More Methods and Arrays

Material from Chapters 5 to 7

that we didn’t cover in 1226

More from Chapters 5 thru 7

More on Arrays create and manipulate arrays of objects multi-dimensional arrays

More on Methods and Constructors understand overloading create and use overloaded methods create and use overloaded constructors

Using Arrays with Classes

Each student has several grades array as an instance variable of Student we added a list of assignments only some assignments graded so far

Several students in a course array of Student objects

Make a grade sheet program list of Students, each with grades

Student.java

ClassList.java

GradeSheet.java

Array Instance Variables

Declare array instance variable privateprivate int[] asgnGrades;

Create extra variables as required size of array (static all objects same size)public static final int NUM_ASGN = 8; used places (static all objects same # used)private static int gradedAsgns = 0;

Create the array object in the constructorasgnGrades = new int[NUM_ASGN];

Array Getters and Setters

Protect our data from their grubby paws! getter returns a copy of the arraypublic int[] getAsgnGrades() { return Arrays.copyOf(asgnGrades, asgnsGraded);} setter copies values OR saves a copypublic void setAsgnGrades(int[] newGrades) { if (areValidGrades(newGrades)) { asgnGrades = Arrays.copyOf(newGrades, NUM_ASGN); }}

Arrays Class

We’ve seen Arrays before: Arrays.toString has some useful methods for arrays

» copyOf, equals, fill, sort, and others

copyOf method copies the array tell it how big the new array should be getter: array should only be graded assignmentsreturn Arrays.copyOf(asgnGrades, asgnsGraded); setter: array needs space for all assignmentsasgnGrades = Arrays.copyOf(newGrades, NUM_ASGN);

Partly Full Array

Eight assignments, but only two graded! NUM_ASGN == 8; asgnsGraded == 2

(Generally) process only the used part loop for full array:for (int i = 0; i < NUM_ASGN; ++i) { ... }

» OR for (int i = 0; i < asgnGrades.length; ++i) { ... }

loop for partly full array:for (int i = 0; i < asgnsGraded; ++i) { ... }

Exercise

Create an array for up to 100 numbers (int) create a constant for the maximum

Ask user for how many numbers they have assume they give a good #

Read that many numbers into the array

Arrays of Objects

Generally have many students at once lots of objects of same type treated the same an array!

Arrays of objects a little bit more complicated than arrays of primitives but not much!

Creating an Array of Students

Create the Student array variable and objectStudent[] student = new Student[MAX];

Student[] student creates the array variable new Student[MAX] creates the array object

Do we now have MAX Student objects? NO! We just have that many Student variables

An array is a collection of variables each variable needs to have a Student object

created for it!ClassList.java

Creating an Array of Students

Loop reads a name, creates a Studentfor (int s = 0; s < student.length; s++) { S.o.p(“Enter Student’s name: ”); String name = kbd.nextLine(); student[s] = new Student(name);}

using “s” because it’s a mnemonic for “student” recall that Student constructor accepts a name,

assigns a student number

ClassList.java

Arrays of Objects

Create the array… …then the array elements

/

/

/

/

/

/

student

A00000001

“Alice”

STUDENT_NUMBER

studentName

&

A00000002

“Bill”

STUDENT_NUMBER

studentName

&

A00000003

STUDENT_NUMBER

&

Printing Class List

Now we have all the objects created, we can just use them as usualfor (int s = 0; s < student.length; s++) { S.o.pln(student[s].getANumber() + “ ” + student[s].getName());}

student[s] is a Student» getANumber() the student's # in A00000000 format» getName() the student's name

ClassList.java

Exercise

Create an array of ten Thingamajigs, sized from 1 to 10 Note: new Thingamajig(n) creates a

Thingamajig of size n Write a loop to add up the sizes of the

Thingamajig in the array use the getSize() method!

Assigning Grades to a Student

Client assigns grades to a student create student objectStudent stu = new Student(“Student, Lowell E.”); assign gradesstu.setAsgnGrade(1, 95);stu.setAsgnGrade(2, 100);stu.setAsgnGrade(3, 72);…stu.setAsgnGrade(6, 90);

Reading One Student’s Grades

Client can set grade for studentfor (a = 1; a <= Student.NUM_ASGN; a++) {

System.out.print("Enter A" + a + " grade: ");int g = kbd.nextInt(); kbd.nextLine();stu.setAsgnGrade(a, g);

} loop: for each assignment

» prompt for and read a grade» set the student’s grade for that assignment

Reading Many Students’ Grades

Enter grades for each assignment for each assignment, read each student’s gradefor (int a = 1; a <= Student.NUM_ASGN; a++) {

System.out.println("Enter A" + a + " grades: ");for (int s = 0; s < numStu; s++) {

System.out.print(student[s].getStudentName() + "> ");

int g = kbd.nextInt(); kbd.nextLine();student[s].setAsgnGrade(a, g);

}} GradeSheet.java

Calculating the Class Average

Add up averages of all students ask each student for their average

Divide by number of studentsint sum = 0;for (int s = 0; s < numStu; s++) {

sum += student[s].getPctGrade();}double avg = (double)sum / (double)numStu;

GradeSheet.java

Exercise

Suppose we have an array of professors and we want to find their average salary array named professors numProfs professors in the array getSalary method returns a prof’s salary

Write the code

Base Types of Arrays

Base type of an array can be anything declare by adding [] after base typeString[] word;

Base type of an array can even be an array! declare by adding [] after base type! (SAME)int[][] matrix;

Create by specifying both dimensionsdouble[][] sales = new double[ROWS][COLS];

Picturing Arrays of Arrays

Normal way of drawing arrays & matrices arr is an int[] mat is an int[][]

00000000

arr

00000000

mat

00000000

00000000

00000000

A matrix is an array of arrays where every row is the same length.

One matrix; two matrices.

Extracting from Arrays of Arrays

Normal way of drawing arrays & matrices arr is an int[] arr[3] is an int mat is an int[][] mat[5] is an int[]

00000000

arr

00000000

mat

00000000

00000000

00000000

arr[3]

mat[5]

Extracting from Matrices

Matrix is like a list of lists mat is an int[][] mat[5] is an int[] mat[5] is an int[] mat[5][2] is an int

00000000

mat

00000000

00000000

00000000mat[5]

mat[5][2]

Matrix Elements

Can think of it as a table first say what row you want, then what column

00000000

00000000

00000000

00000000

mat[5][2] = 7; 0 1 2 301234567

7

Declaration Exercises

Declare matrices for: 30 rows and 10 columns of integers the names of the months in up to 10 languages monthly sales figures for up to 25 years

Processing a Matrix

Usually nested loops Usually row by rowfor (int row=0; row<numRows; row++) for (int col=0; col<numCols; col++) ...a[row][col]...

Sometimes column by columnfor (int col=0; col<numCols; col++) for (int row=0; row<numRows; row++) ...a[row][col]...

But rows always come first when selecting from the array

It’s Called Row Processing...

Because you’re doing one row at a time for each row, do each column in that row

0 1 2 3 4 50 3 5 2 9 8 81 5 5 4 6 7 42 2 3 5 3 5 43 4 4 7 9 2 14 3 2 3 8 8 5

Total

31

35

22

27

29

Likewise Column Processing...

Processes one column at a time for each column, do each row in that column

0 1 2 3 4 50 3 5 2 9 8 81 5 5 4 6 7 42 2 3 5 3 5 43 4 4 7 9 2 14 3 2 3 8 8 5

Total 1917 21 35 30 22

Exercise

Row processing or column processing? Calculate annual sales totals Calculate average sales by month

// Up to 25 years of monthly salespublic static final int MAX_YRS = 25;public static final int NUM_MOS = 12;double[][] sales = new double[MAX_YRS][NUM_MOS];int numYrs = 0; // current # yearsint y, m; // yr/month index

Annual Sales

Row processingdouble annualSales;...for (y=0; y<numYrs; y++) { annualSales = 0.0; for (m=0; m<NUM_MOS; m++) annualSales += sales[y][m]; ...}

Years are rows

Months are columns

Average Sales by Month

Column processingdouble aveSales;...for (m=0; m<NUM_MOS; m++) { double total = 0.0; for (y=0; y<numYrs; y++) total += sales[y][m]; aveSales = total / numYrs; ...}

Years are rows

Months are columns

Higher Dimensions

Can do 3D, 4D, 5D, etc.int[][][] a = new int[600][400][4];int[][][][] b = new int[10][20][30][40];int[][][][][] c = new int[100][366][24][60][60];

Pretty rare Analogous to 2D case

an array to keep one int value for each second in each minute of each hour in each day in each year of a century (not counting leap seconds :-)

Game of Life

Cells on a board two-dimensional array of cells

Generations pass Each generation depends on one before it

keep track of previous generation want two boards – use an arraychar[][][] board = new char[2][ROWS][COLS];

Life Boards

board[0]

board[1]

Overloading

Consider printlnSystem.out.println(“This is a string”);System.out.println(42);System.out.println(3.14);System.out.println();

What’s the argument type for println? String, int, double, nothing? it’s actually 4 methods with the same name!

Picking a Method

Java figures out which method by looking at the arguments in the method callSystem.out.println(“This is a string”);public void println(String s) { ... }System.out.println(42);public void println(int n) { ... }System.out.println(3.14);public void println(double d) { ... }System.out.println();public void println() { ... }

Overloaded Methods (Stubs)

Need the argument types to be differentpublic void println(String s) {}public void println(int n) {}public void println(double d) {}public void println() {} can’t repeat the same argument type sequencepublic void println(String t) {}

» not even if the parameter names are different!» Java can’t choose between them

Matching Methods

Java looks for an exact match doThis(10) looks for doThis(int)

If no exact match, looks for a close match no doThis(int)? look for doThis(double)!

Must be able to assign value to variable can assign an int value to a double variable cannot assign double value to int variable

» so doThis(10.5) will not match doThis(int)

Multiple Parameters

Arguments match parameters in order OK to have same types in different order

doThis(double x, String s)doThis(String s, double x)

Or different numbers of argumentsdoThis(double x)doThis(String s, double x, String t) so long as Java can tell them apart!

Exercise

Match the method calls to the definitionsi. doThis(“Alpha”, “Beta”);ii. doThis(“Gamma”);iii. doThis(20.5);iv. doThis(15); available definitions:a) doThis() { ... }b) doThis(double x) { ... }c) doThis(String s) { ... }d) doThis(String s, String t) { ... }

Writing Overloaded Methods

Mostly methods with the same name are doing more-or-less the same thing recall: name says what it does/returns

Try to re-use code print a title, underlined with ‘-’sprintTitle(“My Title”); print a title, underlined with ‘=’sprintTitle(“My Big Title”, “=”); “default” argument

printTitle Code

Old code used hyphen to underline title change it to use parameter characterpublic static void printTitle(String title, char under) { System.out.println(“\n” + title); for (int i = 0; i < title.length; ++i) { System.out.print(under); } System.out.println(“\n”);}

add parameter

change ‘-’ to new parameter

printTitle Code

For old behaviour: printTitle(“Blah”, ‘-’) but we want printTitle(“Blah”) like before!

So make another method: header is the “short” version we want body is the long version we would have neededpublic static void printTitle(String title) { printTitle(title, ‘-’);}

“Default Parameters” in Java

Actually multiple methods! one method has the most parameters

public static void printTitle(String title, char under)» it has the code for completing the task

other methods have less parameterspublic static void printTitle(String title)» they call the method with the most parameters» they “fill in” the missing arguments printTitle(title, ‘-’);

if underline char is missing

fill it in with ‘-’

Exercise

Suppose we have a method that prints a rectangle (of given dimensions) using *s revise it to use any character given by the client add another (overloaded) method to make old

code work (that is, so that any call that doesn’t say what to use will use a *)

Overloaded Constructors

Same for constructors as for methods consider ScannerScanner kbd = new Scanner(System.in);public Scanner(InputStream is) { ... }Scanner line = new Scanner(“10 20 30”);public Scanner(String s) { ... } different arguments to “the” constructor different constructors!

Default Constructor Arguments

Student constructor has two-argumentsStudent stu = new Student(“Dent, Stu”, 85); name and percentage grade

Student grades start off at 0, usually make it easy to create Student w. pctGrade = 0Student s = new Student(“Tudiaunt, A.”); pctGrade not specified, so set it to 0

Exercise

Given these constructorsa) public Thing()b) public Thing(String a, int c)c) public Thing(String a, String b)d) public Thing(String a, String b, int c)

Which constructor gets called?i) Thing thing1 = new Thing(“Cat”, “Hat”);ii) Thing thing2 = new Thing(“Thing”, 2);iii) Thing thing3 = new Thing();

Secondary Constructors

Our original constructor works OK want our new constructors to work just as well could copy and paste code… …but it’s better to call code

» less work to change it later! want to call our first constructor, adding a 0 Student(“Pat”) same as Student(“Pat”, 0)

» so tell Student(String) to call Student(String, int)

Calling a Constructor

Special way to do it: use “this” instead of “Student” (class name)public Student(String n) { this(n, 0);}

» Note: no “new”, either» Note: nor “return”

Note the arguments: String and int» calls Student(String n, int g), with g set to 0

n = “Chris”, g = 0

n = “Chris”

Constructor Calling

Client uses one argument constructorStudent s = new Student(“Chris”); Java starts one-argument constructorpublic Student(String n) {

» n is “Chris”

this(n, 0); one-argument constructor calls two-argumentpublic Student(String n, pctGrade g) {

» n is “Chris”, g is 0» 2-arg constructor does what needs doing

Secondary Constructors

For now: one constructor is the primary constructor

» the one that has arguments for all settable IVs» it does all the work

other constructors just call the primary one» one line: this(…)» fill in a value for each settable IV that’s missing

Exercise

This constructor is defined:public Thing(String a, String b, int c)

Write new constructors:Thing t1 = new Thing(“aVal”, 10);

» same as Thing(“aVal”, “”, 10)Thing t2 = new Thing(“aVal”, “bVal”);

» same as Thing(“aVal”, “bVal”, 0)Thing t3 = new Thing();

» same as Thing(“Undefined”, “”, 0)

Questions