outline method overloading printf method arrays declaring and using arrays arrays of objects array...

OutlineMethod OverLoading

printf method

Arrays

Declaring and Using Arrays

Arrays of Objects

Array as Parameters

Variable Length Parameter Lists

split() Method from String Class

Integer & Double Wrapper Classes

Two-Dimensional Arrays

Copyright © 2012 Pearson Education, Inc.

Method Overloading• Method overloading is the process of giving a

single method name multiple definitions in a class

• If a method is overloaded, the method name is not sufficient to determine which method is being called

• The signature of each overloaded method must be unique

• The signature includes the number, type, and order of the parameters


Method Overloading• The indexOf method of String class is overloaded


int indexOf(char ch)

Returns the index of the first occurrence of char ch in this string.

Returns -1 if not matched.

int indexOf(char ch, int fromIndex)

Returns the index of the first occurrence of char ch in this string

after fromIndex. Returns -1 if not matched.

int indexOf(String s)

Returns the index of the first occurrence of String cs in this string.

Returns -1 if not matched.

int indexOf(String s, int fromIndex)

Returns the index of the first occurrence of String s in this string

after fromIndex. Returns -1 if not matched.

Method Overloading• The abs method of Math class is overloaded


double abs(double d) float abs(float f) int abs(int i) long abs(long l)

Method Overloading• The compiler determines which method is being invoked by

analyzing the parameters

float tryMe(int x){ return x + .375;}

float tryMe(int x, float y){ return x*y;}

result = tryMe(25, 4.32)

Invocation


Overloading Methods

• The return type of the method is not part of the signature.

• That is, overloaded methods cannot differ only by their return type

• Constructors can be overloaded

• Overloaded constructors provide multiple ways to initialize a new object

– Recall PrintWriter constructors


Overloading Example• See MyMaxOverloaded.java



printf method

Arrays


Arrays of Objects

Array as Parameters






System.out.printf ("format string", data)• The PrintWriter class has a printf method

compatible with the fprintf method in C or Matlab.• In the format string you can use format specifiers

as well as leading text for the data you want to print.

• There should be exactly one data for each format specifier other than %n


Common Format Specifiers

• %d for integers

• %f for floating-point numbers

• %e for floating-point numbers (scientific notation)

• %s for string

• %c for char

• %b for boolean

• %n to advance to next lineCopyright © 2012 Pearson

Education, Inc.

Additional characters for format specifiers

• First of all, you can specify the length of data. If data doesn't fit, the space reserved is extended. Otherwise, numbers are aligned right, and strings are aligned left. To override this, you can additionally use a minus character.

• For numbers you can use zero character to fill the gap with 0's instead of blanks.

• For floating-point numbers you can use .digit to specify decimal places to digit.

• See Example_printf.java



printf method

Arrays


Arrays of Objects

Array as Parameters






Arrays• Let’s say we need to hold the temperature values of each

day in Ankara in the past year

• Hard way: Create 365 different variables to hold each day’s temperature

double tempDay1, tempDay2, …, …,tempDay365;

Ughhhhh!!!

Very difficult to even declare, use and manipulate!

• Easy way: use an array to hold all days temperatures


How to Declare an Array• Create an array variable called temperatures

• Declared as follows:

double[] temperatures = new double[365];

• This sets up a location in memory for 365 double variables at once


Arrays Key Features• Arrays provide an easy way to hold related variables at once

(for example, temperature for the past year, gas prices for the last 30 days)

• It has some ordering, we can refer to array elements based on that ordering

• Homogenous, all data within a single array must share the same data type (for example, you can create an array of integer or boolean values, but not both)

• The size of an array is fixed once it is created.


Array Elements Type• The element type can be a primitive type or an object

reference

• Therefore, we can create an array of integers, an array of characters, an array of boolean, an array of String objects


Declaring Arrays• First you declare an array reference variable

int [] myFirstArray; //declares an array //variable for

ints

• Then you instantiate the array, that is allocate the necessary amount of memory for the array and let the variable hold the starting address of the array.

myFirstArray = new int [10]; //allocates memory

• We can combine the declaration and instantiation lines into one line as follows:

int [] myFirstArray = new int [10];

Declaring Arrays• The scores array could be declared as follows:

int[] scores = new int[10];

• The type is int (an array of integers)

• The name of the array is scores

• The size of the array is 10

• All positions of the new array will automatically be initialized to the default value for the array’s type.


0 0 0 0 0 0 0 0 0 0

Declaring Arrays• Some other examples of array declarations:

int[] weights = new int[2000];

double[] prices = new double[500];

boolean[] flags;flags = new boolean[20];

char[] codes = new char[1750];


Arrays are Objects• In Java, the array itself is an object that must be

instantiated• Another way to depict the scores array:

scores 79

87

94

82

67

98

87

81

74

91Copyright © 2012 Pearson Education, Inc.

The name of the arrayis an object reference

variable

Array BasicsArray1.java


Array Elements• Refers to the individual items represented by the

array. For example,– an array of 10 integers is said to have 10 elements– an array of 5 characters has 5 elements– and so on…

Array Index• Refers to one particular element’s position number in the array or

more formally, as a subscript

int[] scores = new int[10];


0 1 2 3 4 5 6 7 8 9

89 91 84 62 67 98 87 81 74 91

An array of size N is indexed from 0 (zero) to N-1

scores

The entire arrayhas a single name

Each value has a numeric index

This array holds 10 values that are indexed from 0 to 9

Array Element

• A particular value in an array is referenced using the array name followed by the index in brackets

scores[2]

refers to the value 94 (the 3rd value in the array)


0 1 2 3 4 5 6 7 8 9

79 87 94 82 67 98 87 81 74 91

BasicArray.javaThe following example demonstrates the use of indices.

See BasicArray.java


Arrays• An array element can be assigned a value, printed,

or used in a calculation:

scores[2] = 89;

scores[first] = scores[first] + 2;

mean = (scores[0] + scores[1])/2;

System.out.println ("Top = " + scores[5]);


See Array2.java

Array Naming Considerations• The rules for naming variables apply when selecting array

variable names

– Composed of letter, digit, $ and underscore characters– Cannot start with a digit– Begins with a smallcase letter by convention


Array Length and Bounds• Once an array is created, it has a fixed size

• An index used in an array reference must specify a valid element.

• That is, if the array length is N, the index value must be in range 0 to N-1

• You will get the run-time error, ArrayIndexOutOfBoundsException if you use an invalid index


Bounds Checking• For example, if the array codes can hold 100

values, it can be indexed from 0 to 99

• It’s common to introduce off-by-one errors when using arrays:

for (int index=0; index <= 100; index++)codes[index] = index*50 + epsilon;

problem


Array Length• Each array object has a public constant (not a method)

called length that stores the size of the array

• It is referenced using the array name:

scores.length

• length holds the number of elements (not the largest index!)

• Length is not a method so there is no parenthesis at the end unlike String class length() method


Basic Array Examples

Array3.java

ReverseOrder.java



//********************************************************************// ReverseOrder.java Author: Lewis/Loftus//// Demonstrates array index processing.//********************************************************************

import java.util.Scanner;

public class ReverseOrder{ //----------------------------------------------------------------- // Reads a list of numbers from the user, storing them in an // array, then prints them in the opposite order. //----------------------------------------------------------------- public static void main (String[] args) { Scanner scan = new Scanner (System.in);

double[] numbers = new double[10];

System.out.println ("The size of the array: " + numbers.length);

continue


continue

for (int index = 0; index < numbers.length; index++) { System.out.print ("Enter number " + (index+1) + ": "); numbers[index] = scan.nextDouble(); } System.out.println ("The numbers in reverse order:");

for (int index = numbers.length-1; index >= 0; index--) System.out.print (numbers[index] + " ");

System.out.println (); }}


continue

for (int index = 0; index < numbers.length; index++) { System.out.print ("Enter number " + (index+1) + ": "); numbers[index] = scan.nextDouble(); } System.out.println ("The numbers in reverse order:");

for (int index = numbers.length-1; index >= 0; index--) System.out.print (numbers[index] + " "); }}

Sample RunThe size of the array: 10Enter number 1: 18.36Enter number 2: 48.9Enter number 3: 53.5Enter number 4: 29.06Enter number 5: 72.404Enter number 6: 34.8Enter number 7: 63.41Enter number 8: 45.55Enter number 9: 69.0Enter number 10: 99.18The numbers in reverse order:99.18 69.0 45.55 63.41 34.8 72.404 29.06 53.5 48.9 18.36

Array Initializers• An array initializer combines the declaration,

creation, and initialization of an array in one statement using the following syntax:

elementType [] arrayReferenceVariable = {value0, value1,... , valueK};


Initializer Lists• An initializer list can be used to instantiate and fill

an array in one step

• The values are delimited by braces and separated by commas

• Examples:

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

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


Initializer Lists• Note that when an initializer list is used:

– the new operator is not used– no size value is specified

• The size of the array is determined by the number of items in the list

• An initializer list can be used only in the array declaration

• See Primes.java



//********************************************************************// Primes.java Author: Lewis/Loftus//// Demonstrates the use of an initializer list for an array.//********************************************************************

public class Primes{ //----------------------------------------------------------------- // Stores some prime numbers in an array and prints them. //----------------------------------------------------------------- public static void main (String[] args) { int[] primeNums = {2, 3, 5, 7, 11, 13, 17, 19}; System.out.println ("Array length: " + primeNums.length);

System.out.println ("The first few prime numbers are:");

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

System.out.print (primeNums[i] + " "); } }}


//********************************************************************// Primes.java Author: Lewis/Loftus//// Demonstrates the use of an initializer list for an array.//********************************************************************

public class Primes{ //----------------------------------------------------------------- // Stores some prime numbers in an array and prints them. //----------------------------------------------------------------- public static void main (String[] args) { int[] primeNums = {2, 3, 5, 7, 11, 13, 17, 19}; System.out.println ("Array length: " + primeNums.length);

System.out.println ("The first few prime numbers are:");

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

System.out.print (primeNums[i] + " "); } }}

OutputArray length: 8The first few prime numbers are:2 3 5 7 11 13 17 19

Example:Find the frequency of the result of throwing a dice 6000 times

•See RollDice.java


for-each loops• for-each loop (enhanced for loop) enables you to traverse

the array sequentially without using an index variable

for (double d: array)

System.out.println(d);

is equivalent to

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

double d = array [i];

System.out.println(d);

}


Another exampleint sum = 0;

int[] list = {1, 2, 3};

for (int value : list){

sum += value;

}


sumlist

value

01 2 3

1


int[] list = {1, 2, 3};


sum += value;

}


sumlist

value

11 2 3

2


int[] list = {1, 2, 3};


sum += value;

}


sumlist

value

31 2 3

3


int[] list = {1, 2, 3};


sum += value;

}


sumlist

61 2 3

Example • InitializerList.java demonstrates the

usage of array initializers and for-each loops.



printf method

Arrays


Arrays of Objects

Array as Parameters






Arrays of Objects• The elements of an array can be object references

• The following declaration reserves space to store 5 references to String objects

String[] array = new String[5];

• It does NOT create the String objects themselves

• Initially an array of objects holds null references

• Each object stored in an array must be instantiated separately


Arrays of Objects• The array array when initially declared:

• At this point, the following line of code would throw a NullPointerException:

System.out.println(array[0].length() );

• See ArrayOfStrings.java

array -

-

-

-

-


Arrays of Objects• Keep in mind that String objects can be created

using literals

• The following declaration creates an array object called verbs and fills it with five String objects created using string literals

String[] verbs = {"play", "work", "eat", "sleep", "run"};

See StringArr.java



printf method

Arrays


Arrays of Objects

Array as Parameters






Arrays as Parameters• An entire array can be passed as a parameter to a

method

• Like any other object, the reference to the array is passed, making the formal and actual parameters aliases of each other

• Therefore, changing an array element within the method changes the original

• An individual array element can be passed to a method as well, in which case the type of the formal parameter is the same as the element type


Examples of Array as ParametersArray4.java

MakeHot.java

ArrParametersAndReturns.java



printf method

Arrays


Arrays of Objects

Array as Parameters






Variable Length Parameter Lists• Suppose we wanted to create a method that

processed a different amount of data from one invocation to the next

• For example, let's define a method called average that returns the average of a set of integer parameters

// one call to average three valuesmean1 = average (42, 69, 37);

// another call to average seven valuesmean2 = average (35, 43, 93, 23, 40, 21, 75);


Variable Length Parameter Lists• Using special syntax in the formal parameter list, we

can define a method to accept any number of parameters of the same type

• For each call, the parameters are automatically put into an array for easy processing in the method

public double average (int ... list){ // whatever} element

typearrayname

Indicates a variable length parameter list




public double average (int ... list){ double result = 0.0;

if (list.length != 0) { int sum = 0; for (int num : list){ sum += num;

} result = (double)sum / list.length; }

return result;}

Example• See VarArgsDemo.java


Example


Write method called distance that accepts a variable number of doubles (which each represent the distance of one leg of a trip) and returns the total distance of the trip.

See Trip.java

Variable Length Parameter Lists• A method that accepts a variable number of parameters can

also accept other parameters, but variable number of parameters can exist only once and as the last parameter

• The following method accepts an int, a String object, and a variable number of double values into an array called nums

public void test (int count, String name, double ... nums){ // whatever}



printf method

Arrays


Arrays of Objects

Array as Parameters






split() Method from String Class public String[] split(String delimiter)•Method takes a String delimiter

•Returns a String array

•The split method in the String class returns an array of strings consisting of the substrings split by the delimiters.


Aaa bbb ccc

String str = "Aaa, bbb, ccc";

String[] arr = str.split(", ");

arr =

delimiter

Examples: String str = "Java#HTML#Perl“;

String [] tokens = str.split("#");

for (String token: tokens)

System.out.println (token.toUpperCase());


Examples: String str = “Java#HTML#Perl”;

String [] tokens = str.split("#");

for (String token: tokens)

System.out.println (token.toUpperCase());

Generates the following output:

JAVA

HTML

PERL


Examples: String str = "Hi How are you?";

String [] tokens = str.split("\t");

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

System.out.println (tokens[i]);


Examples: String str = "Hi How are you?";

String [] tokens = str.split("\t");

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

System.out.println (tokens[i]);

Generates the following output:

Hi

How are you?



printf method

Arrays


Arrays of Objects

Array as Parameters






Wrapper Classes• A wrapper class represents a particular primitive type as an

object. For example, Integer class represents a simple integer value.

• An Integer object may be created as

Integer obj = new Integer(40);

• All wrapper classes are in the java.lang package (no need to import).


Wrapper Classes in the java.lang Package

Primitive type Wrapper class

byte Byte

short Short

int Integer

long Long

float Float

double Double

char Character

boolean Boolean

void Void


Integer ClassUseful methods from the Integer class:•Integer (int value) : creates an Integer object storing the specified value•Integer (String str) : creates an Integer object storing the integer value extracted from the string, str•static int parseInt(String str) : returns the int corresponding to the value stored in the specified string, str•static String toBinaryString(int num) : returns a string representation of the specified integer value in base 2•static String toString(int num) : returns a string representation of the specified integer value in base 10•static Integer valueOf(String str) : returns a Integer object holding the int value represented by the argument string str.

Constants from the Integer class:Integer.MIN_VALUE returns the minimum int value, -231

Integer.MAX_VALUE returns the maximum int value, 231 -1


Double ClassSome methods from the Double class:•Double (double value) : creates a Double object storing the specified value•Double (String str) : creates a Double object storing the double value in string, str•static double parseDouble(String str) : returns the double corresponding to the value stored in the specified string, str•static String toString(double num) : returns a string representation of the specified double value •static Double valueOf(String str) : returns a Double object holding the double value represented by the argument string str.

Constants from the Double class:Double.MIN_VALUE returns the minimum double value

Double.MAX_VALUE returns the maximum double value


Example for split() and reading from a text file

• See WrapperSplitFile.java



printf method

Arrays


Arrays of Objects

Array as Parameters






Two-Dimensional Arrays• A one-dimensional array stores a list of elements

• A two-dimensional array can be thought of as a table of elements, with rows and columns

onedimension

twodimensions


Two-Dimensional Arrays• To be precise, in Java a two-dimensional array is an array of

arrays

• A two-dimensional array is declared by specifying the size of each dimension separately:

int[][] table = new int[12][50];

• A array element is referenced using two index values:

value = table[3][6]

• The array stored in one row can be specified using one index




Expression Type Description

table int[][] 2D array of integers, orarray of integer arrays

table[5] int[] array of integers

table[5][12] int integer

Two-Dimensional Arraysint[][] matrix = new int[2][7]; //creates a 2-by-7 array

matrix[0][2] = 5;


0 0 0 0 0 0 0

0 0 0 0 0 0 0

0 0 5 0 0 0 0

0 0 0 0 0 0 0

Multidimensional Arrays• You can think of it as array of arrays

for (int i=0; i < 2; i++){

for (int j=0; j<7; j++){

matrix[i][j]=1;

}

}


1 1 1 1 1 1 1

1 1 1 1 1 1 1


• See TwoDArray.java



//********************************************************************// TwoDArray.java Author: Lewis/Loftus//// Demonstrates the use of a two-dimensional array.//********************************************************************

public class TwoDArray{ //----------------------------------------------------------------- // Creates a 2D array of integers, fills it with increasing // integer values, then prints them out. //----------------------------------------------------------------- public static void main (String[] args) { int[][] table = new int[5][10];

// Load the table with values for (int row=0; row < table.length; row++) for (int col=0; col < table[row].length; col++) table[row][col] = row * 10 + col;

// Print the table for (int row=0; row < table.length; row++) { for (int col=0; col < table[row].length; col++) System.out.print (table[row][col] + "\t"); System.out.println(); } }}


//********************************************************************// TwoDArray.java Author: Lewis/Loftus//// Demonstrates the use of a two-dimensional array.//********************************************************************

public class TwoDArray{ //----------------------------------------------------------------- // Creates a 2D array of integers, fills it with increasing // integer values, then prints them out. //----------------------------------------------------------------- public static void main (String[] args) { int[][] table = new int[5][10];

// Load the table with values for (int row=0; row < table.length; row++) for (int col=0; col < table[row].length; col++) table[row][col] = row * 10 + col;

// Print the table for (int row=0; row < table.length; row++) { for (int col=0; col < table[row].length; col++) System.out.print (table[row][col] + "\t"); System.out.println(); } }}

Output0 1 2 3 4 5 6 7 8 910 11 12 13 14 15 16 17 18 1920 21 22 23 24 25 26 27 28 2930 31 32 33 34 35 36 37 38 3940 41 42 43 44 45 46 47 48 49

Example & Exercise• See GradeExam.java

• Exercise: Modify the code such that there are two methods:– gradeAStudent : grades one student– gradeAllStudents: grades all students


Example• See FindNearestPoints.java



• See SodaSurvey.java



//********************************************************************// SodaSurvey.java Author: Lewis/Loftus//// Demonstrates the use of a two-dimensional array.//********************************************************************

import java.text.DecimalFormat;

public class SodaSurvey{ //----------------------------------------------------------------- // Determines and prints the average of each row (soda) and each // column (respondent) of the survey scores. //----------------------------------------------------------------- public static void main (String[] args) { int[][] scores = { {3, 4, 5, 2, 1, 4, 3, 2, 4, 4}, {2, 4, 3, 4, 3, 3, 2, 1, 2, 2}, {3, 5, 4, 5, 5, 3, 2, 5, 5, 5}, {1, 1, 1, 3, 1, 2, 1, 3, 2, 4} };

final int SODAS = scores.length; final int PEOPLE = scores[0].length;

int[] sodaSum = new int[SODAS]; int[] personSum = new int[PEOPLE];

continue


continue

for (int soda=0; soda < SODAS; soda++) for (int person=0; person < PEOPLE; person++) { sodaSum[soda] += scores[soda][person]; personSum[person] += scores[soda][person]; }

DecimalFormat fmt = new DecimalFormat ("0.#"); System.out.println ("Averages:\n");

for (int soda=0; soda < SODAS; soda++) System.out.println ("Soda #" + (soda+1) + ": " + fmt.format ((float)sodaSum[soda]/PEOPLE));

System.out.println (); for (int person=0; person < PEOPLE; person++) System.out.println ("Person #" + (person+1) + ": " + fmt.format ((float)personSum[person]/SODAS)); }}


continue

for (int soda=0; soda < SODAS; soda++) for (int person=0; person < PEOPLE; person++) { sodaSum[soda] += scores[soda][person]; personSum[person] += scores[soda][person]; }

DecimalFormat fmt = new DecimalFormat ("0.#"); System.out.println ("Averages:\n");

for (int soda=0; soda < SODAS; soda++) System.out.println ("Soda #" + (soda+1) + ": " + fmt.format ((float)sodaSum[soda]/PEOPLE));

System.out.println (); for (int person=0; person < PEOPLE; person++) System.out.println ("Person #" + (person+1) + ": " + fmt.format ((float)personSum[person]/SODAS)); }}

OutputAverages:

Soda #1: 3.2Soda #2: 2.6Soda #3: 4.2Soda #4: 1.9

Person #1: 2.2Person #2: 3.5Person #3: 3.2Person #4: 3.5Person #5: 2.5Person #6: 3Person #7: 2Person #8: 2.8Person #9: 3.2Person #10: 3.8

outline method overloading printf method arrays declaring and using arrays arrays of objects array...

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