Java basics variables

Objectives
1. Understand how to use variables

2. Recognise the different datatypes

1. Use a range of arithmetic operators

2. Learn how to call a subroutine

3. Learn how to round a number

4. Become more confident writing your original
JAVA programs

Using Variables
1. What will the output be for this code? The answer is: 17

public class AddingExample
{ Before you can use them in your code
public static void main(String[] args) you must declare your variables. This
{ includes writing two things:
int num1; // declaring the variables
int num2; The DATATYPE of the variable (e.g int)
int result; The IDENTIFIER (name) of the variable

num1 = 10; // assigning values to the variables Once declared you
num2 = 7; can then just use the
result = num1 + num2; identifier to change
the value or use it in
System.out.println(“The answer is: ” + result); calculation etc
}
}

Using variables
When you declare a variable the computer: MEMORY
• Allocates some memory large enough to hold the data num1
• Assigns that memory block the identifier you entered
result

num2

When you assign a value with the = sign MEMORY
• The value is stored in the memory location for that variable num1 10
result 17

num2 7

Datatypes
There are several datatypes you can use
Data type Description Example Storage required

Stores a collection of characters Varies depending
String “Computer”
(numbers, text etc.) on no. of chars
A single character (text, number,
char „6‟, „F‟, etc 1 byte
symbol etc)

int A whole number 5 2 bytes

double A decimal number 2.5 4 bytes

boolean Stores either TRUE or FALSE TRUE 1 byte

String greeting = “Hello"; The first two lines of code to the left declare
a variable AND then assign a value to them.
boolean passed = false;
The last example would have a value
double percentageScore; assigned later in the program

Creating text variables
1. What is the name of the class?
public class StringVariables
StringVariables
{
2. What are the names of the public static void main(String[] args)
variables? {
greeting, name String greeting = “Hello";
System.out.println( greeting );
3. What is the data type for these
variables String name = “Computing Students";
String (which means text) System.out.println( name );
}
4. What will the output be for this }
code?
Hello When this program is run:
Computing Students
• A variable called „greeting‟ is made and the value
Hello is assigned to it on the same line.
• The value assigned to greeting is printed out
• This is repeated for a new variable called „name‟

More complex text variables
1. What will the output be for this code? Hi, my name is Billy

public class MoreMessages When used like this
{ the ‘=‘ is called the
assignment operator
public static void main(String[] args)
{
String myName = “Billy";
System.out.println("Hi, my name is " + myName);
}
}

The ‘+’ symbol is used to join
pieces of text together

Numerical Variables
1. What will the output be for this code? The answer is: 17

public class AddingExample
{
public static void main(String[] args)
{
int num1; // declaring the variables
int num2;
int result;

num1 = 10; // assigning values to the variables
num2 = 7;
result = num1 + num2;

System.out.println(“The answer is: ” + result);
}
}

Ex 2.1 – Simple Arithmetic
Aim: Create a simple program that adds two numbers

Description
Write a program called Arithmetic1.java that calculates the sum
3.5 + 3.75.

HINT
Think about the correct
datatypes.

Look at the previous slide
for some guidance.

Difficulty rating

Skills: Use of variables, arithmetic & datatypes

Arithmetic Operations
Operation Symbol Meaning Examples

Addition + 13 + 2 = 15

Subtraction - 13 – 5 = 8

Multiplication * 6 * 6 = 36

13 / 5 = 2.6
The result can be a decimal
Ordinary Division / 15 / 3 = 5.0
number
2 / 9 = 0.222

13 DIV 5 = 2
the result is just the integer part
Quotient (division) DIV 15 DIV 3 = 5
of the actual answer
2 DIV 9 = 0

13 DIV 5 = 3
Remainder The result is the remainder of the
MOD 15 DIV 3 = 0
(division) calculation
2 DIV 9 = 2

Ex 2.2 – Multiple Arithmetic
Aim: Create a program that performs a range of arithmetic calculations

Description
Write a program called Arithmetic2.java that takes 2 variables
with the values 15 and 5 and produces the following output:

Output
HINT
You will need 3 variables
(that you can reuse for
each calculation)

1. num1
2. num
3. answer

Difficulty rating

Skills: Use of variables &arithmetic statements

Ex 2.3 – Kelly Koms Telephone bill
Aim: Create a program that works out a person’s phone bill

Description
Write a program that breaks down and works out the total of a persons
phone bill. It should show them the output below:

Output
Use the data:
352 texts at 8p 116 mins at 12p

HINT
It is best to have 3 separate variables for the different
totals.

Look at slide 7 for how to output Strings and variables
(Use “n” to space out your output)

Difficulty rating

Skills: Use of arithmetic & concatenating Strings

Ex 2.4 – BMI
Aim: Create a program that works out a person’s BMI

Description
Write a program called SimpleBMI.java that works out the BMI of
someone who is 1.80m and 70kg.

The calculation is: BMI = weight / ( height 2 )

Output

On the next slide we will look at how to round up the value to two decimal places. Don‟t
worry about this until you have completed this program

Difficulty rating

Skills: Use of variables &arithmetic statements

Rounding Numbers

This is where the subroutine „round‟ is
being called by using it‟s name. In
brackets it has the value to be rounded
(bmi) and the number of decimal places
to round it to (2)

The code circled in green is a subroutine called ‘round’ that rounds a value to a set number of
decimal places. This subroutine requires two pieces of data (parameters) before it will work.
double d – The value to be rounded,
int decimalPlace – the number of decimal places to round to.
It will ‘return’ a value that has been rounded up

Remember a subroutine won‟t do anything unless it is „called‟ inside the main subroutine. (line 13)

More on calling subroutines
You can see the code to
the left has 3 subroutines.
Starting on lines 9, 18 & 27.

There is only 1 line of code
in the main subroutine.

Calling a subroutine

When we 'call a subroutine
we use it‟s IDENTIFIER (it‟s
name). As seen on line 29

When the code is
executed it will go to the
main method. When it gets
to line 29 the computer will
execute lines 9 – 16.

As subtractingSubRoutine
is NOT CALLED anywhere it
(remember this is the will NOT GET EXECUTED.
ONLY subroutine that
automatically executes
when the program is run).

Ex 2.5 – Exam mark
Aim: Create a program that works out a students % score for different 3
tests
Description
Write a program called Test.java that works out the % score of 3 different
tests.

Output
Use the data:
Test 1: 10.5/20 Test 2: 17/20 Test 3: 77/98

HINT
Do the first test and try to run and compile it. Then
do the other tests.

Notice there is something different between the
first test score and the last 2.

Difficulty rating

Skills: Use of arithmetic, datatypes & rounding

Some important things to note
When writing code it is good to break up a larger programs into
small subroutines as this makes it easier to write, debug and
understand.
(For now most of your programs are small enough be written
directly in the main subroutine).

Before using variables you must declare them first. This involves
supplying the datatype and it‟s identifier.

Some important things to note
When working out an arithmetic calculation and storing as a
double at least one of the variables involved in the calculation
has to be stored as a double (even if it is in fact just an integer.

int num1 = 3; This would NOT work as
Int num2 = 5; neither of the variables being
double result = num1 / num2 divided are stored as doubles

int num1 = 3; This would work as num2 is
stored as a double (even
double num2 = 5; though 5 is an integer we still
double result = num1 / num2 have to store it as a double)

Java basics variables

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Java basics variables