Transcript Introduction

Java Basics
Java
 High-level language

More readable for humans


Need to be translated to machine language for execution
 Compilers
CPU-independent

translation can target different CPUs (machine languages)
 Designed by Sun Microsystems in 1995

Sun was bought by Oracle in 2010
 Designed with internet in mind

Can run in a web browser
Storing Data
 To store data
 we need to allocate space in the memory

Declare (specify)
 Type
• what kind of data

Name
• we can refer to it later
• Essentially a named location in the memory
Types
 int

(signed) integer
 double

double-precision floating point number
 boolean

true or false
 char

character
Names (“Identifiers”)
 Starts with a letter
 After that, can include


letter
digit
 Can these be names?




numberOfStudents
five5
55
5five
 Case sensitive

Balance and balance are different names
 Meaningful names improve readability

reduce mistakes
The Famous/Weird Semicolon
 Semicolon



Is similar to a period after a sentence in English
End of one instruction
Period is used to mean something else in Java
 Allocating space (“declaration”):
int
double
boolean
char
numberOfStudents;
temperature, humidity, pressure;
sunny, hurricane;
letterGrade;
 They are usually called variables similar to math

How do we vary/change the value?
Changing Values
 Assignment
 =
 Equal sign, but doesn’t mean equal as in math
 x = 97.5;


Means assign 97.5 to x (or store 97.5 in x)
Doesn’t mean we state x is equal to 97.5
Changing Values
 Assignment
 =
 Equal sign, but doesn’t mean equal as in math
 x = 97.5;


Means assign 97.5 to x (or store 97.5 in x)
Doesn’t mean we state x is equal to 97.5
 x = 97.5 + x;


Why is this impossible in math?
What does this mean in Java?
Changing boolean and char variables
boolean sunny;
sunny = false;
char letterGrade;
letterGrade = ’A’;
Initializing Variables
 Combining
 Declaring a variable (allocating space) and
 Assigning an initial value
int
double
char
boolean
numberOfStudents = 15;
gpa = 3.14;
letterGrade = ’A’;
sunny = true;
Manipulating Data
 Operators
 Arithmetic
 Relational
 Logical
Arithmetic Operators
+
*
/
%


modulo/reminder
5 % 2 is 1
 ++x , x++
 Increment x (int)
 Yields a number
Arithmetic: Division with Integers
 Math: 5 / 2
is 2.5
 Java
 “integer division”—both values/operands are integers
5 / 2
5 / 2


If we want a floating point value (2.5)


has an integer value -- floor of 5/2
is 2 [sometimes this is useful]
5 / 2.0 ,
5.0 / 2 , or …
Be careful
int x = 5 / 2.0 ;
 x has 2 because 2.5 can’t fit into an int variable

Relational Operators
<
 <=
>
 >=
 ==
 !=
 Yields true or false value
 5<2
yields false
 not stating 5 is less than 2 (in math), which is impossible


x == 2

Means what?
Logical Operators
 &&

and
 ||

or
!

not
 Yields true or false value
 true && false is false
 !(5 > 2) is false
Precedence/Ordering of Operators
x < y + z
 (x < y) + z
 x < (y + z)
Precedence/Ordering of Operators
x < y + z
 (x < y) + z
 x < (y + z)
 x < y + z && y < z
 x < (y + z) && y < z
((x < (y + z)) && y) < z
 (x < (y + z)) && (y < z)

Precedence/Ordering of Operators
 Quite natural
 Arithmetic (calculate numbers) before
 Relational (compare numbers) before
 Logical (combine boolean--true/false values)
 If not sure, add parentheses
Comments
 Ignore by the compiler
 Improves readability, fewer mistakes
// describe something that is not obvious
/*
this is a
multi-line comment
*/
Math Constants and Functions
 Math.PI, Math.E
 Math.abs(x)
 Math.sqrt(x), Math.pow(x, exp)
 Math.log(x), Math.log10(x)
 Math.sin(x), Math.cos(x), Math.tan(x) // radians
 Math.asin(x), Math.acos(x), Math.atan(x)
 Math.random() // 0 <= num < 1
Input from the Keyboard
 We’ll usually provide templates for input
Scanner
keyboard = new Scanner(System.in);
x = keyboard.nextInt();
y = keyboard.nextDouble();
Output to the Screen
 System.out.println( … );


Print the parameter followed by a new line
Examples:
System.out.println(15);
System.out.println(x);
System.out.println(“Hello!”);
 System.out.print( … );

Print the parameter without a new line
// “string”