programming refresher module - www …alcc/pref/lectures/pref-2016-17week01... · programming...

PROGRAMMING REFRESHER MODULE

Delivered by Ana Cavalcanti and Alvaro Miyazawa

Prepared by Dr Lilian Blot

The Rules

• Everything in PREF (Week 1) is optional.

– From week 2 of APRC and JAPC, we assume that all that has been taught in week one is mastered.

– The material will not be covered in any other lecture.

• From Week 2:

– The practical sessions are mandatory, you must attend.

– Lectures are always optional (but strongly recommended).

A n a . C a v a l c a n t i @ y o r k . a c . u k

A l v a r o . M i y a z a w a @ y o r k . a c . u kContact

BE AWARE

• Earlier editions will confuse you!

– 5th or earlier edition are bad!

• Collections have changed

• Integer, Double and others have changed.

– 3rd or earlier is very bad!!

• Swing has changed

Textbook

• Java How to Program: Early Objects Version (10th

Edition) by Paul and Harvey Deitel.

• The pages and sections of APRC and JAPC may match just the 9th Edition.

• 6th-8th Editions

Okay.

Why Java?

• One language for one degree programme

• Cross-platform

• Good open-source tools

• Simpler than many rivals (for instance, C++)

TODAY’S OBJECTIVES

• Java Basics part 1

• Writing our first Java program

• Types and Variables

• Control Structure

– A Selection statement: IF-ELSE IF-ELSE

In this section we will be writing our first Java program

– A classic “Hello World”

Section 1First Program

NOTE

All names have rules to follow:

• No punctuation (except underscore) and no spaces

• Do not start with a number.

• Java file name must be same as class name.

• Convention: Each word in class name starts in uppercase.

Class

public class HelloWorld {

. . .

}

• At least one class per java file

– Starts with keywords public class

– Followed by class name

• The name of the file must be the name of the class followed by the extension .java.

NOTE

The main method always has same signature, whereas other methods differ. We will look into that a little bit later this week.

Class


public static void main(String[ ] args) {

. . .

}

}

• Java classes are structured into methods.

• Each Java application must have one main method.

.

NOTE

Statements are always terminated by semicolon. However they may span over several line of codes when they are complex expressions.

Class


public static void main(String[ ] args) {

System.out.println("Hello World!");

}

}

• Statements consist of construct and expression:

– Construct is the command.

– Expression is the data to be enacted upon.

.

Section 2 – The Eclipse IDE

Eclipse Integrated Development Environment (IDE)

• Coloured text (“syntax highlighting”)

• Background compilation

• Syntax errors notified as you type

• Auto completion

• Task setting

• Project Package explorer

• Many other views

Objectives

• What is a variable?

• What is a type?

• How to declare a variable?

• What are the different types in Java?

– Primitives

– Objects

Section 3Declarations, Variables & Types

NOTE

• All declarations begin with a type (primitive or class).

• One or more variables can be declared at same time.

• Any variable can be initialised at declaration.

• Variable names:– No spaces– Only punctuation symbol is

the underscore e.gfirst_Name

– Must not be same as a reserved word.

• Conventions:– Should start with a lowercase

letter.– Each subsequent word begins

in uppercase.

Variable Declaration

• Formattype identifier [= initialValue];

– Note the square brackets denote optional initialisation

• Examplesint num;

int num, result;

int num=0, result;

NOTE

• Constant names follow the same rules as variables, but different conventions:– Name is entirely written in

uppercase,– With an underscore

separating different words in the name.

• These conventions make the code easier to read, and therefore should be followed.

Constant Declaration

• If the value stored in a memory location will not change during program run, store the value in a constant.

• Constants must be declared and initialised at same time.

• Formatfinal type identifier = initialValue;

• Examplefinal double INTEREST_RATE = 0.2;

Data Type Examples

• Some data types start with an upper case letter and some with a lower case letter.

– Types starting with a upper case letter are Objects.

– Types starting with lower case letter are Primitives.

• Java supports 8 primitive types.– Integer types (byte, short, int, long)

– Floating point types (float, double)– A character type (char)– A Boolean type (boolean)

Type of data Examples Data Type

A single character (letter) ‘a’ char

A word or words“apple”

or

“two aces”

String

Combination of characters and numbers

“CS 103” String

Nothing (Empty String) “ ” String

Integers63 or

0 or

-63

int

Real number (number with decimal point)

3.14 or

-3.14double

True or Falsetrue

or

false

boolean

Data Type Examples

• Java also supports the String type– Technically this is a class type (discussed

next week).– But is (mostly) used as if it were a

primitive type.

Note:

• char literal values use single quotes ‘ ’ and• String literal values use double quotes “ ”

Type of data Examples Data Type

A single character (letter) ‘a’ char

A word or words“apple”

or

“two aces”

String

Combination of characters and numbers

“CS 103” String

Nothing (Empty String) “ ” String

Integers63 or

0 or

-63

int

Real number (number with decimal point)

3.14 or

-3.14double

True or Falsetrue

or

false

boolean

BE AWARE

Any value can be converted to a String, but not all String values can be parsed to a primitive.

Type Conversion

• Type casts can be applied to convert a variable from one type to another type– Format: (dataType) variableName– Example: double result = 5.2;

int rounded = (int) result;• Rounded is assigned the int value 5.

• We can convert String values to primitive type values by means of parse methods– Example: int num = Integer.parseInt("9");

• Primitive values can be converted to string by toString() method in wrapper classes.– Example: String five = Integer.toString(5);– Or by concatenation: String five = "" + 5;

Objectives

• Build the program Hello <name>

• Program Specification

– Ask the user to enter their name, then print the words “Hello <name>!” to the console.

Section 4Input, Libraries & Packages

Simple view of the program

The Code

public class HelloName {

public static void main(String[] args) {

//create scanner object

Scanner sc = new Scanner(System.in);

//prompt user

System.out.print("Please enter your name:\t");

//get input and print string

System.out.println("Hello " + sc.next() +

"!");

}

}

Processing

Input

Output

Simple view of the program

The Modified Code

import java.util.Scanner;

public class HelloName {

public static void main(String[] args) {

//create scanner object

Scanner sc = new Scanner(System.in);

//prompt user

System.out.print("Please enter your name:\t");

//get input and print string

System.out.println("Hello " + sc.next() +

"!");

}

}

Processing

Input

Output

Tell the compiler where to find the code for a Scanner,

and add it to our program. java.util.Scanner is a Java

library.

Objectives

• What are the Java operators?

• What is an expression?

• How to declare a variable?

Section 5Operators & Expressions

NOTE

• As in Mathematics, when more than one operator appears in an expression, the order of evaluation depends on the rules of precedence.– the result of 3 + 4 * 2

is 11, not 14.

• Parentheses have the highest precedence:– the result of (3 + 4) * 2 is 14, not 11

– use of parentheses is encouraged in long and complex expressions.

Expressions

• An expression is a combination of values, variables, and operators

• Operators are special symbols that represent computations like addition and multiplication.

• The values the operator uses are called operands.

• Expressions can be combined to create a more complex expression.

• For example:<expression_1> operator <expression_2>

(age * 10) + (heightCM / 100)

(“Dr” + name) + (“>” * 3)

Mathematical Operators

• On the table on the right you can see mathematical operators and their precedence.

• The ( + ) operator is also the concatenation of two String operator

– “Dr” + “ “ + “Blot” is a valid expression. The result is the String “Dr Blot”.

• Note the special case for the Assignment Operator ( = ):

– The left-hand side of the assignment operator has to be a variable, not an expression.

Operator Name

P

r

e

c

e

d

e

n

c

e

+, -Unary plus and minus (for example, -10)

*, /, %Multiplication, division, integer division, and remainder

+, -Binary plus and minus (for instance, 3-10)

= Assignment operators

24

Assignment and Expressions

• Example

String firstname = “Lilian”;

String lastname = “Blot”;

String title = “Dr”

String name = firstname+ “ “ +lastname;

name = title + “ “ + name;

Code

25


• Example



String title = “Dr”;

Code

“Lilian”firstname

lastname

title“Dr”

“Blot”

Name space Memory

26


• Example





Code


lastname

title“Dr”

“Blot”

firstname

lastname

“Lilian” + “ “ + “Blot”

“Lilian Blot”name

Name space Memory

27


• Example





name = title + “ “ + name;

Code


lastname

title“Dr”

“Blot”

“Lilian Blot”name

“Dr” + “ “ + “Lilian Blot”

title

name

‘Dr Lilian Blot’

Name space Memory

Main Scanner Class Methods

Method Description

nextInt()Reads the next input value as an

integer

nextDouble()Reads the next input value as a

double

nextBoolean()Reads the next input value as a

boolean

next() Reads the next input value as a String

hasNextInt()Returns true if next input value is an

integer

hasNextDouble()Returns true if next input value is a

double

hasNextBoolean()Returns true if next input value is a

boolean

hasNext()Returns true if there is another input

value to read

Today’s Problem

• We would like to write a program taking a measurement in meters (respectively, Feet, inches) as input and convert it to Feet, inches (respectively, meters).

– We need to find a way for the user to tell us which conversion he/she wants to do.

– Depending on the user SELECTION we need to do one operation or the other.

– We haven’t seen such a structure yet.

29

30

• Straight line programs: sequence of commands executed one after the other one, and only once.

– Cannot do many interesting things.

• A Control Structure for Branching:

– Selection statement if - else if - else.

Section 6Control Structure part I

31

Motivation

• Straight line programs: sequence of commands executed one after the other, and only once.

– Cannot do many interesting things if any.

• Branching: A program can decide which statements to execute based on a condition.

– If condition is True, execute A1 to An

– If condition is False, execute B1 to Bk

Statement B1

…

Statement Bk

Statement A1

…

Statement An

True False

Condition

32

Motivation






Statement B1

…

Statement Bk

Statement A1

…

Statement An

True False

Condition

33

Motivation






Statement B1

…

Statement Bk

Statement A1

…

Statement An

True False

Condition

34

Motivation






Statement B1

…

Statement Bk

Statement A1

…

Statement An

True False

Condition

35

Motivation






Statement B1

…

Statement Bk

Statement A1

…

Statement An

True False

Condition

36

Motivation






Statement B1

…

Statement Bk

Statement A1

…

Statement An

True False

Condition

37

Motivation






Statement B1

…

Statement Bk

Statement A1

…

Statement An

True False

Condition

38

Motivation






Statement B1

…

Statement Bk

Statement A1

…

Statement An

True False

Condition

39

Motivation






Statement B1

…

Statement Bk

Statement A1

…

Statement An

True False

Condition

40

Motivation






Statement B1

…

Statement Bk

Statement A1

…

Statement An

True False

Condition

41

Motivation






Statement B1

…

Statement Bk

Statement A1

…

Statement An

True False

Condition

42

Motivation






Statement B1

…

Statement Bk

Statement A1

…

Statement An

True False

Condition

43

Schema

Code

…

if (condition) {

Statement A1;

…

Statement An;

} else {

Statement B1;

…

Statement Bk;

}

…

Statement B1

…

Statement Bk

Statement A1

…

Statement An

True False

Condition

44

Schema

Code

…

if (condition) {

Statement A1;

…

Statement An;

} else {

Statement B1;

…

Statement Bk;

}

…

Statement B1

…

Statement Bk

Statement A1

…

Statement An

True False

Condition

45

Schema

Code

…

if (condition) {

Statement A1;

…

Statement An;

} else {

Statement B1;

…

Statement Bk;

}

…

Statement B1

…

Statement Bk

Statement A1

…

Statement An

True False

Condition

46

Schema

Simple if-statement

…

if (condition) {

Statement A1;

…

Statement An;

}

statement X;

statement Y;

…

Statement A1

…

Statement An

True

False

Condition

Statement X

Statement Y

…

47

Schema

if-else if-else statement

…

if (cond_1) {

Statement A;

} else if (cond_2) {

Statement B;

}

…

} else if (cond_n) {

Statement C;

} else {

Statement D;

}

…

Statement A

True False

cond_1

cond_2

cond_n

Statement B

Statement C Statement D

True

True

False

False

48

Schema

Nested if-else statements

…

if (cond_1) {

Statement A;

} else {

Statement B1;

Statement B2;

if (cond_2) {

Statement C;

} else {

Statement D;

}

}

…

Statement A

True False

cond_1

cond_2

Statement B1

Statement B2

Statement C Statement D

True False

49

Note

• The ternary operator makes the code more difficult to read. It should be used only when the conditions are simple, and the statements are very short.

• Use it sparingly.

Ternary Operator

• Format:

(condition) ? true-code : false-code

• ?

– Simplified (short hand) alternative to the if-elsestatement.

– Must be used as part of a statement.

– Useful for assigning one of two values depending on some condition.

• Example

grade = (mark >= 50) ? "pass" : "fail";

• A boolean expression returns true or false.

• The returned value is of the Java booleantype (a primitive type), a data type that can take only two values, true or false (case sensitive).

Boolean Expression

NOTE

When comparing to a variable, you must ensure the types are compatible. A compiler error will occur if you try to compare a intto a String.

Beware, confusing the Assignment operator = and the Equality operator == is a common mistake.

Comparing Numbers

• We can compare numbers using comparators such as: >, <, >=, <=, ==, !=.

• Values in variables can be compared too, and the result can be stored in a variable.

boolean a = 10 < 7; // a contains the value false

boolean b = 10 >= 7; // a contains the value true

code

int small = 6;

int big = 2000;

boolean is_greater = (big >= small); // true

code

52

Examples

• Assume: int num = 10;

Expression Result Explanation

(num == 10) true num is equal to 10

(num == “A”) error Different data-types

(num != ‘A’) error Different data-types

(num != 11) true num is not equal to 11

(num > 9) true Greater than test – num is larger value

(num >= 11) false Greater/Equal test – num is smaller

(num < 11) true Less than test – num is smaller

(num <= 10) true Less/Equal test – num is equal to 10

Truth Tables

Boolean Expression composition

• Boolean expressions can be combined to form complex expressions.

• composition can be done using three operators:

– && (and), || (or), ! (not)

• Refer to the truth tables on the right to see the result of composition of two boolean expressions.

&& True False

True True False

False False False

|| True False

True True True

False True False

!True False

False Truediscount = ((age < 26) || (isStudent == true));

resit = !((average >= 40) && (compensable < 40));

Code sample

Operator Precedence (cont.)

• In the table on the right you can see the rule of precedence for the booleanoperators.

• There are more operators available in Java. You are encouraged to read about them.

Operator Name

P

r

e

c

e

d

e

n

c

e

+, -Unary plus and minus (e.g. -10)

!not

*, /, %

Multiplication, division, integer division, and remainder

+, -Binary plus and minus (e.g. 3-10)

<, <=, >=Comparison

==, !=Equality, inequality

&&and

||or

=, +=, -=, *=, /=, %=

Assignment operators

Summary

• You should now be able to develop very simple programs, taking input from user and producing an output.

• You should be familiar with the concepts of Values, Variables and Data Types.

• You should understand Operators and the ‘rules of precedence’, and be able to create more complex expressions using them.

55

Exercises

• Give a truth table that shows the (boolean) value of each of the following Boolean expressions, for every possible combination of “input” values.

a) !(P && Q)

b) (!P) && Q

c) (!P) || (!Q)

d) (P && Q) || R

e) (P || R) && (Q || R)

Hint: including columns for intermediate expressions is helpful.

56

Exercises

• Here is some practice on writing complex Boolean expressions. Your task is to convert the English description into a Java Boolean expression. The variables to be used in the expressions are:

– dogSize with String values "small", "medium", and "large“.– dogHasSpots with Boolean values true or false.– dogAge with positive integer values.– dogColor with String values "white", "black", "red", and "brown“.– catSize with String values "small", "medium", and "large“.– catHasSpots with Boolean values true or false.– catColor with String values "white", "black", "orange", and "tabby“.– catEyeColor with String values "green", "brown", "blue“.– catAge with positive integer values.

• An old dog is one with an age greater than seven years while an old cat has an age greater than or equal to nine years. A cat and a dog are young if they are younger than three years old.

57

Exercises

• For the time being, and for simplicity we consider that we can compare two Strings using:

– !=, ==, <, >, >=, <=.

– these ARE NOT the correct Java operators for comparing Strings; we will see the proper comparator during the next lecture.

• Write a Boolean expression that captures

– Dogs that are large and have spots or cats that are white but do not have blue eyes.

– Young cats and dogs of small size and with brown eyes.

– Old cats and dogs, where cats are white and dogs are not black.

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