.lengthhas a value of 5

Pitfall: Array Index Out of Bounds Pitfall: Array Index Out of Bounds

• Array indices always start with 0, and always end with the integer that is one less than the size of the array

The most common programming error made when using arrays – The most common programming error made when using arrays

is attempting to use a nonexistent array index

• When an index expression evaluates to some value other than those allowed by the array declaration, the index is said to be out of bounds

– An out of bounds index will cause a program to terminate withAn out of bounds index will cause a program to terminate with a run‐time error message

– Array indices get out of bounds most commonly at the first or last iteration of a loop that processes the array: Be sure to test last iteration of a loop that processes the array: Be sure to test for this!

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Initializing Arrays Initializing Arrays

• An array can be initialized when it is declared

– Values for the indexed variables are enclosed in braces, and separated by commas

– The array size is automatically set to the number of values i th b

in the braces

int[] age = {2, 12, 1};

Givenageabove age lengthhas a value of 3 – Given ageabove, age.lengthhas a value of 3

Initializing Arrays Initializing Arrays

A th f i iti li i i b i f l

• Another way of initializing an array is by using a forloop

double[] reading = new double[100];

int index;

for (index = 0;

index < reading.length; index++) di [i d ] 42 0

reading[index] = 42.0;

• If the elements of an array are not initialized explicitly, they will automatically be initialized to the default value for their will automatically be initialized to the default value for their base type

Pitfall: An Array of Characters Is Not a String Pitfall: An Array of Characters Is Not a String

A f h i ll li f

• An array of characters is conceptually a list of characters, and so is conceptually like a string

H f h i bj

• However, an array of characters is not an object of the class String

h [] {'A' 'B' 'C'}

char[] a = {'A', 'B', 'C'};

String s = a; //Illegal!

• An array of characters can be converted to an

• An array of characters can be converted to an object of type String , however

Pitfall: An Array of Characters Is Not a String Pitfall: An Array of Characters Is Not a String

Th l i h h h i l

• The class String has a constructor that has a single parameter of type char[]

St i St i ( )

String s = new String(a);

– The object swill have the same sequence of characters as the entire arraya ("ABC"), but is an independent copy the entire array a ( ABC ), but is an independent copy

• Another String constructor uses a subrange of a character array instead

character array instead

String s2 = new String(a,0,2);

– Given aas before, the new string object is "AB", g j

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Pitfall: An Array of Characters Is Not a String Pitfall: An Array of Characters Is Not a String

f h d h hi

• An array of characters does have some things in common with String objects

– For example, an array of characters can be output using println

System.out.println(a);

– Given a as before, this would produce the output

ABC

Arrays and References Arrays and References

• Like class types, a variable of an array type holds a reference f

– Arrays are objects

A variable of an array type holds the address of – A variable of an array type holds the address of

where the array object is stored in memory

A t ( ll ) id d t b l

– Array types are (usually) considered to be class types

Arrays are Objects Arrays are Objects

A b i d ll i f i d d i bl

• An array can be viewed as a collection of indexed variables

• An array can also be viewed as a single item whose value is a collection of values of a base type

collection of values of a base type

– An array variable names the array as a single item double[] a;

– A newexpression creates an array object and stores the object in memory

new double[10][ ]

– An assignment statement places a reference to the memory address of an array object in the array variable

a = new double[10];

Arrays Are Objects Arrays Are Objects

• The previous steps can be combined into one statement

double[] a = new double[10];

• Note that theNote that the newnewexpression that creates an arrayexpression that creates an array invokes a constructor that uses a nonstandard syntax

• Not also that as a result of the assignment statement above acontains a single value: a memory address or above, acontains a single value: a memory address or reference

• Since an array is a reference type, the behavior of arrays with respect to assignment (=), equality testing (==), and parameter passing are the same as that described for classes

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Pitfall: Arrays with a Class Base Type Pitfall: Arrays with a Class Base Type

Th b t f b l t

• The base type of an array can be a class type

Date[] holidayList = new Date[20];

• The above example creates 20 indexed variables of

• The above example creates 20 indexed variables of type Date

– It does not create 20 objects of the class Datej – Each of these indexed variables are automatically

initialized to null

A tt t t f th t thi i t ld

– Any attempt to reference any them at this point would result in a "null pointer exception" error message

Pitfall: Arrays with a Class Base Type Pitfall: Arrays with a Class Base Type

Lik th bj t h f th i d d i bl i

• Like any other object, each of the indexed variables requires a separate invocation of a constructor using new(singly, or perhaps using a forloop) to create an object to reference

holidayList[0] = new Date();

. . .

holidayList[19] = new Date();

OR

for (int i = 0; i < holidayList.length; i++) holidayList[i] = new Date();

holidayList[i] new Date();

• Each of the indexed variables can now be referenced since each holds the memory address of a Dateobject

Array Parameters Array Parameters

• Both array indexed variables and entire arrays can be used as arguments to methods g

– An indexed variable can be an argument to a method in exactly the same way that any variable method in exactly the same way that any variable of the array base type can be an argument

Array Parameters Array Parameters

d bl 0 0

double n = 0.0;

double[] a = new double[10];//all elements //are initialized to 0 0 //are initialized to 0.0 int i = 3;

• Given myMethody which takes one argument of type g yp double, then all of the following are legal:

myMethod(n);//n evaluates to 0.0

myMethod(a[3]);//a[3] evaluates to 0.0 myMethod(a[i]);//i evaluates to 3,

//a[3] evaluates to 0 0 //a[3] evaluates to 0.0

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Array Parameters Array Parameters

A t t th d b ti

• An argument to a method may be an entire array

• Array arguments behave like objects of a class

Therefore a method can change the values stored in the – Therefore, a method can change the values stored in the

indexed variables of an array argument

• A method with an array parameter must specify the y p p y base type of the array only

BaseType[]

– It does not specify the length of the array

Array Parameters Array Parameters

• The following method, doubleElements, specifies an array of doubleas its single argument:

public class SampleClass public class SampleClass {

public static void doubleElements(double[] a) {

{

int i;

for (i = 0; i < a.length; i++) a[i] = a[i]*2;

. . . }

} . . . }

Array Parameters Array Parameters

• Arrays of double may be defined as follows:

double[] a = new double[10];

double[] b = new double[30];

• Given the arrays above, the method doubleElements from class SampleClassp can be invoked as follows:

SampleClass.doubleElements(a);

SampleClass.doubleElements(b);

Note that no square brackets are used when an entire array is – Note that no square brackets are used when an entire array is

given as an argument

– Note also that a method that specifies an array for a parameter

t k f l th t

can take an array of any length as an argument

Pitfall: Use of = and == with Arrays Pitfall: Use of = and == with Arrays

• Because an array variable contains the memory address of the array it names, the assignment operator (=) only copies this memory address operator (=) only copies this memory address

– It does not copy the values of each indexed variable – Using the assignment operator will make two array g g p y

variables be different names for the same array

b = a;

– The memory address inais now the same as the – The memory address in ais now the same as the

memory address in b: They reference the same array

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Pitfall: Use of = and == with Arrays Pitfall: Use of = and == with Arrays

• A for loop is usually used to make two different arrays have the same values in each indexed position:

int i;

for (i = 0;

(i < a.length) && (i < b.length); i++) b[i] = a[i];

b[i] = a[i];

– Note that the above code will not make ban exact copy ofa unlessaandbhave the same length copy of a, unless aand bhave the same length

Pitfall: Use of = and == with Arrays Pitfall: Use of = and == with Arrays

• For the same reason, the equality operator (==) only tests two arrays to see if they are stored in

h l i i h '

the same location in the computer's memory

– It does not test two arrays to see if they contain the same values

same values

(a == b)

– The result of the above booleanexpression will be p trueif aand bshare the same memory address (and, therefore, reference the same array), and falseotherwise

falseotherwise

Pitfall: Use of = and == with Arrays Pitfall: Use of = and == with Arrays

• In the same way that an equals method can be defined for a class, an

equalsArray method can be defined for a type of array yp y

– This is how two arrays must be tested to see if they contain the same elements y

– The following method tests two integer arrays to see if they contain the same integer y y g values

Pitfall: Use of = and == with Arrays Pitfall: Use of = and == with Arrays

public static boolean equalsArray(int[] a, int[] b) {

if (a.length != b.length) return false;

l else {

int i = 0;

while (i < a length) while (i < a.length) {

if (a[i] != b[i]) return false;

return false;

i++;

} } }

return true;

}

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Arguments for the Method main Arguments for the Method main

Th h di f h i h d f h

• The heading for the main method of a program has a parameter for an array of String

It i ll ll d b ti

– It is usually called argsby convention

public static void main(String[] args)

– Note that sinceNote that since argsargsis a parameter it could be replacedis a parameter, it could be replaced by any other non‐keyword identifier

• If a Java program is run without giving an argument p g g g g to main, then a default empty array of strings is automatically provided

Arguments for the Method main Arguments for the Method main

H i th t t th t i

• Here is a program that expects three string arguments:

public class SomeProgram pub c c ass o e og a {

public static void main(String[] args) {

{

System.out.println(args[0] + " " + args[2] + args[1]);

} } }

• Note that if it needed numbers, it would have to , convert them from strings first

Arguments for the Method main Arguments for the Method main

If i th t th i th d b

• If a program requires that the main method be provided an array of strings argument, each element must be provided from the command line when the must be provided from the command line when the program is run

java SomeProgram Hi ! there

– This will set args[0]to "Hi", args[1]to "!", and args[2]to "there"

– It will also setIt will also set args.lengthargs lengthto 3to 3

• When SomeProgram is run as shown, its output will be:

Hi there!

Methods That Return an Array Methods That Return an Array

I J th d l t

• In Java, a method may also return an array

– The return type is specified in the same way that an array parameter is specified

parameter is specified

public static int[]

incrementArray(int[] a, int increment) {

{

int[] temp = new int[a.length];

int i;

f (i 0 i < l th i++) for (i = 0; i < a.length; i++) temp[i] = a[i] + increment;

return temp;

}

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Partially Filled Arrays Partially Filled Arrays

Th t i d d f i t l

• The exact size needed for an array is not always known when a program is written, or it may vary from one run of the program to another

from one run of the program to another

• A common way to handle this is to declare the array to be of the largest size that the program could possibly need

• Care must then be taken to keep track of how much

f th i t ll d

of the array is actually used

– An indexed variable that has not been given a meaningful value must never be referenced

Partially Filled Arrays Partially Filled Arrays

• A variable can be used to keep track of how many elements are currently stored in an array

For example given the variablecount the elements – For example, given the variable count, the elements

of the array someArraywill range from positions someArray[0]through someArray[count – 1]

1]

– Note that the variable countwill be used to process the partially filled array instead of p y y

someArray.length

– Note also that this variable (count) must be an argument to any method that manipulates the argument to any method that manipulates the partially filled array

Accessor Methods Need Not Simply Return Instance Variables

h i i bl i

• When an instance variable names an array, it is not always necessary to provide an accessor method that returns the contents of the entire array

• Instead, other accessor methods that return a variety of information about the array and its y y elements may be sufficient

The "for each" Loop The for each Loop

• The standard Java libraries include a number of collection classes

– Classes whose objects store a collection of valuesj

• Ordinary forloops cannot cycle through the elements in a collection object

Unlike array elements collection object elements are not – Unlike array elements, collection object elements are not

normally associated with indices

• However, there is a new kind of forloop, first available in Java 5 0 called aforeach loop or enhancedforloop in Java 5.0, called a for‐each loop or enhanced forloop

• This kind of loop can cycle through each element in a collection even though the elements are not indexed

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The "for each" Loop The for each Loop

• Although an ordinary forloop cannot cycle through the elements of a collection class, an enhanced forloop can cycle through the elements of an arrayy g y

• The general syntax for a for‐each loop statement used with an array is

for (ArrayBaseType VariableName : ArrayName) for (ArrayBaseType VariableName : ArrayName)

Statement

• The above for‐each line should be read as "for each V i bl N i A N d th f ll i "

VariableNamein ArrayNamedo the following:"

– Note that VariableNamemust be declared within the for‐

each loop, not before

( )

– Note also that a colon (not a semicolon) is used after VariableName

The "For Each" Loop The For‐Each Loop

• The for‐each loop can make code cleaner and less error prone

• If the indexed variable in aforloop is used only as a way

• If the indexed variable in a forloop is used only as a way to cycle through the elements, then it would be

preferable to change it to a for‐each loop

– For example:

for (int i = 0; i < a.length; i++) a[i] = 0 0;

a[i] 0.0;

– Can be changed to:

for (double element : a)

l t 0 0

element = 0.0;

• Note that the for‐each syntax is simpler and quite easy to understand

Methods with a Variable Number of Parameters Methods with a Variable Number of Parameters

S i i h J 5 0 h d b d fi d h

• Starting with Java 5.0, methods can be defined that take any number of arguments

E i ll i i i l d b ki i

• Essentially, it is implemented by taking in an array as argument, but the job of placing values in the array is done automatically

done automatically

– The values for the array are given as arguments – Java automatically creates an array and places the – Java automatically creates an array and places the

arguments in the array

– Note that arguments corresponding to regular parameters g p g g p are handled in the usual way

Methods with a Variable Number of Parameters Methods with a Variable Number of Parameters

• Such a method has as the last item on its parameter list a vararg specification of the form:

Type ArrayName Type... ArrayName

– Note the three dots called an ellipsis that must be included as part of the vararg specification syntax

• Following the arguments for regular parameters are any number of arguments of the type given in the vararg specification

specification

– These arguments are automatically placed in an array – This array can be used in the method definition – Note that a vararg specification allows any number of

arguments, including zero

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Method with a Variable Number of Parameters (Part 1 of 2)

Method with a Variable Number of Parameters (Part 2 of 2)

Privacy Leaks with Array Instance Variables Privacy Leaks with Array Instance Variables

If h d d h f

• If an accessor method does return the contents of an array, special care must be taken

J t h t f t i t

– Just as when an accessor returns a reference to any private object

public double[] getArray() public double[] getArray() {

return anArray;//BAD!y;//

}

– The example above will result in a privacy leak

Privacy Leaks with Array Instance Variables Privacy Leaks with Array Instance Variables

• The previous accessor method would simply return a reference to the array anArrayitself

• Instead, an accessor method should return a referenceInstead, an accessor method should return a reference to a deep copy of the private array object

– Below, both aand countare instance variables of the class containing thegetArraymethod

containing the getArraymethod public double[] getArray() {

double[] temp = new double[count];

for (int i = 0; i < count; i++) temp[i] = a[i];p[ ] [ ]

return temp }

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Privacy Leaks with Array Instance Variables Privacy Leaks with Array Instance Variables

If i i i bl i h h l i

• If a private instance variable is an array that has a class as its base type, then copies must be made of each class object in the array when the array is copied:y y p

public ClassType[] getArray() {

ClassType[] temp = new ClassType[count];

for (int i = 0; i < count; i++)

temp[i] = new ClassType(someArray[i]);

return temp;

}

Sorting an Array Sorting an Array

• A sort method takes in an array parameter a , and rearranges the elements in a, so that after the method call is finished, the elements of a are sorted in ascending order

• A selection sort accomplishes this by using the following algorithm:

for (int index = 0; index < count; index++) Place the indexth smallest element in a[index]

Selection Sort (Part 1 of 2)

Selection Sort (Part 1 of 2) Selection Sort (Part 2 of 2) Selection Sort (Part 2 of 2)

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SelectionSort Class (Part 1 of 5) SelectionSort Class (Part 1 of 5)

public class SelectionSort public class SelectionSort {

/**

Precondition: count <= a.length;

The first count indexed variables have values.

Action: Sorts a so that a[0] <= a[1] <=

... <= a[count - 1].

*//

SelectionSort Class (Part 2 of 5) SelectionSort Class (Part 2 of 5)

public static void sort(double[] a int count) public static void sort(double[] a, int count) {

int index, indexOfNextSmallest;

for (index = 0; index < count - 1; index++) {//Place the correct value in a[index]:

indexOfNextSmallest =

indexOfSmallest(index, a, count);

interchange(index,indexOfNextSmallest, a);

//a[0]<=a[1]<=...<=a[index] and these are

// [ ] [ ] [ ]

//the smallest of the original array

//elements. The remaining positions contain //the rest of the original array elements //the rest of the original array elements.

} }

SelectionSort Class (Part 3 of 5) SelectionSort Class (Part 3 of 5)

/**

Returns the index of the smallest value among a[startIndex], a[startIndex+1], ...

a[numberUsed - 1]

*/

private static int indexOfSmallest(int startIndex, double[] a, int count) {

{

double min = a[startIndex];

int indexOfMin = startIndex;

int index;

SelectionSort Class (Part 4 of 5) SelectionSort Class (Part 4 of 5)

for (index startIndex + 1;

for (index = startIndex + 1;

index < count; index++) if (a[index] < min)

{

min = a[index];

indexOfMin = index;

//min is smallest of a[startIndex] through //a[index]

} }

return indexOfMin;

}

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SelectionSort Class (Part 5 of 5) SelectionSort Class (Part 5 of 5)

/**

/

Precondition: i and j are legal indices for the array a.

Postcondition: Values of a[i] and a[j] have Postcondition: Values of a[i] and a[j] have

been interchanged.

*/

i t t ti id i t h (i t i i t j private static void interchange(int i, int j,

double[] a) {

double temp;

temp = a[i];

a[i] = a[j];j

a[j] = temp; //original value of a[i]

} }

}

Enumerated Types Enumerated Types

St ti ith i 5 0 J it t d t

• Starting with version 5.0, Java permits enumerated types

– An enumerated type is a type in which all the values are given in a (typically) short list

• The definition of an enumerated type is normally placed outside of all methods in the same place that named constants are defined:

enum TypeName {VALUE_1, VALUE_2, …, VALUE_N};

– Note that a value of an enumerated type is a kind of named constant and so, by convention, is spelled with all uppercase letters

and so, by convention, is spelled with all uppercase letters

– As with any other type, variables can be declared of an enumerated type

Enumerated Types Example Enumerated Types Example

• Given the following definition of an enumerated type:

enum WorkDay {MONDAY TUESDAY enum WorkDay {MONDAY, TUESDAY,

WEDNESDAY, THURSDAY, FRIDAY};

• A variable of this type can be declared as follows: yp

WorkDay meetingDay, availableDay;

• The value of a variable of this type can be set to one of the values listed in the definition of the type, or else to the special value null :

meetingDay = WorkDay THURSDAY;

meetingDay = WorkDay.THURSDAY;

availableDay = null;

Enumerated Types Usage Enumerated Types Usage

• Just like other types, variable of this type can be declared and initialized at the same time:

WorkDay meetingDay = WorkDay.THURSDAY;y g y y ; – Note that the value of an enumerated type must be prefaced

with the name of the type

• The value of a variable or constant of an enumeratedThe value of a variable or constant of an enumerated type can be output using println

– The code:

System out println(meetingDay);

System.out.println(meetingDay);

– Will produce the following output:

THURSDAY As will the code:

– As will the code:

System.out.println(WorkDay.THURSDAY);

– Note that the type name WorkDayis not output

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Enumerated Types Usage Enumerated Types Usage

• Although they may look like Stringvalues, values of an enumerated type are not Stringvalues

• However they can be used for tasks which could be

• However, they can be used for tasks which could be done by Stringvalues and, in some cases, work better

– Using a Stringgvariable allows the possibility of setting the variable to a nonsense value

– Using an enumerated type variable constrains the possible values for that variable

– An error message will result if an attempt is made to give an enumerated type variable a value that is not defined for its type

Enumerated Types Usage Enumerated Types Usage

T i bl f d

• Two variables or constants of an enumerated type can be compared using the equals method or the

== operator

• However, the == operator has a nicer syntax

if ( ti D il bl D )

if (meetingDay == availableDay)

System.out.println("Meeting will be on schedule.");

schedule. );

if (meetingDay == WorkDay.THURSDAY) System.out.println("Long weekend!);y p ( g );

An Enumerated Type

An Enumerated Type Some Methods Included with Every Enumerated

Type (Part 1 of 3)

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Some Methods Included with Every Enumerated Type (Part 2 of 3)

Some Methods Included with Every Enumerated Type (Part 3 of 3)

The values Method The values Method

T h f ll i l f d i i f

• To get the full potential from an enumerated type, it is often necessary to cycle through all the values of the type

• Every enumerated type is automatically provided with theEvery enumerated type is automatically provided with the static method values()which provides this ability

– It returns an array whose elements are the values of the enumerated

i i h d i hi h h l li d i h

type given in the order in which the elements are listed in the definition of the enumerated type

– The base type of the array that is returned is the enumerated type

The Method values (Part 1 of 2)

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The Method values (Part 2 of 2) The Method values (Part 2 of 2)

Programming Tip: Enumerated Types in switch Statements

E t d t b d t t l it h

• Enumerated types can be used to control a switch statement

– The switches tc control expression uses a variable of an enumerated co t o e p ess o uses a a ab e o a e u e ated type

– Case labels are the unqualified values of the same enumerated type

• The enumerated type control variable is set by using the static

• The enumerated type control variable is set by using the static method valueOfto convert an input string to a value of the enumerated type

– The input string must contain all upper case letters, or be converted to all upper case letters using the toUpperCasemethod

Enumerated Type in a switch Statement (Part 1 of 3)

Enumerated Type in a switch Statement (Part

2 of 3)

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Enumerated Type in a switch Statement (Part 3 of 3)

Multidimensional Arrays Multidimensional Arrays

It i ti f l t h ith

• It is sometimes useful to have an array with more than one index

• Multidimensional arrays are declared and created in

• Multidimensional arrays are declared and created in basically the same way as one‐dimensional arrays

– You simply use as many square brackets as there are p y y q indices

– Each index must be enclosed in its own brackets

double[][]table = new double[100][10];

int[][][] figure = new int[10][20][30];

Person[][] = new Person[10][100];

Multidimensional Arrays Multidimensional Arrays

M l idi i l h b f

• Multidimensional arrays may have any number of indices, but perhaps the most common number is two

two

– Two‐dimensional array can be visualized as a two‐

dimensional display with the first index giving the row, and dimensional display with the first index giving the row, and the second index giving the column

char[][] a = new char[5][12];

– Note that, like a one‐dimensional array, each element of a multidimensional array is just a variable of the base type (in this case,char)

(in this case, char)

Multidimensional Arrays Multidimensional Arrays

I J t di i l h i

• In Java, a two‐dimensional array, such as a , is actually an array of arrays

– The arrayThe array aacontains a reference to a one‐dimensionalcontains a reference to a one dimensional array of size 5 with a base type of char[]

– Each indexed variable (a[0],a[1], etc.) contains a reference to a one dimensional arra of si e 12 also ith a reference to a one‐dimensional array of size 12, also with a base type of char[]

• A three‐dimensional array is an array of arrays of y y y

arrays, and so forth for higher dimensions

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Two‐Dimensional Array as an Array of Arrays (Part 1 f 2)

of 2)

Two‐Dimensional Array as an Array of Arrays (Part 2 f 2)

of 2)

Using the length Instance Variable Using the length Instance Variable

h [][] h [30][100]

char[][] page = new char[30][100];

• The instance variable lengthdoes not give the total number of indexed variables in a two‐dimensional array

of indexed variables in a two dimensional array

– Because a two‐dimensional array is actually an array of arrays, the instance variable lengthgives the number of first indices (or "rows") in the array

in the array

• page.lengthis equal to 30

– For the same reason, the number of second indices (or "columns") for i " " i i b f i l thf th t " " i bl a given "row" is given by referencing lengthfor that "row" variable

• page[0].lengthis equal to 100

Using the length Instance Variable Using the length Instance Variable

• The following program demonstrates how a nested for loop can be used to process a two‐

dimensional array

– Note how each lengthinstance variable is used

int row, column;

f ( 0 < l th ++)

for (row = 0; row < page.length; row++)

for (column = 0; column < page[row].length;

column++) column++) page[row][column] = 'Z';

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Ragged Arrays Ragged Arrays

h i di i l d

• Each row in a two‐dimensional array need not have the same number of elements

– Different rows can have different numbers of columns

• An array that has a different number of elements per row it is called a ragged array p gg y

Ragged Arrays Ragged Arrays

d bl [][] d bl [3][5]

double[][] a = new double[3][5];

• The above line is equivalent to the following:

double [][] a;

a = new double[3][]; //Note below a[0] = new double[5];[ ] [ ];

a[1] = new double[5];

– Note that the second line makes athe name of an array with room for 3 entries, each of which can be an array of doublesthat can be of any lengthy g

– The next 3 lines each create an array of doubles of size 5

Ragged Arrays Ragged Arrays

double [][] a;

a = new double[3][];

• Since the above line does not specify the size of a[0] , a[1] , or a[2], each could be made a different size instead:

a[1] = new double[10];

Multidimensional Array Parameters and Returned Values

• Methods may have multidimensional array parameters

– They are specified in a way similar to one‐dimensional arrays

– They use the same number of sets of square brackets as they have dimensions

public void myMethod(int[][] a) public void myMethod(int[][] a) { . . . }

– The parameterThe parameter aais a two‐dimensional arrayis a two dimensional array

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Multidimensional Array Parameters and Returned Values

h d h l idi i l

• Methods may have a multidimensional array type as their return type

– They use the same kind of type specification as for a multidimensional array parameter

public double[][] aMethod() { . . . }

– The method aMethod returns an array of double

A Grade Book Class A Grade Book Class

A l f i i l

• As an example of using arrays in a program, a class GradeBook is used to process quiz scores

• Objects of this class have three instance variables

– grade: a two‐dimensional array that records the grade of each student on each quiz

– studentAverage: an array used to record the average quiz score for each student

– quizAverage: an array used to record the average – quizAverage: an array used to record the average

score for each quiz