Introduction to Java and DrJava

1
Introduction to Java and DrJava
Notes adapted from Introduction to Computing and
Programming with Java A Multimedia Approach by
M. Guzdial and B. Ericson, andinstructor
materials prepared by B. Ericson.
2
Learning Goals

• Understand at a conceptual level
• What is Java again?
• What is object-oriented about Java?
• What is DrJava?
• What are Java primitive types and how do they
  work?
• What are strings?
• What are Java math and relational operators?
• What are variables?

3
What is Java Again?

• Java is a programming language developed by Sun
  Microsystems
• It was made to overcome problems and limitations
  with other languages
• It was also targeted to allow dynamic content and
  applications to execute in browsers surfing the
  Internet, something unheard of at the time
• This meant that it had to be cross platform to
  run on all the different types of computers out
  there
• To make this all easier, it was decided that Java
  should be an object-oriented language

4
What is Object-Oriented About Java?

• In Java, the focus is on objects
• Objects are persons, places, or things that can
  do actions or be acted upon in a Java program
• All objects have
• Properties (aka characteristics)
• Behaviours (aka actions)
• Every object belongs to a specific class
• Objects that belong to the same class share
  properties and behaviours

5
An Objects Example

• Consider the following restaurant example
• When customers enter the restaurant, a greeter
  welcomes and seats them at a table
• A waiter takes their order and one or morechefs
  prepare the order
• The waiter brings the drinks and food, and when
  the customers are done, the waiter creates and
  brings them their bill
• On their way out, the customers pay the bill

6
An Objects Example

• What people were involved in this scenario?
• Customers, a greeter, a waiter, and chefs
• What other things were doing actions or were
  being acted upon?
• Table, order, drink, food, and bill
• Each of the entities involvedin this scenario is
  an object
• The objects in this scenario worked together to
  get the job done, which in this case was
  feeding the customers

7
An Objects Example

• What types of objects are they?
• Suppose, for example that there were two chefs
  working on the order, Alice and Bob
• In this case, we could call chefs a class,with
  both Alice andBob being instancesor objects
  that belongto the class chefs

8
An Objects Example

• Like all chefs, Alice and Bob would have common
  properties and behaviours.
• They would both have a name, a set of dishes they
  knew how to prepare, a number of years of
  experience, and so on as properties.
• They would both be able totalk, prepare
  ingredients,cook dishes, and so on as
  behaviours.
• Even though they areboth chefs, they arealso
  still individuals.

9
An Objects Example

• Objects and classes in an object-oriented
  programming language like Java work similarly to
  this example
• In a Java program, all of the objects work
  together to get the task at hand done, even
  though each object plays a different role
• Each object belongs to a class, and all objects
  in the same class share properties and
  behaviours, but are still distinct entities
• We will be seeing a lot more on objects very
  soon

10
What is DrJava?

• DrJava is a free integrated development
  environment for doing Java programming
• From Rice University
• It is written in Java
• It has several window panes in it
• For creating programs (definitions pane)
• For trying out Java code (interactions pane)
• Listing of open files (files pane)

• To try out DrJava, we will begin with doing some
  fairly simple math in Java ...

11
Math Operators in Java ( / - )

• Addition
• 3 4
• Multiplication
• 3 4
• Division
• 3 / 4
• Subtraction
• 3 4
• Negation
• -4
• Modulo (Remainder)
• 10 2 and 11 2

12
Math Operators Exercise

• Lets open DrJava and do the following in the
  interactions pane
• Subtract 7 from 9
• Add 7 to 3
• Divide 3 by 2
• Divide 4.6 by 2
• Multiply 5 by 10
• Find the remainder when 10 is divided by 3

13
Why is the Result of 3 / 2 1?

• Java is what is called a strongly typed
  language
• Each value has a type associated with it
• This tells the computer how to interpret the
  number
• It is an integer, floating point, letter, and so
  on
• The compiler determines the type if it isnt
  specified (as in the case of literals)
• 3 is an integer
• 3.0 is a floating point number (has a fractional
  part)
• The result of an operation is in the same type as
  the operands
• 3 and 2 are integers so the answer is an integer 1

14
Casting in Java

• There are ways to solve the problem of 3 / 2
  having a result of 1
• You can make one of the values floating point by
  adding .0 to the number, as in
• 3.0 / 2
• 3 / 2.0
• The result type will then be floating point
• Or you can cast one of the values to the
  primitive types float or double
• (double) 3 / 2
• 3 / (float) 2

15
Casting Exercise

• Use casting to get the values right for a
  temperature conversion from Fahrenheit to Celsius
• Celsius is 5/9 (Fahrenheit 32)
• Try it first with a calculator
• Try it in DrJava without casting
• Try it in DrJava with casting

16
Data Types in Java

• As mentioned earlier, Java is a strongly typed
  language, so it wants everything to have a type
• In Java, there are two kinds of data types
• Primitive data types
• Used to store very simple data valuesin main
  memory these are not treatedas objects, mainly
  for efficiency reasons
• Reference data types
• Used to refer to objects (more on this soon)

17
Java Primitive Data Types

• Integers (numbers without fractional parts) are
  represented by
• The types int or byte or short or long
• 235, -2, 33992093, etc.
• Floating point numbers (numbers with fractional
  parts) are represented by
• The types double or float
• 3.233038983 -423.9, etc.
• A single character is represented by
• The type char
• a b A etc.
• True and false values are represented by
• The type boolean
• true or false

18
Why so Many Different Types?

• They take up different amounts of space
• They have different precisions
• Usually use int, double, and boolean
• byte uses 8 bits (1 byte) 2s compliment
• short uses 16 bits (2 bytes) 2s compliment
• int uses 32 bits (4 bytes) 2s compliment
• long uses 64 bits (8 bytes) 2s compliment
• float uses 32 bits (4 bytes) IEEE 754
• double uses 64 bits (8 bytes) IEEE 754
• char uses 16 bits (2 bytes) Unicode format

19
Sizes of Primitive Types
byte
short
int
long
float
double
char
20
Floating Point Numbers

• Numbers with a fractional part
• 6170.20389
• Stored as binary numbers in a scientific notation
  -52.202 is -.52202 x 102
• The sign (1 bit)
• The digits in the number (mantissa)
• The exponent (8 bits)
• Two types
• float 6-7 significant digits accuracy
• double 14-15 significant digits accuracy

21
Operator Order

• The default evaluation order is
• Parentheses
• Negation
• Multiplication, division, and modulo (remainder)
  from left to right
• Addition and subtraction from left to right
• The default order can be changed
• By using parentheses
• (3 4) 2 versus 3 4 2
• It is also sometimes useful for parentheses to
  emphasize the default operator precedence

22
Math Operator Order Exercise

• Try 2 3 4 5
• Add parentheses to make it clear what is
  happening first, and emphasize the
  defaultoperator precedence
• How do you change it so that 2 3 happens
  first?
• How do you change it so that it multiplies the
  result of 2 3 and the result of 4 5?

23
Printing Output to the Console

• One of the things you often want to do in a
  program is output the value of something
• In Java the way to print to the console is to use
  
• System.out.println()
• Will print out the value of the thing in the
  parentheses and a new line
• System.out.print()
• To print just the thing in the parentheses
  without a new line

24
Console Output Exercise

• Use System.out.println() to print the results of
  an expression to the console
• System.out.println(3 28)
• System.out.println(14 7)
• System.out.println(10 / 2)
• System.out.println(128 234)
• Try using System.out.print() instead
• What is the difference?

25
Comparison (Relational) Operators

• Greater than gt
• 4 gt 3 is true
• 3 gt 3 is false
• 3 gt 4 is false
• Less than lt
• 2 lt 3 is true
• 3 lt 2 is false
• Equal
• 3 3 is true
• 3 4 is false
• Not equal !
• 3 ! 4 is true
• 3 ! 3 is false

• Greater than or equal gt
• 4 gt 3 is true
• 3 gt 3 is true
• 2 gt 4 is false
• Less than or equal lt
• 2 lt 3 is true
• 2 lt 2 is true
• 4 lt 2 is false

26
Comparison Operators Exercise

• In DrJava
• Try out the comparison operators in the
  interactions pane
• with numbers
• 3 lt 4
• 4 lt 4
• 5 lt 4
• 6 6.0
• with characters (single alphabet letter)
• Put single quote around a character
• a lt b
• b lt a
• a a

27
Strings

• Java has a type called String
• A string is an object that has a sequence of
  characters in Unicode, used to represent text
• Java recognizes strings as beginning and ending
  with " (a double quote)
• They can have no characters (the null string "")
• They can have many characters
• "This is one long string with spaces in it."

28
Strings

• Everything in a string is printed out as it was
  entered in the first place
• Even math operations "128 234"
• Try doing a System.out.println("128 234")
• One exception to this is the \ character, which
  is used to embed special charactersinto a string
  in Java. Examples include
• \" Used to put a " in a string
• \\ Used to put a \ in a string
• \t Used to put a tab in a string
• \n Used to force a new line in a string

29
Strings

• Java also knows how to add (or concatenate)
  strings to other strings
• It returns a new string with the characters of
  the second string after the characters of the
  first
• Try System.out.println("Java is" "great")
  versusSystem.out.println ("Java is" " "
  "great")
• Java also can add numbers and strings, by first
  converting the number to a string, and then doing
  the traditional concatenation
• Try System.out.println("The total is " 10)
• Compare to System.out.println("The total is "
  5 5 )

30
Variables

• Weve used Java to do calculations and
  concatenate strings, but we havent stored the
  results
• The results are in memory somewhere, but we dont
  know where they are, and we dont know how to
  get them back
• To solve this problem, we use variables

31
Variables

• Variables are named locations in memory used for
  storing values
• Variables are created and named by declaring
  them
• To declare a variable in Java you specify a type
  for the variable, and give it a name
• Remember that Java is strongly typed and wants to
  have a known type for everything
• Providing a type lets Java know how much memory
  to set aside for the variable and whatit can do
  with that variable in the future

32
Variable Declarations

• In general, variables are declared like this
• type name
• A simple and more complex example
• int numPeople
• double bill, tip
• In this case, we have declared three variables
  one integer variable (numPeople) and two double
  precision floating point variables (bill and tip)
• Java allows multiple variables to be declaredat
  the same time

33
Storing Values in Variables

• Values are stored to variables in Java using
  assignment statements
• In general, assignment statements look like
• name expression
• We do not read as equals, but rather we assign
  the value from the expression on the right side
  to the variable named on the left
• As an example
• numPeople 2
• Here, we assign the value 2 to the integer
  variable numPeople that we declared earlier

34
Storing Values in Variables

• Java also allows us to declare variables and
  assign initial values to them at the same time
• For example, we can combine the declaration of
  our integer variable numPeople with the
  assignment we didon the previous slide
• int numPeople 2

35
Using Variables

• In essence, a variable can be used wherever a
  literal value of the same type could be used
• The variable must be declared first though
• And, in most cases, the variable must appear on
  the left side of an assignment statement first
  too, to ensure that it has a valid value stored
  in it
• This gives us a lot of flexibility in
  usingvariables in our Java programs
• Consider the example Java code on the following
  slide

36
Using Variables Example

• int numPeople 2
• System.out.println(numPeople)
• double bill 32.45
• System.out.println(bill)
• double tip bill 0.2
• System.out.println(tip)
• double total bill tip
• System.out.println(total)
• double totalPerPerson total / numPeople
• System.out.println(totalPerPerson)

37
Tracing Through Code

• Quite often, you will find it useful to trace
  through code to see what it does when you do not
  have access to a computer
• On midterm and final exams, for example
• A good way to do this is to draw and label boxes
  for each variable in the code and follow through
  with changes to them

2
6.49
32.45
38.94
38
Named Constants

• In our using variables example two slides ago, we
  used three literal constant values
• 2, 32.45, and 0.2
• Sometimes you will hear this kind of constant
  called a magic number
• This term denotes a literal constant which may
  mean something to the programmer, but means
  nothing to someone else who is reading the code
• Using magic numbers in programming is typically
  considered to be a poor practice because it can
  make code difficult to read and change

39
Named Constants

• Instead, we used named constants that have
  meaningful names to them
• To create a named constant, we declare a variable
  and assign a value to it at the same time, but
  add the keyword final to signify that this
  variable is a named constant
• As an example
• final double TIP_RATE 0.2
• It can then be used like
• double tip bill TIP_RATE

40
Common Errors with Variables and Constants

• Simple typos
• The most common error is to simply mistype
  something Java could get quite confused over
  something like that
• Example duble total bill tip
• Example double total bil tip
• Case sensitivity
• Java is case sensitive and will generally treat
  issues with case as it would typos
• Example doublE total bill tip
• Example double total Bill tip

41
Common Errors with Variables and Constants

• Redeclaring a variable
• Once a variable has been declared, attempting to
  declare another variable with the same name can
  cause issues, depending on where you do it
• Example int numPeople 2 int
  numPeople
• Reassigning a constant
• A constants value cannot be changed.
• Example final double TIP_RATE 0.2 TIP_RATE
  0.1

42
Common Errors with Variables and Constants

• Loss of precision
• Java will automatically convert integer values to
  floating point values as necessary
• Java will not automatically convert floating
  point values to integers, however, as this could
  result in a loss of precision you must cast
  instead
• Example int age 5.6
• Uninitialized variables
• In most cases, Java wants variables to be
  initialized before they are used
• Example int bill, tip tip bill 0.2

43
Reference Variables

• All of the variables we have discussed so far
  have been for primitive types
• These variables are used to store values of the
  various primitive types
• Reference variables (or object variables) are
  variables that are used to refer to objects
• They do not store the objects themselves
• Instead, they store the location of the
  objectsso that they can be found when necessary
• Thats why we say they refer to objects, andcall
  them reference variables

44
Using Reference Variables

• Reference variables are declared in much the same
  way other variables are declared
• For example, as mentioned earlier, strings are
  objects. To declare a reference variable to
  refer to a string, we can use the following
  declaration
• String test
• Note that this does not actually create a String
  object, but declares a variable named test that
  can later be used to refer to a String object
• By default, however, it does not refer to
  anything, and is said to be a null reference

45
Using Reference Variables

• To have a reference variable refer to an object,
  we use an assignment statement
• For example
• test "Hi"
• Java will create a String object containing the
  text Hi and has the variable test refer to it
• Another example
• test new String("Bye")
• The new operator is the traditional method used
  in Java to create new objects, so this statement
  will create a String object containing the text
  Bye and have test refer to it instead
• We will see more about Strings and new later on

46
Reference Variables Example

• String test
• System.out.println(test)
• test "Hi"
• System.out.println(test)
• test new String("Bye")
• System.out.println(test)

47
Multiple References to Objects

• In Java, it is possible to have multiple
  references to the same object. Consider this
• String name1 new String("Suzanne Clark")
• String name2 name1
• In this case, name1 and name2 refer to the same
  object in memory
• We would call this identity equality

48
Multiple References to Objects

• It is important to note that the two references
  are independent what you do to one does not
  affect the other
• Following the example on the previous slide, now
  consider this
• name1 null
• This change only affects the reference variable
  name1, and not name2

null
name1
Suzanne Clark
name2
49
Multiple References to Objects

• Suppose we did things a little differently in the
  first place and created two objects as in
• String name1 new String("Suzanne Clark")
• String name2 new String("Suzanne Clark")
• In this case, name1 and name2 refer to two
  different objects with the same contents
• We would call this state equality

Suzanne Clark
name1
Suzanne Clark
name2
50
Java Naming Conventions

• In Java, Class names start with an uppercase
  letter
• System, String, Picture
• Named constants are in all uppercase letters,
  with underscores separating words
• TIP_RATE
• All other names start with lowercase letters but
  uppercase the first letter of each additional
  word
• picture, fileName, thisIsALongName

51
Java Naming Conventions

• Java code will compile if you dont follow these
  conventions, but it may be hard for other
  programmers to understand your code
• So, it is a good idea to follow these
  conventions!
• As an example, can you identify which of these
  are primitive types, and which are the names of
  classes, just by following conventions?
• char
• Double
• Math
• double
• Integer
• String

52
A Semicolon () ends a Statement

• Java programs are made up of statements
• Like sentences in English
• As you may have already noticed, Java statements
  end in a semicolon not a period
• Missing semicolons in a Java program could lead
  to a lot of syntax errors in your code
• DrJavas interaction pane, however, prints the
  result of statements without a semicolon
• But not the result of statements with a semicolon
• Use System.out.println() to force output

53
Comments

• To make code readable, Java allows you to embed
  comments in your code that describewhat your
  code does
• Comments are ignored by Java, but can be ofgreat
  use to other people reading your code
• Commenting your code is considered to be a very
  good programming practice!
• Java allows commenting in a couple of ways
• / Everything between these symbols is a comment
  /
• // Everything on the line following this is a
  comment

54
Summary

• Java is object-oriented
• Java has both primitive and reference types
• Java has typical math and relational operators
• Java uses variable to store primitive values and
  references to objects
• You can print out things using System.out.println(
  value)