1. Computer Science 3 Hobart College
Java Data Types
Computer Science 3
Hobart College

2. Primitive data types Java has eight primitive data types:
char: used to store a single character e.g. ‘G’, ‘&’, ‘a’, ‘ ’
boolean: used to store true or false e.g. true, false
int, byte, short, long: used to store whole numbers e.g. 42,-5, 0
float, double: used to store numbers with fraction parts, as well as very large number numbers and very small numbers e.g. 42.0, 7.1, , 7.2 x 109, 0.0

3. Primitive data types 9 827 boolean char int 2.00 false float int 3.8
Pair the following data types to their examples 9
827
boolean
char
int
2.00
false
float
int
3.8
boolean U
char
double %
true

4. Declare, Instantiate, Initialise
To create and use a primitive data type we…
specify the data type,
give it a name (identifier), and
set an initial value (or leave it as its default value)
int age = ;
data type identifier value
Note: when declaring objects, we use a capital letter for the object type
When declaring variables or primitive data types, we use a lower case letter for the data type

5. Mathematics
Data types that deal with numbers can be used to calculate the solutions to mathematical equations
E.g.
int result = 6 + 2;
The variable ‘result’ will then contain the value 8

6. Mathematics We can use the following operators + Add - Subtract
* Multiply
/ Divide
% Modulus – remainder after dividing

7. Mathematics
Remember that when you do a calculation you must have a variable ready to store the result.
For each of the following calculations, determine the value that would be stored in the result.
int resultA = 7 * 2;
int resultB = 23%5;
int resultC = 16/4;
int resultD = resultA + resultC * resultB;

8. Mathematics
When a calculation involves an int and a float, the answer will be a float.
If you try to store the answer in a variable of the wrong data type, you may get a cast error.
E.g. “Cannot cast ...”

9. Type Casting
We can cast a value to formally change it into another data type.
When a double is cast to an int, the whole number is kept, and any faction part ignored.
Eg. (int) 4.7 => 4
When an int cast to a double, the fraction part (.0) is added.
E.g. (double) 4 => 4.0

10. Type Casting
To cast a data type, you put the type in brackets before the value you want to cast.
The two examples from the previous slide:
4.7  4
int resultA = (int) 4.7;
4  4.0
double resultB = (double)4;

11. Type Casting
In order of operations, e.g. BODMAS, casting is the same as brackets…
int result = (int)5.5 * 3;
this will first truncate 5.5, giving 5
then calculate 5 * 3 = 15
and set result to 15
If we want to calculate 5.5 * 3 first, we need to do this:
int result = (int) (5.5 * 3);
this calculates 5.5 * 3 (16.5) first, then truncates the result (16)

12. Displaying values
To display a value on the screen, we typically use the drawString() method.
The first parameter of the drawString is a text string.
g.drawString(“Hello world”,20,20);
Strings can be added together with the (+) operator. This is called concatenation.
g.drawString(“Hello ” + "world”,20,20);

13. Displaying values
The easiest way to convert a numerical value to a string is simply to concatenate it to another string, even an empty one. The numerical value is automatically cast as a string.
int result = 42;
g.drawString(“” + result,20,20);
This displays “42” at (20,20).
If you put the variable name in the quotes, it will simply print out the variable name
g.drawString(“result”,20,20);
This displays “result” at (20,20).

14. Displaying output What is displayed after the following code is run?
int result = 42;
double a = 2;
double b = 1.9;
g.drawString(“The result is: “ + result,20,20);
g.drawString(“A is “ +a+ ” and B is ” +(int)b ,20,40);
g.drawString(“3 + 2 is “+3+2,20,60);