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