cs303euser defined methods1 cs 303e class 7 part 1: designing complex programs the sooner you start...

CS303E User Defined Methods 1

CS 303E Class 7 Part 1:

Designing Complex Programs

The sooner you start to code, the longer your program will take.-Roy Carlson, U Wisconsin


Tally Grades(pp. 96–110)

Request: A program to tally grades on a test.

Analysis (details of input and output):Input: scores on a test.Output: Number of As, Bs, Cs, Ds, Fs (tallies) and

running average.

Interface: Fields, buttons, etc. needed for I/O.Fields and labels for input (score) and outputs

(tallies and average).Buttons to tally another score and to reset all.


Design — Variables

Window components.

Global variables:Needed? Yes:

Click Tally button for each score — “user loop”.

Tallies, average, other variables must retain their values between button clicks.

Global variables: tallyA, tallyB, …, tallyF, numberOfTests, totalOfTests, average.


Method buttonClicked

Determines which button was clicked and calls the appropriate method:

buttonClicked

if (reset button clicked)

resetGlobalVariables ();

else

processScore ();

set focus to score input field;


Method resetGlobalVariables

resetGlobalVariablesset tallies to 0; // tallyA, …set totals to 0; // totalOfTests, …set average to 0;display 0 in score field;displayTalliesAndAverage ();

Method call for last action because it will be needed again when processing scores.


Method displayTalliesAndAverage

displayTalliesAndAverage

display the tallies and average on the screen;

Computation done in global variables, but display done in fields or other window components.


Method processScoreprocessScore

if (score is invalid)display error message;

else{ get score value from field;

if (score < 0 or score > 100)display error message;

else{ updateTallies (score);

updateAverage (score); displayTalliesAndAverage ();

}}

Separate methods for computations of a few lines or more.


Update Methods

updateTallies (int score);update the proper tally for score;

updateAverage (score);update the totals for score;compute the average;

Simple enough that pseudocode isn’t needed.


Coding — Step 1 (pp. 99–101)

Code:• the import, class, and method main.• all the window objects — labels, fields, and

buttons declared.• method buttonClicked.

Omit global variables — not needed yet.

Stub out the methods buttonClicked calls.

Run and debug.• What should you see?


Coding — Step 2( pp. 100-101)

Code the methods called by buttonClicked.

Add global variables — needed now.

Stub out the methods called by methods added above.



Coding — Step 3(pp. 101-104)

Code the rest of the methods, removing all stubs.



Preconditions and Postconditions

Preconditions:Statement of what must be true before a method

can be invoked — inputs required in parameters or globals.

Postconditions:Statement of what the method will guarantee to

be true after it is executed if the preconditions are met — values computed and returned or changed, errors detected.


Example

Text pp. 94-96:

// Preconditions: number is an integer >= 0.

// Postconditions: The number of divisors in

// number is returned.

private int computeCount (int number)

{ . . . ;

return count;

}


Call Stack — Run-time Errors

Exception occurred during event dispatching:java.lang.ArithmeticException: / by zero at TallyGrades.updateAverage(TallyGrades.java) at TallyGrades.processScore(TallyGrades.java) at TallyGrades.buttonClicked(TallyGrades.java) at BreezyGUI.GBFrameButtonListener.actionPerformed

(GBFrame.java:241)

at java.awt.Button.processEvent(Button.java:281) at . . .

Press Enter to continue


CS 303E Class 7 Part 2

Characters, Strings, and the Math class

"The time has come, "the Walrus said, "To talk of many things: of shoes - and ships - and sealing wax - of cabbages - and kings - And why the sea is boiling hot - And whether pigs have wings." - Lewis Carroll, 1871, in Through the Lookinglass.


Primitive Data Types

Numeric types:

int, double, and others

Character type:

char

Boolean type.


Characters — type char

Type char represents the 64,768 characters (2 bytes) in the Unicode system.

The ASCII character set (representing English keyboard characters) is shown in Appendix D.

Char constants use single quotes: 'A'.


Wrapper Classes and Objects

pages 371 - 373 in text All primitive data types have a class associated

with various methods. These wrapper objects can also store one

primitive data type value for each object. The usefulness of this is described later.

The wrapper class for a char is the Character class.


Character Operations

char letter = 'a', digit = '4';

System.out.println (Character.isLetter (letter));

System.out.println (Character.digit (digit, 10));

System.out.println (Character.digit (letter, 16));

int i = letter;

System.out.println(i);

Note: Character char


Type Conversions

Each character value maps to an integer. For the ASCII character set, these numbers

range from 0 to 127. Use (char) i to get a character value from

an int. Use (int) ch to get the ASCII value from a char.

examples of casting.


Casting for ints

• Java will not assign a value of a more inclusive type to a variable of a less inclusive type unless the code explicitly converts the type.

• Each primitive type can be cast to any other primitive type, but information may be lost.

int i = 5;double d = 3.5;i = (int) d; // Cast operationSystem.out.println (i); // Displays 3

(int) truncates by dropping the fractional part.


Strings (page 126)

String str = "Hey Joe!";

System.out.println (str.length());

System.out.println (str.charAt(4));

System.out.println (str.indexOf('J'));

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

A string is an array of chars:

'H' 'e' 'y' ' ' 'J' 'o' 'e' '!'

0 1 2 3 4 5 6 7


Standard String Processing Loop

for (int i = 0; i < str.length(); i++){ char ch = str.charAt(i); <process ch>}

'H' 'e' 'y' ' ' 'J' 'o' 'e' '!'

0 1 2 3 4 5 6 7


Defining a String Method

Write a method that tests a String to see whether or not it represents an integer. A String represents an integer if it is not empty and contains just decimal digits.

// Input parameter: a String// Returns: true if the String is not empty and// contains just digits or false otherwise

boolean validInt (String str){ . . .}


Defining a String Method

// Input parameter: a String// Returns: true if the String is not empty and// contains just digits or false otherwise.

public boolean validInt (String str){ if (str.equals("")) return false; // Empty for (int i = 0; i < str.length(); i++) { char ch = str.charAt(i); if (! Character.isDigit(ch)) // Not digit return false; } return true; // Got through}


Equality

Use == and != with primitive types and window objects.

Use equals and ! … equals with all other types, such as String.

String a = "xyz", b = "xyz";

if (a == b) ... // Always false

if (a.equals(b)) ... // Use this instead


Lexicographical Order

"Ann" < "Bill" < "bill" // Good idea, bad Java

String str1 = "Ann", str2 = "Bill";

str1.compareTo("Ann"); // Returns 0

str1.compareTo(str2) // Returns int < 0

str2.compareTo(str1) // Returns int > 0

if (str1.compareTo(str2) < 0) // str1 comes before str2


Palindrome, pp. 129–130 — Example of String processing

public void buttonClicked (Button buttonObj){ String aString = stringField.getText(); aString = aString.toUpperCase(); if (isPalindrome (aString)) messageBox ("Yes, you entered a palindrome."); else messageBox ("No, you did not enter a palindrome.");}

private boolean isPalindrome (String s){ int lastPosition = s.length() - 1; int middlePosition = lastPosition / 2; int forward = 0; int backward = lastPosition; while (forward <= middlePosition){ if (s.charAt (forward) != s.charAt (backward)) return false; forward++; backward--; } return true;}


The Math Class(pp. 134–135)

Contains several useful methods and constants:abs (number) Returns the absolute value of number.sqrt (number) Returns the square root of number.

pow (x, y) Returns xy (all double).PI Constant — double value closest to .

double side = . . .;

double area = Math.sqrt (side);

Use the class name Math before the method name.


Rounding

Math.round rounds to the nearest whole number, as a long.

int i = 5;double d = 3.5;

i = (int) Math.round (d); // Round operationSystem.out.println (i); // Displays 4


Random NumbersProblem: Generate a random integer between 1 and 6 for rolling dice.

Math.random() // Returns a double, d, where 0 <= d < 1

Math.random() * 6 // Returns a double, d, where 0 <= d < 6

(int) (Math.random() * 6) // Returns an int, i, where 0 <= i <= 5

(int) (Math.random() * 6) + 1 // Returns an int, i, where 1 <= i <= 6


Floating point arithmetic

Limited precision:float a = 1;

float b = 100000000; // 100,000,000

float c, d, e;

c = b + 5; // Yields 100,000,008

d = b + (a+a+a+a+a); // Yields 100,000,008

d = b + a+a+a+a+a; // Yields 100,000,000

Less problem with double than float.


CS 303E Class 7 Part 3: Data Structures I:

Objects and Classes"I have a cat named Trash. In the current politicalclimate it would seem that if I were trying to sell him (at least to a Computer Scientist), I would not stress that he is gentle to humans and is self-sufficient, living mostly on field mice. Rather, I would argue that he is object-oriented."- Roger King


What Is a Data Structure?

A data structure is a construct that collects several data items together to be treated as a unit.

Examples:• a string (a collection of characters)• a bank account (a name, ID, and balance)


What is an Object?

An object is a data structure that collects information describing some thing so that a program can manipulate it — a collection of data that can be treated as a unit.

Examples:• a person (name, ID, address, phone number, etc.)• a bank account (name, ID, balance, etc.)• a window (labels, fields, text areas, buttons, etc.)


An Object Contains its own Data

Each object has a separate area in memory and has space for a value for each datum.

E.g., each object of type Person might have space for the following data:

name:

ID:

address:

phoneNumber:

int

Strings


What Is a Class?

A class is a specifcation of —• the data needed in objects of the class type, and• methods for manipulating those objects.

That is, a class is a set of related methods and data.

Examples of classes:• BreezyGUI classes Label, IntegerField,

DoubleField, Button, etc.• String and Math classes


Example — Class DoubleField

Specifies —• all the data needed for an object (window

component) of type DoubleField such as the component’s location and extent, its initial value, the characters it contains, etc., and

• the methods for manipulating objects of type DoubleField, such as addDoubleField, getNumber, setNumber, setPrecision, etc.


Classes and Objects

A class defines the methods and a template for the data for a set of objects.

An object is an instance of a class.

E.g.:

DoubleField

balance transaction interestRate

Class:

Objects:


Software Design with Classes

Like methods, classes are a convenient tool for structuring code — collecting of data (objects) and related methods.

Classes are often useful when we need data structures in a program.

A large system consists of several interacting classes.


Clients and Servers

Code that uses a class is also called a client.

Code that implements a class is also called a server.

To the client, the class provides an abstract data type (ADT), which is a black box that hides information about the details of the class from the client and provides only an interface to objects of the class type.


One File per Class

Java requires a separate file for each class.

Your code will have: A file with code that defines a class. A separate file for code that uses the class. It

will create objects and manipulate them using methods in the class.


Example — Class Student(pp. 159–180)

Analysis (design):

What attributes (data) and behavior (methods) are needed by users for each object in the class (each student)?

Attributes (data) (p. 159):• student name (type String) and• three test scores (each of type int).


Declaring class Student and data

public class Student extends Object{ // Instance variables // Each Student object will have a // name and three test scores: private String name; private int test1; private int test2; private int test3;


Declaring the class and data

public class ClassName extends Object{ // Declare Fields or Instance variables: private type name1; private type name2, name3; // etc.


An Object of type Student

Each object of type Student occupies a separate area in memory and has space for a value for each instance variable:

name:

test1:

test2:

test3:

a String

3 ints


Instantiation —Creating an object of a class

typeCode like the following goes in a program to use a class to create objects of the class type:

String sentence = "A sentence fragment.";

Frame frm = new AccountManager();

george = new Student();


Class Student - Behavior

What can be done with a Student object from the client’s perspective?• instantiation -- create an object of type Student• set the name and the test scores• get the name and the test scores• get the student’s average, highest score, lowest

score, etc.


Methods - Constructors

Constructor methods create or instantiate objects:

// Constructor method // Initialize a new student's name to the

empty // string and his test scores to zero: public Student() { name = ""; test1 = 0; test2 = 0; test3 = 0; }


Design the method’s interface.E.g.:

setName (aString) -- returns void

getName () -- returns String

These are object methods -- the object is implied and supplied before the method name with a dot. Calls:Student stu;stu.setName ("Bill Jones");String stuName = stu.getName();


Implement the Method // Set a student's name

public void setName (String nm)

{ name = nm;

}

// Get a student's name

public String getName ()

return name;

}

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