© Xiaoying Gao, Peter Andreae

Class, method, statements

COMP 102 #3 2014T2

Xiaoying Sharon GaoComputer Science

Victoria University of Wellington

© Xiaoying Gao, Peter Andreae

COMP 102 3:2

Menu• More examples and a slightly big program• Semantics (meaning) and Syntax (grammar rules)• Class, Methods, Statements• Variables, Types, Expressions

Announcements:• Assignment 1 is due next Wednesday 11am.

• Use lab computer, BlueJ, submit assignments • Go to the labs even you have not signed up.

• Lab A: Thursday 3-4, 4-5, Fri 12-1• Lab B: Monday 4-5, Tuesday 12-1, 2-3, Wed 9-10

• Optional tutorial• Friday 9-10, CO219 with Tim, starting week 3

© Xiaoying Gao, Peter Andreae

Example 2

/** An example for simple string Inout/Output. The way to generate loginName is much more simplified */ import ecs100.*;public class Example2{ public void printMessage(){ String course = "COMP102"; String name = UI.askString("type your first name: "); String family = UI.askString("your family name? "); UI.println("Hello "+ name + ", Welcome to " + course); String loginName = family.substring(0,2) + name; UI.println("your Login name is " + loginName); } }

COMP 102 3:3

© Xiaoying Gao, Peter Andreae

DrawLabel

import ecs100.*;public class DrawLabel {

public void drawSimple() { double x = 100; double y = x + 200; double w = 223; double h = w/2; UI.drawRect(100,200,50,50); UI.drawRect(x, y, w, h); }}

COMP 102 3:4

© Xiaoying Gao, Peter Andreae

COMP 102 3:5

A Java Program

Task: Write a temperature conversion program: C ⇔ F

1: Specification: what is it supposed to do?

• Write a program that will let the user do three things:• print out the conversion formula• let user enter temperature in Fahrenheit, and print out in

Celsius.• let user enter temperature in Celsius, and print out in

Fahrenheit

2: Design:• For calculate actions:

• Ask user for the value to be converted• Calculate new values out of old value (F-32)*5/9 or

C*9/5+32• Print out the answer

© Xiaoying Gao, Peter Andreae

COMP 102 3:6

Designing the Java program

3: Editing

• Need to write this design in the Java language.

• We need an object (a "temperature calculator") to do

the actions

⇒ we need a class to describe the object

• The object needs three actions

⇒ define three methods

⇒ specify what the methods will do

© Xiaoying Gao, Peter Andreae

Writing the Java codeimport ecs100.*;/** Program for converting between temperature scales */public class TemperatureCalculator{

/** Print conversion formula */public void printFormula ( ) {

UI.println("Celsius = (Fahrenheit - 32) *5/9");}/** Convert from fahrenheit to centigrade */public void fahrenToCelsius(){

double fahren = UI.askDouble("Farenheit:");double celsius = (fahren - 32) * 5 / 9;UI.println(" -> " + celsius + " C");

}/** Convert from centigrade to fahrenheit */public void celsiusToFahren(){

double celsius = UI.askDouble("Celsius:");double fahren = celsius * 9 / 5 + 32;UI.println(" -> " + fahren + " F");

}}

COMP 102 3:7

© Xiaoying Gao, Peter Andreae

Compiling and Running

4: Compiling• If there are syntax errors (invalid Java)

then the compiler will complain and list all the errors⇒ read the error message to work out what's wrong⇒ fixing syntax errors until it compiles without complaint

• BlueJ makes this process easier

5: Running and Testing• Must run the program and test it on lots of different

input.

• BlueJ makes it easy to run individual methods.

COMP 102 3:8

© Xiaoying Gao, Peter Andreae

COMP 102 4: 9

Writing your own programs

How?

• Use other programs as models• Very useful strategy• Limiting• Hard to work out what's wrong

• Understand the language – rules and vocabulary• Unlimited• Able to understand and reason about your program

© Xiaoying Gao, Peter Andreae

COMP 102 4: 10

Class syntax pattern

〈 import statements〉

public class

〈method descriptions〉

{

}

〈 classname〉

Comments can be added anywhere

import comp102.*;

import java.awt.Color;

© Xiaoying Gao, Peter Andreae

COMP 102 4: 11

Comments

Three kinds of comments:

• Documentation comments

eg /** Program for converting between temperature scales */

• end-of-line comments

eg double fahren = celsius * 9 / 5 + 32; // compute answer

• anywhere comments

eg /* double fahren = celsius * 9 / 5 + 32; UI.println(" -> " + fahren + " F"); */

/** 〈 text of comment〉 */

// 〈 text of comment〉

Top of class, Before each method

at end of any line

/* 〈 text of comment〉 */

multi-line, or middle of line, or …

© Xiaoying Gao, Peter Andreae

COMP 102 4: 12

Method Definitions

/** Print out the conversion formulas */

public void printFormula ( ) {

UI.println("Celsius = (Fahrenheit - 32) *5/9");

}

〈 Comment〉 〈 Header〉 〈 Body〉{ }

public void ( )〈 name〉

instructions to perform this action

Specifying the information the action needsMaybe empty

© Xiaoying Gao, Peter Andreae

COMP 102 4: 13

“Statements” (instructions)

(Single instructions are called “statements” for silly historical reasons!)

Two important kinds of statements:

• method call statement:• tell some object to perform one of its methods.

eg: tell the UI object to ask the user for a numbereg: tell the UI object to print out a stringeg: tell the UI object to draw an oval

• assignment statement• compute some value and put it in a place in memory.

© Xiaoying Gao, Peter Andreae

COMP 102 4: 14

Method Calls

UI.println( "Celsius = (Fahrenheit - 32) *5/9" );

• Method call Statement: who . what ( data to use ) ;

UI . println ( “Celsius = (Fahre…” ) ;

• Meaning of Statement:• Tell the object

to perform the method using the argument values provided

〈 object〉 〈methodname〉 〈 arguments〉. ( ) ;

© Xiaoying Gao, Peter Andreae

COMP 102 4: 15Objects and their methods in Java• What objects are there?

Predefined• UI a "User Interface" window with

several panes println(….), askString(…), drawRect(…), clearGraphics()

• System representing the computer system

currentTimeMillis()

• Math methods for mathematical calculations

random(), sin(…)

Others• The object(s) defined by your program• New objects that your program creates

Some method calls return a value

© Xiaoying Gao, Peter Andreae

COMP 102 4: 16

Data types

There are lots of different kinds of values:

• Numbers • Integers ( int or long) 42 -194573203• real numbers ( double or float ) 16.43 6.626e-34• …

• Characters ( char ) 'X' '4'

• Text ( String ) " F -> "

• Objects

• …

© Xiaoying Gao, Peter Andreae

COMP 102 4: 17

Variables

int x = 100;

UI.println(“x is ”, x);

x = x + 1;

UI.println(“x is ” +x);

• A variable is a place that can hold a value.• Must specify the type of value that can be put in the variable

⇒ “Declare” the variable.• Must put a value into a variable before you can use it

⇒ “assign” to the variable• Can use the value by specifying the variable’s name • Can change the value in a variable

(unlike mathematical variable)

© Xiaoying Gao, Peter Andreae

COMP 102 4: 18

Assignment Statements

double fahren = UI.askDouble("Farenheit:");double celsius = (fahren - 32) * 5 / 9;

• Assignment Statement: where = what ;

name-of-place = specification-of-value ;double celsius = (fahren - 32) * 5 / 9;

Compute the value and put it in the place

〈 variable〉 〈 expression〉= ;