600.107 Intro to Java

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600.107 Intro to Java

• Review Session
• TA Jai Madhok
• Email jaimadhok_at_jhu.edu

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Chapter 7 User Defined Methods

• Look up the common methods form the Math class,
  String class, Character class, Random class
• How can Public static methods be called?
  className.methodName(parameters)
• User defined methods Value returning or Void
• Void methods can be stand-alone statements
• Syntax of value returning methods
• Public/private static(or not) returnType name
  (Parameters)

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• Method cannot return multiple values
• Void methods generally print/display stuff
• Is nesting of methods allowed?
• Static methods can be accessed only by static
  methods
• Know scope of identifiers within a class
• Method Overloading, know what this is and how can
  u implement this while writing classes? Can
  return types of overloaded methods be different?
• Know how to work with random objects
• Sample Problem on Chapter 7 (Difficulty level
  Easy)
• Write a method that can be used by other
  users in their classes to check if an
  alphanumeric code is a palindrome or not. The
  method must take in the code and its length as
  parameters. (You may assume that the code doesnt
  have spaces in between)

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Chapter 8 User Defined Classes

• Encapsulation combining data with operations on
  the data Core of OOP
• An object combines data and operations on the
  data
• Instance Variables are the non static data
  members of the class
• Do not use public instance variables on the exam
  while writing classes unless the question demands
  it.
• Constructors execute when an object of the class
  is created. Special methods with no Return type
  (NOT VOID)
• Constructors can be overloaded, they must have
  different signatures

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• Shallow copying Vs Deep Copying of data.
• Dont worry too much about the technicality just
  the how to implement the stuff
• Accessor Vs Mutator methods get Vs set
• Using the . member access operator to refer to
  instance variables.
• toString is inherited and prints the class name
  and the hash code of the object by default
• You need to over ride this method. Different
  from overloading
• The implicit reference this

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Using a Static Variable as a counter in a class

• Why would you use a static data member?
• As ID for object
• To keep track of how many objects have been
  instantiated so far
• Know that static data members are common to all
  the objects in the class

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Chapter 9 Array Manipulation

• General Syntax
  dataType or name new
  dataTypesize or mn
• arrayName.length gives the length of the array
• Array indexing begins from 0
• What is the ArrayIndexOutOfBoundsException?
• In case of a 2D array arrayName0.length gives
  the number of columns in the 2D grid assuming
  its an uniform grid
• Can have arrays of user defined objects. Ex
  Array of Boats
• know to write methods to compare, copy and find
  elements in an array using simple for loops

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Example Equal Arrays?

• public static boolean isEqual(int list1, int
  list2)
• int len1 list1.length int len2 list2.length
• if (len1!len2)
• return false
• for(int a0 altlen1a)
• if(list1a!list2a)
• return false
• return true

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Chapter 10 Applications of Arrays

• Typical questions are the ones where you have to
  tell what an array looks like in the ith
  iteration of the for loop involved in the code
• So you need to know what happens at each step in
  sorting/searching
• Algorithms you need to know Bubble sort,
  insertion sort, merge sort (recursive), linear
  search, binary search

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Simple Sorting Algorithms 0(N2)

• O(N2) sorting algorithms.
• In bubble sort, a next position to fill is
  compared with all later positions, swapping
  out-of-order values.
• In selection sort, the smallest value in the
  remaining positions is computed and swapped with
  the value in the next position.
• In insertion sort, the next value is moved
  backward (swapped with the value in the previous
  position in the region of sorted values) until it
  reaches its correct position.

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Complex Sorting Algorithms 0(Nlog2N)

• Merge Sort, heap sort, quick sort
• Just look at the code, you will NOT be expected
  to write code that implements these algorithms

Binary Search on a sorted list only!!

• O(log N)
• Done using a recursive algorithm
• Efficient

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Chapter 11 Inheritance and Polymorphism

• Inheritance is-an relationship
• A sub-class extends the definition of a super
  class
• Private members of the super class cannot be
  accessed directly by the sub class
• Sub class can override methods of the super
• All data members of the super class are members
  of the sub class as well
• Multiple inheritance is NOT allowed in java
• Use super and . to access overridden methods of
  the super class

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• Protected members of the super class can be
  accessed by the sub class
• A constructor of a sub class cannot initialize
  the data members of the super class
• Needs to call the constructor of the super class
  with the reserve word super
• The instanceOf operator is used to determine if
  an object is an instance of a particular class
  type

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Class Object

• Indirectly the super class of every class in Java
• Important Method that you NEED to know how to
  over write is
• public boolean equals(Object obj)

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Comparable Interface

• Any class that implements this interface should
  override the method
• public int compareTo (Object o)
• The next slide has an example of how this
  method should be written correctly.
• The practice tests posted online also have an
  example.

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• Say you were doing this for the Waiter class
• public int compareTo(Object o)
• //check if o is a Waiter to begin with
• if(o instanceof Waiter)
• //Begin by typecasting o to a Waiter
• //Do the appropriate computation to
  determine which Waiter wins the comparison
• else
• //Handle appropriately depending on
  program specifications

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Chapter 12 Exceptions

• Idea of an exceptional event, what do when such
  an event occurs
• Try catch finally blocks
• Throw and throws
• Throwable class and its sub classes
• Kinds of Exceptions (Just the basic types)

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Chapter 14 Recursion

• Base cases must be defined appropriately in order
  to prevent infinite recursion
• Direct Vs indirect

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Examples

• Assume that the following declaration has been
  made at the beginning of the main class
  declaration
• static random rand new Random()
• a. Assuming that ch1 and ch2 are initialized
  character variables, with ch1ltch2 and both
  uppercase, write a statement to put a character
  between ch1 and ch2(inclusive) into a third
  variable ch3. 6 points
• b. Write the header of a method that could appear
  in a main driver program which has 2 character
  parameters and returns a random upper case letter
  between those values
  3 points
• c. Write a recursive statement that could be
  added to the definition of the above method in
  order to handle the case where the first actual
  parameter comes later in the alphabet than the
  second one. You can still assume they are both
  uppercase. 3 points
• d. Write javadocs for the method above
  5 points

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Potential Types of Questions

• Recursive Method writing (can be hard)
• Tracing through a chunk of code where in a
  recursive method is called several times. The
  method definition will be given to you
• Code tracing with arrays
• Writing methods, java docs and appropriate method
  calls for them, dont take this lightly.
• Completing class definition using the comments
  and hints given

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• Writing syntactically correct statements to get
  some specific output from the class definition
  you completed just as in the sample practice
  exams
• Questions on inheritance, maybe writing a
  sub-class for a given super class
• Valid/ invalid statements
• True/false
• Fill in the blanks
• Multiple choice
• Other surprise stuff

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General Stuff

• Be careful, especially in the multiple choice
  questions
• Read the summaries of the chapters in the text
  book
• Shoot me an email if you have any questions. I
  will respond until midnight today within a short
  time
• Do the reviews posted on the website under exam
  conditions
• Write down stuff for partial credit in case of
  code tracing type of questions
• Further questions Email me at jaimadhok_at_gmail.com
  
• Good Luck!

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• public static String vowels(int len)
• String vAEIOU
• String makeV
• for(int i0iltleni)
• makeVv.charAt(rand.nextInt(5))
• return makeV

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• / Generate a random string of vowels of a
  desired length
• _at_param len is the integer length
• _at_return is the random string generated
• /
• System.out.println(vowels(8))

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• Public PhoneEntry(String n, long c)
• namen
• cellc
• entries

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• Public String getEntry()
• return name
• Public static int getEntries()
• return entries

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• System.out.println(PhoneEntry.getEntries())
• PhoneEntry myself new PhoneEntry(jai,234321432)
  
• System.out.println(pe.getName())
• PhoneEntry cellphonenew PhoneEntry65
• Cellphone0myself

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• public int compareTo (Object o)
• Implements Comparable next to the class header

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• Public boolean equals(Object o)
• If(! O instanceof PhoneEntry)
• System.out.println(ERROR!!)
• System.exit(0)
• PhoneEntry newP (PhoneEntry) o
• If(this.name.equalsIgnoreCase(newP.name))
• If(this.cellnewP.cell)
• Return true
• Return false

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• Public int compareTo(Object obj)
• If(! Obj instanceof Car)
• Car c (Car) obj

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• Public String toString()
• Return super.toString() fagewgewgewgew