Java Programming Basics

1
Java Programming Basics
First, a QUIZ. Then, new stuff.
2
Questions from the last few classes

• Who makes the Java programming language?
• If we are making a Java file that contains a
  public class Lab1, what should file be saved as?
• What is the main method used for?
• What is the import statement used for?
• What did we use to print out a line to the
  command line?
• What are the three steps to creating and running
  a Java program?
• What is an IDE?
• The JVM?

3
Questions you should get the answer to from this
class

• What is a variable?
• What are the primitive data types in Java and how
  much memory does it take to hold each type?
• What math operators can we use in Java?

4
Variable

• Just like in Algebra, a variable is something
  that represents something else
• In Algebra,x and y always had to represent a
  number. You never had to worry about them being
  characters (letters) or anything else.
• In Java, though, we can have variables of all
  different types. Therefore, before we use a
  variable, we have to declare it with both a type
  and an identifier.

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a type and an identifier?

• When we say declare it with a type and an
  identifier, that means we have to say what kind
  of data the variable will be representing, and
  what we are going to call the variable.
• For instance int x
• char y
• String args
• double num

Type
Identifier
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Identifiers

• An identifier (the name of the variable) must
  start with a letter, an underscore ( _ ), or a
  dollar sign ()typically, though, every variable
  you see starts with a letter
• You cant start an identifier with a number, or
  contain an operator ( , -, , etc)
• An identifier cant be a reserved word (those
  blue words in JCreator)
• As a good coding practice, the identifier should
  be something that describes what the variable
  will be used for. For example, if a variable will
  be used to hold an average of several numbers,
  name it something like average, avg, or mean.

7
Types

• There are two kinds of data types in Java
  primitive data types and objects.
• When a variable is going to represent a primitive
  data type, the type is always in lowercase
• int num
• When a variable represents an object, the type is
  generally capitalized
• String word
• Rectangle rec

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Javas Primitive Data Types

• Four of the primitive types represent whole
  numbers of different sizes
• byte -128 to 127 (remember, a byte is 8 bits,
  like 11111111. In order to work with negative
  numbers, the last bit is the sign bit positive
  or negative, so that leaves seven bits to hold
  the value. 011111112 12710 )
• short -32,768 to 32,767
• int -2,147,483,648 to 2,147,483,647
• long -9,223,372,036,854,775,808 to
  9,223,372,036,854,775,807
• Two of the data types represent real numbers
  (numbers with decimals)
• float -3.4E38 to 3.4E38 (6 to 7 digits of
  accuracy)
• double -1.7E308 to 1.7E308 (14 to 15 significant
  digits)
• One character type char, for characters in single
  quotes, like 'a'
• And one boolean (true or false) type

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Javas Primitive Data Types(that you need to
know)

• boolean (1 bit of storage) true or false, 0 or 1
• char (2 bytes of storage) 0 to 65,535, which
  correspond to the Unicode character codes. Digits
  0 to 125 of Unicode correspond to ASCII, which
  can be found at www.asciitable.com.
• int (4 bytes of storage) -2,147,483,648 to
  2,147,483,647
• double (8 bytes of storage) -1.7E308 to 1.7E308

10
Using Primitive Data Types

• public static void main(String args)
• //declares an int (whole number) named val and
  gives it the value 5
• int val 5
• //declares a char (character) named letter and
  gives it the value 'c'
• char letter 'c'
• //declares a decmal number (double) named num w/
  value 0.365252
• double num 0.365252

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Math Operators in Java

• You've seen the used to assign values to
  variables
• There are other important operators in Java you
  can use on numerical variables (ints and
  doubles)
• //addition
• - //subtraction
• //multiplication
• / //division
• () //to specify order of operations

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Using Math Operators in Java

• Order of operations in Java is the same as it is
  in Algebra.
• What does each line of code result in?
• int x 4
• int y 2
• int answer x y
• answer x x y
• answer (x x) y
• answer ((x x) y)/4
• System.out.println("answer "answer)

13
imprecision of double (floating point)
computation

• Since doubles have 8 bytes of storage to store a
  HUGE set of decimal numbers (-1.7E308 to
  1.7E308), it isn't possible to store each and
  every possible number. So doubles in Java can be
  thought of as APPROXIMATIONS of numbers
• when you do calculations on doubles, you can't
  expect EXACT solutions
• double num 0.365252
• System.out.println(num) //prints out "0.365252"
• num num 0.01
• System.out.println(num) //prints out
  "0.37525200000000003"