程序代写代做代考 Java Programming Foundations FIT9131

Programming Foundations FIT9131

FIT9131 Week 3

Programming Foundations
FIT9131

Basic Java constructs:
selection, operators,

variables
Week 3

FIT9131 Week 3

Lecture outline

• selection – if construct
• relational operators
• boolean expressions
• compound statements
• nested ifs
• logical operators
• switch statement
• shorthand arithmetic operators
• local variables
• basic input

FIT9131 Week 3

Reflecting on the naive-ticket-
machine

The naive-ticket-machine is inadequate in several
ways, for instance :
•no check to ensure that the customer has

entered enough money for the ticket;
•no refund given if too much money is entered;
•no check to ensure that the customer enters a

sensible amount of money;
•no check that the ticket price is set to a sensible

amount.

FIT9131 Week 3

Selection construct

We often want a program to do either one thing or another,
depending on whether something is true or not.

We use a selection construct to do this.

A selection construct contains 3 components:
• a condition to be evaluated to true or false
• a consequent (what to do if the condition is true)
• an alternative (what to do if the condition is false).

This component is optional.

Examples in English:
• If you have the flu, stay at home, otherwise go to work.
• If it is raining, take an umbrella.

FIT9131 Week 3

Flow of control (making choices)

A selection construct can change the flow of
control of a program.

Have the
flu?

Stay at
homeGo to

work

Yes

Is it
raining?

Take an
umbrella

Yes

FIT9131 Week 3

Making choices – the IF statement

if (condition)
{

Do these statements if the test gave a true result
}
else
{

Do these statements if the test gave a false result
}

‘if’ keyword
boolean condition to be tested

actions if condition is true

actions if condition is false‘else’ keyword

(the “consequent”)

(the “alternative”)

FIT9131 Week 3

Relational operators
Expressions can be compared using relational

operators. The result is a boolean value (i.e. true
or false).

These are relational operators:
== equal to
!= not equal to
< less than <= less than or equal to > greater than
>= greater than or equal to

A boolean expression is an expression that returns a
boolean value. These are often used as the
conditions in selection constructs.

FIT9131 Week 3

Some boolean expressions
Given the definition:
int age = 20;

What will the following expressions evaluate to?
age == 20
age != 20
age > 20
age >= 20
age < 20 age <= 20 Another example. Given the definition: boolean isStudent = false; The following is also a boolean expression: isStudent FIT9131 Week 3 9 Some boolean expressions Given the definition: int age = 20; What will the following expressions evaluate to: age == 20 Î true age != 20 Î false age > 20 Î false
age >= 20 Î true
age < 20 Î false age <= 20 Î true Another example. Given the definition: boolean isStudent = false; The following is also a boolean expression: isStudent FIT9131 Week 3 10 Some boolean expressions Given the definition: int age = 20; What will the following expressions evaluate to: age == 20 Î true age != 20 Î false age > 20 Î false
age >= 20 Î true
age < 20 Î false age <= 20 Î true Another example. Given the definition: boolean isStudent = false; The following is also a boolean expression: isStudent Î false FIT9131 Week 3 11 Use of relational operators if (age >= 18) …

if (response == ‘Y’) …

if (price <= 0) … if (isStudent) … if (postcode.length() < 4) … FIT9131 Week 3 12 Analysing the insertMoney method Currently the insertMoney method of the TicketMachine class adds the amount of money entered to the balance without checking that it is a sensible value: public void insertMoney(int amount) { balance = balance + amount; } We would like to check that a positive amount is entered before we add it to the balance. If a negative amount is entered then we will display an ERROR message. FIT9131 Week 3 13 Digression – using a Java method When we want to use a Java method, we typically use one of these following terms : • invoke the method • call the method • execute the method They all mean the same thing!! In BlueJ, we would perform this by right-clicking the object, then selecting the method we wish to invoke/call/execute. In later weeks, we will also learn how to do this by writing the code directly. FIT9131 Week 3 14 Improving the insertMoney method Here is a selection construct that would do this: public void insertMoney(int amount) { if (amount > 0)
balance = balance + amount;

else
System.out.println(“Use a positive amount: ” +

amount);
}

Note : the if statement itself does not return a value.

FIT9131 Week 3

The if statement
The alternative for the if statement is optional.

public void insertMoney(int amount) // another version
{

if (amount > 0)
balance = balance + amount;

else
System.out.println(“Use a positive amount: ” +

amount);
if (balance >= price)

System.out.println(“Thanks. Enough money entered”);
}

Example of an “if” statement
without an “else” part

FIT9131 Week 3

Compound statements
Sometimes a consequent or an alternative of an
if has more than one statement in it. For
instance, in English, we may say :

if you feel sick
– go to the doctor (and)
– notify your employer that you will be away

Both these statements are part of the consequent.
There is no alternative in this example.

In Java, we would “group” statements together
into a compound statement, delimited by braces.

FIT9131 Week 3

Compound statements in Java
public void insertMoney(int amount)
{

if (amount > 0)
{

balance = balance + amount;
System.out.println(“Your balance is ” + balance);

}
else

System.out.println(“Use a positive amount! “);

…

} Example of a compound statement , consisting of
2 simple statements (note
the use of the braces to
enclose the statements)

Also note the way the code
has been formatted (via
the indentations)

FIT9131 Week 3

Nested if statements

public void showCategory()
{

if (age >= 14)
if (isStudent)

System.out.println(“Student”);
else

System.out.println(“Adult”);
else

System.out.println(“Child”);
}

Note: In this example we assume that age has been defined as
type int and isStudent has been defined as type boolean.

The consequent of an if,
like the alternative, may be
a simple statement, or a
compound statement, or
another if statement.

Again note the way the
code is indented)

Eg :

FIT9131 Week 3

Bad formatting

public void showCategory()
{
if (age >= 14)
if (isStudent)
System.out.println(“Student”);
else
System.out.println(“Adult”);
else
System.out.println(“Child”);
}

This is the exact same code
from the previous slide.
Which one is more readable?

Make sure you read (and
follow) the FIT9131 Java
Coding Standards!!

FIT9131 Week 3

Another cascaded if statement
example

public void showCategory()
{

if (age < 14) System.out.println("Child"); else if (isStudent) System.out.println("Student"); else System.out.println("Adult"); } FIT9131 Week 3 21 Logical operators Boolean expressions can be combined using the logical operators not (!), or (||) and and (&&). if (!isStudent) ... if (age >= 20 && !isStudent) …
if (age < 15 || isStudent) ... if ((age == 18 && isStudent) || isPensioner) ... The result of an and is true only if both operands are true. The result of an or is false only if both operands are false. A not results in the logical opposite of its operand. FIT9131 Week 3 22 Truth Tables AND true false true true false false false false OR true false true true true false true false These 2 tables show the results when 2 boolean values are operated upon by the AND/OR operators. In programming, we typically call these values “operands”. FIT9131 Week 3 23 Another selection construct Java has another selection construct called a switch statement. The switch statement is a shorthand way of writing a nested if. if and switch statements do not return values. FIT9131 Week 3 24 Flow of control (switch) Day of the week = 1 Display “Monday” Display “Sunday” Yes No Day of the week = 2 Display “Tuesday” Yes No … Day of the week = 3 Yes FIT9131 Week 3 25 Example : Printing the name of a day public void printDayName(int dayOfWeek) { if (dayOfWeek == 1) System.out.println("Sunday"); else if (dayOfWeek == 2) System.out.println("Monday"); else if (dayOfWeek == 3) System.out.println("Tuesday"); else ... System.out.println("Invalid day"); } This example uses a nested if statement. The result is not very tidy (it does work!). FIT9131 Week 3 26 Tidier : switch statement public void printDayName(int dayOfWeek) { switch (dayOfWeek) { case 1: System.out.println("Sunday"); break; case 2: System.out.println("Monday"); break; case 3: System.out.println("Tuesday"); break; case 4: System.out.println("Wednesday"); break; case 5: System.out.println("Thursday"); break; case 6: System.out.println("Friday"); break; case 7: System.out.println("Saturday"); break; default: System.out.println("Invalid day value"); break; } } What happens if we leave out the break statement(s) ? The same example using a switch statement; makes the code much tidier. FIT9131 Week 3 27 When to use a switch statement? A switch statement can be used only when: • the expression in the condition is being compared for equality with the values in the case statements. Note: Prior to Java 7, the expression in the switch condition had to contain an int or a char. As of Java 7, the switch condition can also contain a String type FIT9131 Week 3 28 Multiple return statements A method can return only one value. As soon as it has returned the value, the method has finished. It is possible to write a method with more than one return statement in it, but of course only one of them can be executed on each call! Note that it is good coding practice to only use one return statement in a method. FIT9131 Week 3 29 Example : Multiple return statements in a method This method tests to see if a year is a leap year. It returns true if it is. public boolean isLeapYear(int year) { if (year % 4 > 0)
return false;

if (year % 400 == 0)
return true;

if (year % 100 == 0)
return false;

return true;
}

Note that only ONE of
these return statements
will be executed at any one
time.

FIT9131 Week 3

Alternative code using single
return statement

public boolean isLeapYear(int year)
{

boolean result = true;
if (year % 4 > 0)

result = false;
else

if (year % 400 == 0)
result = true;

else
if (year % 100 == 0)

result = false;
return result;

}

public boolean isLeapYear(int year)
{

if (year % 4 > 0)
return false;

if (year % 400 == 0)
return true;

if (year % 100 == 0)
return false;

return true;
}

Which
version is
clearer?

FIT9131 Week 3

Shorthand operators for integers

The arithmetic operators have shorthand equivalents:

int number = 0;

number += 1 is equivalent to number = number + 1
number -= 1 is equivalent to number = number – 1
number *= 2 is equivalent to number = number * 2
number /= 3 is equivalent to number = number / 3
number %= 3 is equivalent to number = number % 3

FIT9131 Week 3

The ++ operator

Java has another shortcut for the statement:
numberOfLines = numberOfLines + 1;

which uses the ++ (increment) operator:
numberOfLines++;

There is also a –- (decrement) operator.
numberOfLines–;

which is a shortcut for:
numberOfLines = numberOfLines – 1;

It is best to use the
++ and – – operators
in statements by
themselves, not as
part of expressions.

FIT9131 Week 3

Placement of ++ and —

The ++ operator increments a variable, but when
this happens depends on the position of the ++.

• number++ increments number after its original
value is used in an expression.

• ++number increments number before its original
value is used in an expression.

The same applies to the placement of the –-
operator, e.g. –number and number–

FIT9131 Week 3

What will the following code display?

int number = 3;
System.out.println(++number);
number = 3;
System.out.println(number++);
System.out.println(number);

Placement of ++ and —

FIT9131 Week 3

What will the following code display?

int number = 3;
System.out.println(++number);
number = 3;
System.out.println(number++);
System.out.println(number);

4
3
4

Placement of ++ and —

FIT9131 Week 3

Precedence rules for evaluating
expressions

Parentheses (brackets)
Unary operators ++ — !
Binary arithmetic operators * / %
Binary arithmetic operators + –
Relational operators < > <= >=
Relational operators == !=
Logical operator &&
Logical operator ||

Highest
precedence

Lowest
precedence

FIT9131 Week 3

Local variables

public int refundBalance()
{

int amountToRefund;
amountToRefund = balance;
balance = 0;
return amountToRefund;

}

A local variable

No visibility
modifier

Methods can include local variables which exist only as
long as the method is being executed.

• they are only accessible from within the method.

FIT9131 Week 3

Scope and lifetime
The scope of a variable defines the section of the

source code from which the variable can be
accessed:

•A formal parameter is available only within the
body of the constructor or method that declares
it.

•A field can be accessed from anywhere in the
same class.

•The scope of a field can be modified using an
access modifier (also called a visibility modifier).
More about this later …

FIT9131 Week 3

Scope and lifetime

The lifetime of a variable describes how long a
variable can exist before it is destroyed:

•A formal parameter exists for the duration of
the execution of the constructor (or method).

•A field exists while the object to which it
belongs exists.

FIT9131 Week 3

Fields, parameters and local
variables

Definition Scope Lifetime

fields Defined outside
constructors and
methods

Within any methods
or constructors of the
class (if private) in
which they are
defined

Exist while their
object exists

parameters Defined in the
header or the
method or
constructor

Within the body of
the method in which
they are defined

Exist while the
method or
constructor
executes

local
variables

Defined within
the body of a
method or
constructor

Within the block in
which they are
defined, starting from
the line where they
are defined

Exist while the
method or
constructor
executes

FIT9131 Week 3

The System class

The System class contains some “constants” which
represent the standard input device and standard
output device (typically keyboard and screen) and
methods to enable input and output.

The System class is part of the java.lang package.

FIT9131 Week 3

Output to the screen

The standard output device is represented in the
System class as ‘out’. The methods print() and
println() are used to display string arguments
to the standard output device.

//Display a string literal
System.out.print(“Enter cat name”);

//Display a string literal and then a
carriage return
System.out.println(“Enter cat name”);

Note : the “.print(…)”
here means “call the print
method of the System.out
class.

FIT9131 Week 3

Basic input

In the code that we have written so far we have
assigned values to variables in the code.

But how do we read in values from the user?
This can be a complex process, one that is difficult

for novice Java programmers.

Java, however, has some built-in ways that it can
help us get input from the user … we will use the
Scanner class.

FIT9131 Week 3

Basic input

The Scanner is a pre-defined Java class that we
can use to obtain user input from the keyboard.

This requires the following steps:
1. import the Scanner class into our program
2. create a new Scanner object
3. invoke methods to read values from the

Scanner object

FIT9131 Week 3

Digression : creating an object
without using BlueJ

In Java,we use the new keyword to create an object with
our code, for example :

Scanner scannerName = new Scanner(System.in);

Student newStudent = new Student(“andy”, “1234”);

class name object name creating the object

FIT9131 Week 3

Step One: Import the Scanner

In order to use the Scanner class, we need to
explicitly tell Java to “import” it into our
program.

We do this by including an import statement at
the very top of our code, above our class, Eg :

import java.util.*;

public class Welcome
{

…
}

FIT9131 Week 3

Step Two: Create a Scanner object

We then need to create a Scanner object, such as :

Scanner scannerName = new Scanner(System.in);

import java.util.*;
public class Welcome
{

Scanner console = new Scanner(System.in);
}

Note:
1. in this case the Scanner object is named “console”.
2. System.in is the standard input source (keyboard)

FIT9131 Week 3

Step Three: Reading in data

We can now use methods from the Scanner object to read
data in from the terminal. These allow us to specify the
type of data to be read in:

scannerName.nextLine() … for Strings including
whitespace characters

scannerName.next() … for Strings up to the first or
next whitespace character encountered

scannerName.nextInt() … for ints
scannerName.nextDouble() … for doubles
scannerName.nextLine().charAt(0) … for chars

Most of these commands will read the next piece of data, up
until a white space character.

FIT9131 Week 3

Step Three: Reading in data

Typically we will assign what we read in from the
user into a variable (or series of variables)

For example:

Scanner console = new Scanner(System.in);
int age;

System.out.println(“Please enter your age”);
age = console.nextInt();

creating
the scanner
object

using the scanner object to read in
an integer, and then assigns it to a
variable named age

FIT9131 Week 3

Basic input example
So putting it all together :

import java.util.*;
public class InputTest
{

public void displayInput()
{

Scanner console = new Scanner(System.in);

int number1 = 0;
char letter1 = ‘a’;
System.out.println(“Please enter an int and a char ” +

“separated by a space”);
number1 = console.nextInt();
letter1 = console.next().charAt(0);
System.out.println(“You entered ” + number1 +

” and ” + letter1);
}

}

FIT9131 Week 3

Assignment 1 Released
Assignment 1 will be released this week

Progress by next week:
From last week’s material

• All class skeletons can be completed

From this week’s material
• Basic input structures can be completed

Remember to work logically in stages each week.

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