Primitive Data Types
Identifiers
What word does it sound like?
Identifiers
Name that will be used to describe anything a programmer is required to define. classes, methods, constants, variables;
Examples Name of objects
marker, pencil
Methods turnRight, forward, move
Rules for Identifiers
Must start with a letter After the letter, can be any combination
of letters, numbers, or _ No SPACES!!! Cannot be a reserved word (words with
special meanings in java [see handout])
Example Identifiers
Are these okay? myPerson m_person person1 my Person 1person person#1
Example Identifiers
These are fine myPerson m_person person1
These are NOT my Person 1person person#1
Java is Case Sensitive
Person ≠ person ≠ perSon
Good Identifiers
Don’t use single letters Make them descriptive
grades instead of g
Variable names should be meaningful but concise gpa instead of
gradePointAverageForStudentsAtThisSchool
Some Conventions
Class names start with capitals DrawingTool
Variable names start with lowercase marker
Multiple word names have a capital letter at the beginning of each new word turnRight
Constants (value never changes) are in all capitals MAXSCORE
Data Types
Depending on what you want to store in java, you need to tell it what type it is.
Why do you think it matters if something is a number or a letter?
Type Matters
Math You can’t add the number 5 to the word
“Happy”
Depending on the type, java has to make a given amount of space for it.
Primitive Data Types
int – integers – whole numbers -5 0 86
double – decimals 3.14 5.0 -1.2 6.02e23
scientific notation - 6.02e23 = 6.02x10^23
boolean – true or false char – holds one character
‘a’ ‘%’ ‘6’
Invalid Numbers
Don’t do this $5.06 #3.0 86%
You Might Also see
long and short are like int float is like double
Declaring variables
Remember me? DrawingTool marker;
Other variables are the same: int number; number = 86; int number = 86;
You only declare the type once! First time> DrawingTool marker; After> marker.
Ascii
The characters are secretly stored as integer values. Thus ascii value 65 is the captial ‘A’
System.out
One way to print out to the screen System.out.print
Print and don’t skip a line System.out.print(“Hello”);System.out.print(“World”);
– prints HelloWorld
System.out.println Print and skip a line
System.out.println(“Hello”);System.out.println(“World”);
– prints Hello World
Examples int number = 5;
char letter = 'E'; double average = 3.95; boolean done = false; System.out.println("number = " + number); System.out.println("letter = " + letter); System.out.println("average = " + average); System.out.println("done = " + done); System.out.print("The "); System.out.println("End!"); Run output: number = 5letter = Eaverage = 3.95done = falseThe End!
What does + mean?
Inside System.out.println("number = " + number); + means add the words “number = “ to the
value of number
Escape CharactersCharacterJava Escape Sequence Newline '\n'Horizontal tab '\t'Backslash '\\'Single quote '\''Double quote '\"'Null character '\0' System.out.println(“This is a\ntest and only\’ a test.”); Run output: This is atest and only’ a test.
Interesting Differences
System.out.println( 2 + 2);
//Output: 4
System.out.println(“2 + 2”);
//Output: 2 + 2
System.out.println(“2” + “2”);
//Output: 22
Castingchar letter = 'A';int number = 75;System.out.println("letter = " + letter); System.out.print("its ASCII value = ");System.out.println((int)letter); System.out.print("ASCII value 75 = ");System.out.println((char)number); Run output: letter = Aits ASCII value = 65 ASCII value 75 = K
Assignment (=)
The = sign works RIGHT to LEFT only! a = 5;
Means the variable a gets the value 5 5 = a; DOES NOT WORK!!! a = 3;
b = 5;
a = b;
a now equals?
b now equals?
Variables with =
On the LEFT side, mean save the answer here a = 5 + 3;
On the RIGHT side, means look up the value b = 6; a = b + 2;
You can do math
a = 5 + 3; Adding is + Subtracting is – Multiplication is * Division is / Modulus is %
Careful with division
If you divide by an integer, java will round down 15/4 = 3!
If you divide by a decimal, java will give you an exact answer 15/4.0 = 3.75
Careful How you Save
If you save a decimal in to an int, the decimal part of the number will be lost int x = 3.14;
Mod (%)
Remember how you learned division? 13/4 = 3R1
Mod just means give me the remainder from dividing.
13%4 = 1