Just%20Enough%20Java

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Just Enough Java
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What is Java?

• Java is a programming language a language that
  you can learn to write, and the computer can be
  made to understand
• Java is currently a very popular language
• Java is a large, powerful language
• but it is not simple!
• Compared to C, Java is elegant

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Structure of a Java program

• A program, or project, consists of one or more
  packages
• Package directory folder
• A package contains one or more classes
• A class contains one or more fields and methods
• A method contains declarations and statements
• Classes and methods may also contain comments
• Well begin by looking at the insides of methods

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Java structure and Eclipse

• A workspace is where Eclipse keeps projects
• When you use Eclipse to create a project (a
  single program), it creates a directory with
  that name in your workspace
• Within the project, you next create a package
• It is possible to skip this step and use the
  default package
• Finally, you create a class in that package
• For the simplest program, you need only a single
  package, and only one (or a very few) classes

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Simple program outline
class MyClass public static void
main(String args) new
MyClass().run() void run()
// some declarations and statements go here
// this is the part we will talk about today

• Notes
• The class name (MyClass) must begin with a
  capital
• main and run are methods
• This is the form we will use for now
• Once you understand all the parts, you can vary
  things

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Syntax and semantics

• Syntax is the grammar of the language
• The syntax of Java is large, but finite
• Syntax must be absolutely correct
• The computer will point out every syntax error
• Error messages may be helpful or misleading
• Semantics is the meaning of your program
• Semantic errors cause your answers to be wrong
• You may or may not get error messages

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Two aspects of Java

• Java has syntax and semantics
• This is where you begin
• It is possible to learn everything about Javas
  syntax and semantics
• We will cover most of Javas syntax and semantics
• Java comes with many built-in packages
• Packages are sort of like vocabulary bundles
• To be good at Java, you need to learn many
  packages
• There are more Java packages than you can ever
  learn

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What you need to know

• You need to be able to
• Read in data (for now, numbers)
• We will use a Scanner for this
• Save numbers in variables
• We will use declarations to create variables
• Do arithmetic on numbers to get new numbers
• We will use assignment statements
• Test whether or not to do something
• We will use if statements
• Do something repeatedly
• We will use while statements
• Print output
• We will use System.out.print and
  System.out.println
• Not absolutely necessary, but very helpful
• We will use methods

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Declarations, statements, comments

• A declaration gives some information to the
  computer
• A statement tells the computer to do something
• Statements should really be called commands
• Comments are ignored by the computerthey are
  explanations of your program for human beings to
  read
• A method may contain declarations, statements,
  and comments

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Variables

• A variable is a box that holds data
• Every variable has a name
• Examples name, age, address, isMarried
• Variables start with a lowercase letter
• In multiword variables, each new word is
  capitalized
• Every variable has a type of value that it can
  hold
• For example,
• name might be a variable that holds a String
  (sequence of characters)
• age might be a variable that holds an integer
  value
• isMarried might be a variable that holds a
  boolean (true or false) value

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Some Java data types

• In Java, the four most important primitive
  (simple) types are
• int variables hold integer values
• double variables hold floating-point numbers,
  that is, numbers containing a decimal point
• boolean variables hold a true or false value
• char variables hold single characters
• Another important type is the String
• A String is an Object, not a primitive type
• A String is composed of zero or more chars

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Declaring variables

• Every variable that you use in a program must be
  declared (in a declaration)
• The declaration specifies the type of the
  variable
• The declaration may give the variable an initial
  value
• Examples
• int age
• int count 0
• double distance 37.95
• boolean isReadOnly true
• String greeting "Welcome to CIT 591"
• String outputLine

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Reading in numbers

• First, import the Scanner class import
  java.util.Scanner
• Create a scanner and assign it to a
  variable Scanner scanner new
  Scanner(System.in)
• The name of our scanner is scanner
• new Scanner(...) says to make a new one
• System.in says the scanner is to take input from
  the keyboard
• Next, its polite to tell the user what is
  expected System.out.print("Enter a number ")
• Finally, read in the number myNumber
  scanner.nextInt()
• If you havent previously declared the variable
  myNumber, you can do it when you read in the
  number int myNumber scanner.nextInt()

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Printing

• There are two methods you can use for printing
• System.out.println(something)
• This prints something and ends the line
• System.out.print(something)
• This prints something and doesnt end the line
  (so the next thing you print will go on the same
  line)
• These methods will print anything, but only one
  thing at a time
• You can concatenate (join) things with the
  operator
• ExampleSystem.out.println("There are "
  appleCount "
  apples and " orangeCount
  " oranges.")

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Program to double a number

• package numberStuffimport java.util.Scannerp
  ublic class Doubler public static void
  main(String args) new
  Doubler().run() private void run()
  Scanner scanner int number
  int doubledNumber scanner
  new Scanner(System.in)
  System.out.print("Enter a number ")
  number scanner.nextInt() doubledNumber
  2 number System.out.println("Twice "
  number " is " doubledNumber)

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Assignment statements

• Values can be assigned to variables by assignment
  statements
• The syntax is variable expression
• The expression must be of the same type as the
  variable
• The expression may be a simple value or it may
  involve computation
• Examples
• name "Dave"
• count count 1
• area (4.0 / 3.0) 3.1416 radius radius
• isReadOnly false
• When a variable is assigned a value, the old
  value is discarded and totally forgotten

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One-line comments

• A comment is a note to any human reading the
  program comments are ignored by Java
• A single-line comment starts with // and goes to
  the end of the line
• A comment may be put after a statement (on the
  same line) to say something about that one
  statement
• A comment may be put on a line by itself, to say
  something about the following statements
• Example
• // Swap the values of x and ytemp x // save
  old value of x in tempx y // replace
  old value of x with yy temp // this many
  comments is just silly

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Multiline comments

• A multiline comment starts with / and ends with
  /
• Typically, these comments extend over several
  lines
• / Here is an example of a multiline comment. It
  isnt a very inspired one, but I needed an
  example of some sort. /
• Single-line and multiline comments are used to
  make notes to yourself about how a method does
  what it does
• Javadoc comments (next slide) are used to tell
  other programmers what a class is for, or how to
  use a method

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Javadoc comments

• Javadoc comments, also called documentation
  comments or doc comments, are multiline comments
  used to describe the purpose of classes and
  methods
• Javadoc comments start with / and end with /
• Each internal line typically begins with a
• Javadoc comments for a class should contain these
  tags
• _at_version number_or_date
• _at_author your_name (may have more than one of
  these)
• Javadoc comments for a method may contain these
  tags
• _at_param parameter_name description_of_this_param
  eter
• _at_return description_of_value_returned (omit if
  void)
• _at_throws exception description_of_when_problem_oc
  curs

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Methods

• A method is a named group of declarations and
  statements
• void tellWhatYearItIs( ) int year 2009
  System.out.println("Hello in " year "!")
• The name of the above method is tellWhatYearItIs
• The word void says that this method does not
  return a value
• We call, or invoke a method by naming it in a
  statement
• tellWhatYearItIs( )
• This should print out Hello in 2009!

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Organization of a class

• A class may contain data declarations and methods
  (and constructors, which are like methods), but
  not statements
• A method may contain (temporary) data
  declarations and statements
• A common error
• class Example
• int variable // simple
  declaration is OK
• int anotherVariable 5 // declaration with
  initialization is OK
• variable 5 // statement! This
  is a syntax errorvoid someMethod( )
• int yetAnotherVariable //declaration is
  OK
• yetAnotherVariable 5 // statement
  inside method is OK

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Arithmetic expressions

• Arithmetic expressions may contain
• to indicate addition
• - to indicate subtraction
• to indicate multiplication
• / to indicate division
• to indicate remainder of a division (integers
  only)
• parentheses ( ) to indicate the order in which
  to do things
• An operation involving two ints results in an int
• When dividing one int by another, the fractional
  part of the result is thrown away 14 / 5 gives 2
• Any operation involving a double results in a
  double3.7 1 gives 4.7

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Boolean expressions

• Arithmetic comparisons result in a boolean value
  of true or false
• There are six comparison operators
• lt less than
• lt less than or equals
• gt greater than
• gt greater than or equals
• equals
• ! not equals

• There are three boolean operators
• and--true only if both operands are true
• or--true if either operand is true
• ! not--reverses the truth value of its one
  operand
• Example (x gt 0) !(x gt 99)
• x is greater than zero and is not greater than
  99

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String concatenation

• You can concatenate (join together) Strings with
  the operator
• Example fullName firstName " " lastName
• In fact, you can concatenate any value with a
  String and that value will automatically be
  turned into a String
• ExampleSystem.out.println("There are " count
  " apples.")
• Be careful, because also still means addition
• int x 3int y 5System.out.println(x y
  " ! " x y)
• The above prints 8 ! 35
• Addition is done left to right--use parentheses
  to change the order

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if statements

• An if statement lets you choose whether or not to
  execute one statement, based on a boolean
  condition
• Syntax if (boolean_condition) statement
• Exampleif (x lt 100) x x 1 // adds 1 to x,
  but only if x is less than 100
• C programmers take note The condition must be
  boolean
• An if statement may have an optional else part,
  to be executed if the boolean condition is false
• Syntax if (boolean_condition) statement else
  statement
• Exampleif (x gt 0 x lt limit) y x /
  limitelse System.out.println("x is out of
  range " x)

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Compound statements

• Multiple statements can be grouped into a single
  statement by surrounding them with braces,
• Example if (score gt 100) score 100
  System.out.println("score has been
  adjusted")
• Unlike other statements, there is no semicolon
  after a compound statement
• Braces can also be used around a single
  statement, or no statements at all (to form an
  empty statement)
• It is good style to always use braces in the if
  part and else part of an if statement, even if
  they enclose only a single statement
• Indentation and spacing should be as shown in the
  above example

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while loops

• A while loop will execute the enclosed statement
  as long as a boolean condition remains true
• Syntax while (boolean_condition) statement
• Example n 1 while (n lt 5)
  System.out.println(n " squared is " (n
  n)) n n 1
• Result 1 squared is 1 2 squared is 4
  3 squared is 9 4 squared is 16
• C programmers take note The condition must be
  boolean
• Danger If the condition never becomes false, the
  loop never exits, and the program never stops

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Method calls

• A method call is a request to an object to do
  something, or to compute a value
• System.out.print(expression) is a method call
  you are asking the System.out object to evaluate
  and display the expression
• A method call may be used as a statement
• Example System.out.print(2 pi radius)
• Some method calls return a value, and those may
  be used as part of an expression
• Example h Math.sqrt(a a b b)

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A complete program

• public class SquareRoots // Prints the
  square roots of numbers 1 to 10 public static
  void main(String args) int n 1
  while (n lt 10)
  System.out.println(n " " Math.sqrt(n))
  n n 1
• 1 1.02 1.41421356237309513
  1.73205080756887724 2.05 2.23606797749979etc.

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Another complete program
public class LeapYear public static void
main(String args) int start 1990
int end 2015 int year start
boolean isLeapYear while
(year lt end) isLeapYear year 4
0 // a leap year is a year
divisible by 4... if (isLeapYear
year 100 0) // ...but not by
100... if (year 400 0)
isLeapYear true // ...unless its also
divisible by 400 else isLeapYear
false if
(isLeapYear) System.out.println(
year " is a leap year.")
year year 1
1992 is a leap year. 1996 is a leap year. 2000 is
a leap year. 2004 is a leap year. 2008 is a leap
year. 2012 is a leap year.
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The End
If you give someone a program, you will
frustrate them for a day if you teach them how
to program, you will frustrate them for a
lifetime.
--Anonymous