Comp 248 Introduction to Programming

Chapter 6 ArraysPart B

Dr. Aiman HannaDepartment of Computer Science & Software Engineering

Concordia University, Montreal, Canada

These slides has been extracted, modified and updated from original slides of Absolute Java 3 rd Edition by Savitch; which has originally been prepared by Rose Williams of Binghamton University. Absolute Java is published by

Pearson Education / Addison-Wesley.

Copyright © 2007 Pearson Addison-WesleyCopyright © 2007-2013 Aiman Hanna

All rights reserved

Initializer ListsInitializer Lists An An initializer listinitializer list can be used to instantiate can be used to instantiate

and initialize an array in one stepand initialize an array in one step

The values are delimited by braces and The values are delimited by braces and separated by commasseparated by commas

Examples:Examples:

int[] units = {147, 323, 89, 933, 540, int[] units = {147, 323, 89, 933, 540, 269, 97, 114, 298, 476};269, 97, 114, 298, 476};

char[] letterGrades = {'A', 'B', 'C', 'D', char[] letterGrades = {'A', 'B', 'C', 'D', ’F'};’F'};

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Array of CharactersArray of Characters

An Array of Characters Is Not a StringAn Array of Characters Is Not a String

char[] a = {'A', 'B', 'C'};char[] a = {'A', 'B', 'C'};String s = a; //Illegal!String s = a; //Illegal!

However, an array of characters can However, an array of characters can be converted to an object of type be converted to an object of type StringString

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CharArrays1.java ((MS-Word file))

Arrays of ObjectsArrays of Objects The base type of an array can be a class The base type of an array can be a class

typetypeCar[]Car[] carArr = newcarArr = new Car[20]Car[20];;

VERY IMPOTRANT, However, VERY IMPOTRANT, However, This will NOT create 20 Car objects; since This will NOT create 20 Car objects; since each of the 20 elements of the array are each of the 20 elements of the array are initialized to initialized to nullnull

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

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ObjectArrays1.java ((MS-Word file))

ObjectArrays2.java ((MS-Word file))

Array As Method Array As Method ParametersParameters

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

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

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ArrayOperations9.java ((MS-Word file))

Pitfall: Use of Pitfall: Use of == and and ==== with with ArraysArrays

The equality operator The equality operator ((====)) only tests only tests two arrays to see if they are stored in two arrays to see if they are stored in the same location in the computer's the same location in the computer's memorymemory It does not test two arrays to see if they It does not test two arrays to see if they

contain the same valuescontain the same values

The result of The result of if(a == b)if(a == b) will be will be truetrue if if aa and and bb point to the same memory address point to the same memory address (and, therefore, reference the same (and, therefore, reference the same array), and array), and falsefalse otherwise otherwise

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Pitfall: Use of Pitfall: Use of == and and ==== with with ArraysArrays

In the same way that an In the same way that an equalsequals method can be defined for a method can be defined for a class, an class, an equalsArrayequalsArray method method (notice that this is just any name) (notice that this is just any name) can be defined for a type of arraycan be defined for a type of array

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

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Pitfall: Use of Pitfall: Use of == and and ==== with with ArraysArrays

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

if (a.length != b.length) return false;if (a.length != b.length) return false; elseelse {{ int i = 0;int i = 0; while (i < a.length)while (i < a.length) {{ if (a[i] != b[i])if (a[i] != b[i]) return false;return false; i++;i++; }} }} return true;return true;}}

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Methods That Return an Methods That Return an ArrayArray

In Java, a method may also return an In Java, a method may also return an arrayarray The return type is specified in the same way The return type is specified in the same way

that an array parameter is specifiedthat an array parameter is specifiedpublic static public static int[]int[] incrementArray(int[] a, int increment)incrementArray(int[] a, int increment){{ int[] temp = new int[a.length];int[] temp = new int[a.length]; int i;int i; for (i = 0; i < a.length; i++)for (i = 0; i < a.length; i++) temp[i] = a[i] + increment;temp[i] = a[i] + increment; return temp;return temp;}}

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ArrayOperations10.java ((MS-Word file))

Partially Filled ArraysPartially Filled Arrays The exact size needed for an array is not The exact size needed for an array is not

always known when a program is written, always known when a program is written, or it may vary from one run of the or it may vary from one run of the program to anotherprogram to another

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

Care must then be taken to keep track of Care must then be taken to keep track of how much of the array is actually usedhow much of the array is actually used An indexed variable that has not been given a An indexed variable that has not been given a

meaningful value must never be referencedmeaningful value must never be referenced

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Partially Filled ArraysPartially Filled Arrays

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

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ArrayOperations11.java ((MS-Word file))

Privacy Leaks with Array Instance Privacy Leaks with Array Instance VariablesVariables

If a method return the contents of an array, If a method return the contents of an array, special care must be takenspecial care must be takenpublic double[] getArray()public double[] getArray(){{ return anArray;//BAD!return anArray;//BAD!}} The example above will result in a The example above will result in a privacy leakprivacy leak Instead, an accessor method should return a Instead, an accessor method should return a

reference to a reference to a deep copydeep copy of the private array of the private array objectobject

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ArrayOperations12.java ArrayOperations12.java (MS-Word file)(MS-Word file)

ArrayOperations13.java ArrayOperations13.java (MS-Word file)(MS-Word file)

Privacy Leaks with Array Instance Privacy Leaks with Array Instance VariablesVariables

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

public public ClassType[]ClassType[] getArray() getArray(){{ ClassType[]ClassType[] temp = new temp = new ClassTypeClassType[count];[count]; for (int i = 0; i < count; i++)for (int i = 0; i < count; i++) temp[i] = new temp[i] = new ClassTypeClassType(someArray[i]);(someArray[i]); return temp;return temp;}}

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Passing Multidimensional Passing Multidimensional Arrays as Method Arrays as Method

Parameters Parameters Multidimensional arrays can be passed as Multidimensional arrays can be passed as

parameters to methods in the same fashion as for parameters to methods in the same fashion as for one-dimensional arrays. one-dimensional arrays.

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ArrayOperations16.java ArrayOperations16.java (MS-Word file)(MS-Word file)

Multidimensional Array as Multidimensional Array as Returned ValuesReturned Values

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

They use the same kind of type specification They use the same kind of type specification as for a multidimensional array parameteras for a multidimensional array parameterpublic double[][] aMethod()public double[][] aMethod(){ {

. . . . . . }}

The method The method aMethodaMethod returns an array of returns an array of doubledouble

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Ragged ArraysRagged Arrays

Each row in a two-dimensional array Each row in a two-dimensional array need not have the same number of need not have the same number of elementselements Different rows can have different Different rows can have different

numbers of columnsnumbers of columns

An array that has a different number An array that has a different number of elements per row it is called a of elements per row it is called a ragged arrayragged array

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Ragged ArraysRagged Arrays

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

The above line is equivalent to the following:The above line is equivalent to the following:double [][] a; double [][] a;

a = new double[3][]; //Note belowa = new double[3][]; //Note below

a[0] = new double[5];a[0] = new double[5];

a[1] = new double[5];a[1] = new double[5];

a[2] = new double[5];a[2] = new double[5];

Note that the second line makes Note that the second line makes aa the name of an array the name of an array with room for 3 entries, each of which can be an array of with room for 3 entries, each of which can be an array of doublesdoubles that can be of any lengththat can be of any length

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

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Ragged ArraysRagged Arraysdouble [][] a; double [][] a;

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

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

a[0] = new double[5];a[0] = new double[5];

a[1] = new double[10];a[1] = new double[10];

a[2] = new double[4];a[2] = new double[4];

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RaggedArrays1.java RaggedArrays1.java (MS-Word file)(MS-Word file)

RaggedArrays2.java RaggedArrays2.java (MS-Word file)(MS-Word file)

Enumerated TypesEnumerated Types An enumerated type is a type in which all the values are given in a An enumerated type is a type in which all the values are given in a

(typically) short list(typically) short list

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

Example: Example:

enum WeekDays {MONDAY, TUESDAY, WEDNESDAY, THURSDAY, enum WeekDays {MONDAY, TUESDAY, WEDNESDAY, THURSDAY, FRODAY};FRODAY};

The definition of an enumerated type is normally placed outside of The definition of an enumerated type is normally placed outside of all methods all methods

Once an enumerated type is defined, variables can be declared from Once an enumerated type is defined, variables can be declared from this enumerated typethis enumerated type Note that a value of an enumerated type is a kind of named Note that a value of an enumerated type is a kind of named

constant and so, by convention, is spelled with all uppercase constant and so, by convention, is spelled with all uppercase lettersletters

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Enumerated Types Enumerated Types ExampleExample

A variable of this type can be A variable of this type can be declared as follows:declared as follows:WeekDay meetingDay, availableDay;WeekDay meetingDay, availableDay;

The value of a variable of this type The value of a variable of this type can be set to one of the values listed can be set to one of the values listed in the definition of the type, or else in the definition of the type, or else to the special value to the special value nullnull::meetingDay = WorkDay.THURSDAY;meetingDay = WorkDay.THURSDAY;availableDay = null;availableDay = null;

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Enumerated Types UsageEnumerated Types Usage

Just like other types, variable of this type can Just like other types, variable of this type can be declared and initialized at the same time:be declared and initialized at the same time:WeekkDay meetingDay = WorkDay.THURSDAY;WeekkDay meetingDay = WorkDay.THURSDAY; Note that the value of an enumerated type must be Note that the value of an enumerated type must be

prefaced with the name of the typeprefaced with the name of the type The value of a variable can be output using The value of a variable can be output using printlnprintln The code:The code:

System.out.println(meetingDay);System.out.println(meetingDay); Will produce the following output:Will produce the following output:

THURSDAYTHURSDAY As will the code:As will the code:

System.out.println(WorkDay.THURSDAY);System.out.println(WorkDay.THURSDAY); Note that the type name Note that the type name WorkDayWorkDay is not output is not output

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Enumerated Types UsageEnumerated Types Usage Two variables or constants of an Two variables or constants of an

enumerated type can be compared using enumerated type can be compared using the the equalsequals method or the method or the ==== operator operator

However, the However, the ==== operator has a nicer operator has a nicer syntaxsyntaxif (meetingDay == availableDay)if (meetingDay == availableDay)System.out.println("Meeting will be on System.out.println("Meeting will be on schedule.");schedule.");

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

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An Enumerated TypeAn Enumerated Type

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Some Methods Included with Some Methods Included with Every Enumerated Type (Part 1 of Every Enumerated Type (Part 1 of

3)3)

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

3)3)

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

3)3)

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The The valuesvalues Method Method To get the full potential from an enumerated To get the full potential from an enumerated

type, it is often necessary to cycle through all the type, it is often necessary to cycle through all the values of the typevalues of the type

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

It returns an array whose elements are the values of the It returns an array whose elements are the values of the enumerated type given in the order in which the enumerated type given in the order in which the elements are listed in the definition of the enumerated elements are listed in the definition of the enumerated typetype

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

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The Method The Method valuesvalues (Part (Part 1 of 2)1 of 2)

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The Method The Method valuesvalues (Part (Part 2 of 2)2 of 2)

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