CSC 212

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CSC 212

• Object-Oriented Programming and Java
• Part 2

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Announcements

• The end of the Java refresher is nigh! If you
  need more of a review, seek assistance
  immediately.
• I have a cool office. Please drop by and see so
  for yourself (and ask me questions while you are
  there).

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Student Class

• public class Student
• // declare the fields
• // define the constructors
• // define the methods

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The Student Variables

• public class Student
• protected String name, studentID
• protected int years_attended
• private float gpa, credits
• protected static int total_enrollment
• // define the constructors
• // define the methods
• // end of class definition

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Constructors for Student

• Constructors are special methods which create
  instances.
• Typically initialize fields values.
• (They can do more later)

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Additional Constructors

• Classes can have several constructors
• Must differ in parameter lists (signatures).

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The Student Class
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Set and Get Methods

• Provide set and get methods for each field
• Controlling access to fields
• Limits errors and problems (or amount of
  searching when debugging)
• Common design pattern

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Rules of Thumb

• Classes are public
• Fields are private
• Outside access only using get and set methods
• Constructors are public
• Get and set methods (if any) are public
• Other methods on a case-by-case basis

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Naming Conventions

• Variables start with a lower case letter
• When name includes multiple words, combine words
  and use intermediate caps
• int xLoc, yLoc
• char choice
• Classes begin with an upper case letter
• String strVal
• Car bob
• No name can match a Java keyword

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Things to Remember

• delimit logical blocks of statements
• Two ways to define comments
• / up to / defines a block comment
• // defines a single line comment
• Java is case-sensitive
• out, Out, OUt, OUT are all different

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Primitive Types
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Java Operators

• unary increment (k ? k k 1)
• -- unary decrement (k-- ? k k-1)
• ! logical negation (!done)
• remainder
• ! primitive equality/inequality test
• logical and (done valid)
• logical or (done flag)
• assignment (x a b)

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Strings

• String is used like a primitive, but really is a
  class
• For example, one can create a string by String
  s "This is a Java string"
• Strings are stored as zero or more Unicode
  characters.

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String Operators

• Basic string operator is concatenation ()
• Concatenation joins two strings
  togetherStrings joined ? Strings
  joined
• Numbers can be converted back and forth with
  strings
• int x Integer.parseInt(32) x ? 32float y
  Float.parseFloat(7.69) y ?7.69

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toString() Methods

• Generates representations of objects
• Not required, but really useful debugging tool
• For example, the Student class could
  definepublic String toString()
• return Student name studentID
  GPA Float.toString(gpa)

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Arrays

• Store fixed number of elements of the same type
• length field contains size of array

Student roommates new Student3 roommates0
new Student(Al,1000) roommates1 new
Student(Bob,1050) roommates2 new
Student(Carl, 2000) String name_list "
" for (int n0 nltroommates.length n)
name_list name_list roommatesn.getName ()
"" int numbers 1, 2, 3 // initialized
with length 3 numbers2 3 float reals
new float810 // reals is an array of float
arrays
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Static members

• Methods fields can be declared static
• Static field is shared by all class objects
• All objects see the same value
• Static methods and fields can be used without
  creating an instance of the class with the new
  command.
• E.g., ClassName.staticMethod() or x
  ClassName.staticField

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Static members

• Static methods cannot refer to non-static members
  of the same class directly.
• Like non-class objects, must specify which
  instance of the class they are referring.
• Using non-static method or field inside a static
  method is a common compiler error
• Error Can't make static reference to method void
  print() in class test.

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if -- else if -- else if -- -- else

• if (a b)
• ...
• else if (a lt b)
• ...
• else
• ...
• Only boolean tests are allowed
• At most 1 branch followed

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while loop

• while (v ! b)
• Only boolean tests are legal
• Test occurs before loop is entered
• Loop continues until while test is false
• while (true) loops forever
• while (false) never executes code in loop

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do while loop

• do
• while (b lt m)
• do-while performs test after the loop
• Guarantees loop executed at least once
• Block bracing ( ) is required
• Why?

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while vs. do-while loop

• What is the advantage of one over the other?

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for loop

• Just a while loop with aspirations
• Typical use iterate (loop) over set of instances
• for (initialization test modification)
  block of code

Executed before starting loop
Examined before every iteration
Executed after each pass through loop
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switch statements

• int x
• switch (x)
• case 0 System.out.println(x is 0) break
• case 1 System.out.println(x is 1) break
• default System.out.println(x is not 0 or 1)
• Execution starts at first matching case
• Stops at first break statement

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What will this code do?

• int x 0
• switch (x)
• case 0 System.out.println(x is 0)
• case 1 System.out.println(x is 1) break
• default System.out.println(x is not 0 or 1)

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Explicit Control of Execution

• break ltlabelgt
• Exit from any block
• Can exit multiple blocks using ltlabelgt form
• Unlabeled - terminate innermost
• Labeled - terminate the appropriate block

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What will this code do?

• outer
• for (int i 0 i lt 10 i)
• inner
• for (int j 0 j lt I j)
• if (j 5)
• System.out.println(i is Integer.toString(i)
  and j is Integer.toString(j))
• break

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What will this code do?

• outer
• for (int i 0 i lt 10 i)
• inner
• for (int j 0 j lt I j)
• if (j 5)
• System.out.println(i is Integer.toString(i)
  and j is Integer.toString(j))
• break outer

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Explicit Control of Execution

• continue ltlabelgt
• Skip to end of loop body evaluate loop control
  conditional
• Can exit multiple levels using ltlabelgt form
• Only in while, do-while, for loops
• Unlabeled - continue innermost loop
• Labeled - continue to an outer loop

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What will this code do?

• outer
• for (int i 0 i lt 10 i)
• inner
• for (int j 0 j lt I j)
• if (j 2 i)
• if (j 5)
• continue inner else
• System.out.println(i is
  Integer.toString(i) and j is
  Integer.toString(j))

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Explicit Control of Execution

• return ltexpressiongt
• terminate execution and return to invoker
• It is illegal in Java to have code after a return
  statement!
• But only if the code can only be exected after
  the return statement

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Daily Quiz

• Do problem R-1.13 from the book (p. 52)Write a
  Java function that takes an integer n and returns
  the sum of all odd integers smaller than n.