Java Basics

Java High-level language Designed by Sun Microsystems in 1995 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 double boolean char (signed) integer 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 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 numberOfStudents; double temperature, humidity, pressure; boolean sunny, hurricane; char letterGrade; They are usually called variables similar to math How do we vary/change the value?

Changing Values Assignment x = 97.5; = 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 x = 97.5; x = 97.5 + x; = 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 numberOfStudents = 15; double gpa = 3.14; char letterGrade = ’A’; boolean sunny = true;

Manipulating Data Operators Arithmetic Relational Logical

Arithmetic Operators + - * / % ++x , x++ Yields a number 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 has an integer value -- floor of 5/2 5 / 2 is 2 [sometimes this is useful] If we want a floating point value (2.5) 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 && || ! Yields true or false value and or not true && false is false !(5 > 2) is false

&& (and) operator Green (boolean) Square Green && Square F T

|| (or) operator Green (boolean) Square Green || Square F T

! (or) operator Green (boolean) !Green F T

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( … ); System.out.print( … ); Print the parameter followed by a new line Examples: System.out.println(15); System.out.println(x); System.out.println(“Hello!”); // “string” System.out.print( … ); Print the parameter without a new line