Final Review Recitation – 05/01/2009

CS 180

Department of Computer Science,

Purdue University

Final Exam Overview Time and place

10:20am on May 6, at SMTH 108 Types of questions

30 multiple choice questions 5 programming questions on

Objects, inheritance, exception, file I/O, recursion Cover all chapters, with a focus on later chapters that

haven’t been tested FYI: old exams available on course website

Fall 2006 exam I, II, and final

TA evaluation online, open until 11pm, May 2

Object-Oriented Design

Methods Extract small parts of a program and

calculations which will be performed multiple times

Encapsulation Classes, Objects, and Polymorphism

Covered in more detail later

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4

Primitives Numeric: int, double, float, double, … Character: char Logic: boolean They are NOT Objects

Can be compared with ==, !=, <=, >=, etc. Cannot call methods

Explicit type-castingdoublefloatlongintshortbyte

Implicit type-castingbyteshortintlongfloatdouble

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5

Objects Objects are handled with a reference address Two objects should be compared with the .equals() method

Do NOT use == to compare objects Assignment operators assign references – they do not make separate

copies NullPointerException – always make sure your object is not null

before calling methods with it Constructors should be used to initialize class variables Calling methods

Need object Static methods

Can use class name Cannot access non-static methods/variables inside

Keyword: this

Strings Strings are a type of object

Also should NOT be compared with == Can be concatenated with the ‘+’ operator Important String functions

charAt indexOf substring length

Selection Modifies the flow of control of the program if/else construct

Must have a boolean condition to check against { } are important, but not necessary for one line

statements else branch is optional Don’t be confused by indentation..

switch construct Multi-way branch which makes a decision based on a

char, byte, short, or int default case break statement

Repetition for while do-while Pitfalls

Off-by-one errors Infinite looping Different control flows

GUI and Event-driven Programming Common classes

JFrame JPanel JLabel JMenu, JMenuItem JButton

Layouts

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Arrays Linear collection of data

Can hold primitives or Objects, but must all be of the same type

length tells the number of elements in the array Member, not a method

Indexed from 0 to length – 1 Protect against

ArrayIndexOutOfBoundsException Can create multiple dimension arrays Usually use for-loops to iterate through members

of the array

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Exceptions and Assertions

Use a try/catch/finally block to handle exceptions thrown by a program

Use throw statement to notify caller of an error Exception types and hierarchy. Examples of

RunTimeHierarchy? User defined exceptions must extend Exception Use assertions to detect internal errors. Use

exceptions to notify the client programmer of the misuse of our class. When assertions fail, an AssertionError is thrown.

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12

File I/O Many classes

FileInputStream, DataInputStream, FileReader, BufferedReader, Scanner, PrintWriter, DataOutputStream, etc.

ObjectInputStream, ObjectOutputStream Used to write objects to a file Any object serialized by a stream must implement

Serializable Which classes are used to write text and which

are used to write binary? Always close files you open

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Inheritance and Polymorphism Differences between abstract classes and

interfaces Polymorphism can simplify code Extend specific classes from general

classes Use protected keyword to protect

information and methods No need to rewrite methods which are the

same as in a parent class Super constructor is always called as the

first line of constructor

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Dynamic Data Structures and Generics

Inner classes Lists

Node class Circularly linked lists Doubly linked lists

Java Collections Vector ArrayList LinkedList

Recursion Think about what the sub-problem is Only be concerned with the current level of

recursion Two necessary cases

Base case Recursive case

Review ends here.

Now time for exercises!

What is the output?public class A

{

private int x;

public static int doStuff()

{

x = 100;

x /= 3;

x++;

return x;

}

public static void main(String[] args)

{

System.out.println(A.doStuff());

}

}

What is the output?

public class A

{

private int x;

public static int doStuff()

{

x = 100;

x /= 3;

x++;

return x;

}

public static void main(String[] args)

{

System.out.println(A.doStuff());

}

}

Because this method is static, it does not haveaccess to non-static class variables. This code willNot compile because x is non-static.

Types

Given the following classes, which of the following declarations are valid?

public interface I {…}

public interface J extends I {…}

public interface K {…}

public abstract class A {…}

public class B extends A {…} implements J, K

public class C extends B {…}

public class D extends A {…} implements I

A a = new B(); B b = new J();

C c = new B(); B b = new C();

I i = new A(); I i = new B();

I i = new D(); K k = new C();

Types

Given the following classes, which of the following declarations are valid?

public interface I {…}

public interface J extends I {…}

public interface K {…}

public abstract class A {…}

public class B extends A {…} implements J, K

public class C extends B {…}

public class D extends A {…} implements I

A a = new B(); valid – B is a subclass of A

B b = new J(); invalid – cannot instantiate interfaces

C c = new B(); invalid – not all B is the superclass of C

B b = new C(); valid – C is a subclass of B

I i = new A(); invalid – A does not implement I

I i = new B(); valid – A implements J, and J is a type of I

I i = new D(); valid – D impelements I

K k = new C(); valid – C extends B which implements K

Arrays

Write a method which takes in an array of integers and replaces the values of the array with a value ci. Define ci to be the sum of the numbers in indices 0…i in the incoming array.

public void cumulativeArray(int[] a) {

}

Arrays

Write a method which takes in an array of integers and replaces the values of the array with a value ci. Define ci to be the sum of the numbers in indices 0…i in the incoming array.

public void cumulativeArray(int[] a) {

if (a.length <= 1)

return;

for (int k=1; k<a.length; k++)

a[k] = a[k] + a[k-1];

}

Linked Lists Given an appropriate (integer) Node class,

write a recursive method which sums up the numbers in the list.

public int sumList(Node l)

{

}

Linked Lists Given an appropriate (integer) Node class,

write a recursive method which sums up the numbers in the list.

public int sumList(Node l)

{

if (l == null)

retrurn 0;

return l.num + sumList(l.next);

}

Strings

Write a recursive method reverse which takes in a String and returns the reverse of the String. You may NOT use the reverse method in the String class, however you may use other methods available to you.

public String reverseString(String s) {

}

Strings

Write a recursive method reverse which takes in a String and returns the reverse of the String. You may NOT use the reverse method in the String class, however you may use other methods available to you.

public String reverseString(String s) {

if (s == null || s.length() <= 1)

return s;

if (s.length() == 2)

return “” + s.charAt(1) + s.charAt(0);

return s.charAt(s.length()-1) + reverseString(s.substring(1, s.length()-1)) +

s.charAt(0);

}

Linked Lists

Given an appropriate Node class, write a method which removes every other node from the list, starting with the second.

public Node removeEveryOther(Node l)

{

}

Linked Lists

Given an appropriate Node class, write a method which removes every other node from the list, starting with the second.

public Node removeEveryOther(Node l)

{

Node l1 = l;

while (l1 != null && l1.next != null)

{

l1.next = l1.next.next;

l1 = l1.next;

}

return l;

}

Linked Lists

Now write the same method, but recursively.

public Node removeEveryOther(Node l)

{

}

Linked Lists

Now write the same method, but recursively.

public Node removeEveryOther(Node l)

{

// base case

if (l == null || l.next == null)

return l;

// recursive case

Node l1 = removeEveryOther(l.next.next);

l.next = l1;

return l;

}

Recursion Given an appropriate node class, write a

recursive method which reverses a singly linked list.

public Node reverse(Node l)

{

}

Recursion Given an appropriate node class, write a

recursive method which reverses a singly linked list.

public Node reverse(Node l)

{

if (l == null || l.next == null)

return l;

Node restreversed = reverse(l.next);

l.next.next = l;

l.next = null;

return restreversed;

}

Linked Lists

Given an appropriate Node class, one way to detect a cycle in your list is by moving two pointers around the list, one one index at a time and the other two indices at a time. Write a method which determines whether or not there is a cycle in a list.

public boolean hasCycle(Node l)

{

}

Linked Lists

Given an appropriate Node class, one way to detect a cycle in your list is by moving two pointers around the list, one one index at a time and the other two indices at a time. Write a method which determines whether or not there is a cycle in a list.

public boolean hasCycle(Node l)

{

Node l1 = l;

Node l2 = l;

while (l1 != null && l2 != null &

l1.next != null && l2.next != null &&

l2.next.next != null)

{

l1 = l1.next;

l2 = l2.next.next;

if (l1 == l2)

return true;

}

return false;

}

Finally…

Thanks for your hard work this semester!

Good luck with your CS180 final and all other finals and projects!

Have a wonderful summer!!