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 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  =  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 = 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 + - * / %  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 &&  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!”); // “string” System.out.print( … );  Print the parameter without a new line