1. Going further
Enumerated types
Recursion
Collections

2. enum Season {winter, spring, summer, fall};
Enumerated Types
Java allows you to define an enumerated type, which has the set of possible values specified in the definition.
The values are identifiers of your own choosing
The following declaration creates an enumerated type called Season
enum Season {winter, spring, summer, fall};
Any number of values can be listed

3. Enumerated Types
Once a type is defined, a variable of that type can be declared
Season time;
and it can be assigned a value
time = Season.fall;
Enumerated types are type-safe – you cannot assign any value other than those listed

4. Ordinal Values
Each value of an enumerated type has an integer associated with it, called its ordinal value
The first value in an enumerated type has an ordinal value of zero, the second one, and so on
You cannot assign a numeric value to an enumerated type, even if it corresponds to a valid ordinal value

5. Enumerated Types Each variable of an enumerated type is an object
Enumerated typs have several methods
The ordinal method returns the ordinal value of the object
The name method returns the name of the identifier corresponding to the object's value
See IceCream.java

6. Enumerated Types
An enumerated type definition can be more interesting than a simple list of values
Because they are like classes, we can add additional instance data and methods
We can define an enum constructor as well
Each value listed for the enumerated type calls the constructor
See Season.java
See SeasonTester.java

7. Enumerated Types
Every enumerated type contains a static method called values that returns a list of all possible values for that type
The list returned from values is iterable, so a for loop can be used to process them easily
An enumerated type cannot be instantiated outside of its own definition
A carefully designed enumerated type provides a versatile and type-safe mechanism for managing data

8. Recursion

9. Recursion
Recursion is a fundamental programming technique that can provide an elegant solution certain kinds of problems
A recursive definition is one which uses the word or concept being defined in the definition itself
a recursive definition of an English word is often not helpful
In math and computer science, a recursive definition can be an appropriate way to express a concept

10. Recursive Definitions
Consider the following list of numbers:
24, 88, 40, 37
Such a list can be defined as follows:
A LIST is a: number
or a: number comma LIST
That is, a LIST is defined to be a single number, or a number followed by a comma followed by a LIST
The concept of a LIST is used to define itself

11. Recursive Definitions
The recursive part of the LIST definition is used several times, terminating with the non-recursive part:
number comma LIST
, 88, 40, 37
, 40, 37
, 37
number 37

12. Infinite Recursion
All recursive definitions have to have a non- recursive part
If they didn't, there would be no way to terminate the recursive path
Such a definition would cause infinite recursion
This problem is similar to an infinite loop, but the non-terminating "loop" is part of the definition itself
The non-recursive part is often called the base case

13. Recursive Definitions
N!, for any positive integer N, is defined to be the product of all integers between 1 and N inclusive
This definition can be expressed recursively as:
1! = 1
N! = N * (N-1)!
A factorial is defined in terms of another factorial
Eventually, the base case of 1! is reached

14. Recursive Definitions5!
5 * 4!
4 * 3!
3 * 2!
2 * 1! 1 2 6 24
120

15. Recursive Programming
A method in Java can invoke itself; if set up that way, it is called a recursive method
The code of a recursive method must be structured to handle both the base case and the recursive case
Each call to the method sets up a new execution environment, with new parameters and local variables
As with any method call, when the method completes, control returns to the method that invoked it (which may be an earlier invocation of itself)
See Factorial.java

16. Recursive Programming
Note that just because we can use recursion to solve a problem, doesn't mean we should
For instance, we usually would not use recursion to solve the sum of 1 to N problem, because the iterative version is easier to understand
However, for some problems, recursion provides an elegant solution, often cleaner than an iterative version
You must carefully decide whether recursion is the correct technique for any problem

17. Recursive Directory Listing
Suppose you want to list all the files in a directory including files in any subdirectories that might be present.
Algorithm
for each entry in the current directory
if it is a file, print its name
else // must be a directory
do recursive call on the entry

18. Towers of Hanoi
The Towers of Hanoi is a puzzle made up of three vertical pegs and several disks that slide on the pegs
The disks are of varying size, initially placed on one peg with the largest disk on the bottom with increasingly smaller ones on top
The goal is to move all of the disks from one peg to another under the following rules:
We can move only one disk at a time
We cannot move a larger disk on top of a smaller one

19. Original Configuration
Towers of Hanoi
Original Configuration
Move 1
Move 2
Move 3

20. Towers of Hanoi
Move 4
Move 5
Move 6
Move 7 (done)

21. Towers of Hanoi
An iterative solution to the Towers of Hanoi is quite complex
A recursive solution is much shorter and more elegant
See SolveTowers.java
See TowersOfHanoi.java

22. Fractals
A fractal is a geometric shape made up of the same pattern repeated in different sizes and orientations
The Koch Snowflake is a particular fractal that begins with an equilateral triangle
To get a higher order of the fractal, the sides of the triangle are replaced with angled line segments
See KochSnowflake.java
See KochPanel.java

23. Koch Snowflakes < x5, y5> < x1, y1> < x5, y5>
Becomes

24. Other Examples of Recursion
Recursive sort algorithms
Recursive data structures
Chapter 12
COMPSCI 225

25. Collections

26. Collections
A collection is an object that serves as a repository for other objects
A collection usually provides services such as adding, removing, and otherwise managing the elements it contains
Sometimes the elements in a collection are ordered, sometimes they are not
Sometimes collections are homogeneous, containing all the same type of objects, and sometimes they are heterogeneous

27. Dynamic Structures A static data structure has a fixed size
This meaning is different from the meaning of the static modifier
Arrays are static; once you define the number of elements it can hold, the size doesn’t change
A dynamic data structure grows and shrinks at execution time as required by its contents
A dynamic data structure is implemented using links

28. Object References
Recall that an object reference is a variable that stores the address of an object
A reference also can be called a pointer
References often are depicted graphically:
student
John Smith
40725
3.58

29. References as Links
Object references can be used to create links between objects
Suppose a Student class contains a reference to another Student object
John Smith
40725
3.57
Jane Jones
58821
3.72

30. References as Links
References can be used to create a variety of linked structures, such as a linked list:
studentList

31. Queues
A queue is similar to a list but adds items only to the rear of the list and removes them only from the front
It is called a FIFO data structure: First-In, First-Out
Analogy: a line of people at a bank teller’s window
enqueue
dequeue

32. Stacks A stack ADT is also a linear data structure
Items are added and removed from only one end of a stack
It is therefore LIFO: Last- In, First-Out
Example: a stack of plates in a cupboard, a stack of hay bales in a barn
Stacks often are drawn vertically:
pop
push

33. Trees
A tree is a non-linear data structure that consists of a root node and potentially many levels of additional nodes that form a hierarchy
In a general tree, each node can have many child nodes
We often work with binary trees which have no more than two children per node

34. Binary Trees
In a binary tree, each node can have no more than two child nodes
A binary tree can be defined recursively. Either it is empty (the base case) or it consists of a root and two subtrees, each of which is a binary tree
Trees are typically are represented using references as dynamic links, though it is possible to use fixed representations like arrays
For binary trees, this requires storing only two links per node to the left and right child

35. Graphs A graph is a non-linear structure
Unlike a tree or binary tree, a graph does not have a root
Any node in a graph can be connected to any other node by an edge
Analogy: the highway system connecting cities on a map

36. The Java Collections API

37. Collection Classes
The Java standard library contains several classes that represent collections, often referred to as the Java Collections API
Their underlying implementation is implied in the class names such as ArrayList and LinkedList
Several interfaces are used to define operations on the collections, such as List, Set, SortedSet, Map, and SortedMap

38. Generics
As mentioned in Chapter 7, Java supports generic types, which are useful when defining collections
A class can be defined to operate on a generic data type which is specified when the class is instantiated:
LinkedList<Book> myList =
new LinkedList<Book>();
By specifying the type stored in a collection, only objects of that type can be added to it
Furthermore, when an object is removed, its type is already established

39. Beyond java

40. Other Languages
There are a number of other languages with a syntax very similar to that of Java
Java's syntax is based on that of C which is not object-oriented
C++ was designed to be backwards-compatible with C
C# was based on C/C++ without the backwards-compatibility
Many of the scripting languages started from C's syntax
JavaScript is a scripting language that is used inside web pages
It is not related to Java

41. Other Languages: C# Like Java, C# is
A modern, general-purpose object-oriented language
Based on C and C++
C# is part of the .NET family of languages supported by MicroSoft
Multiple languages which can interoperate
Languages compile to a common intermediate language
Common Language Runtime runs programs from all the .NET languages

42. C# Program Structure A program consists of one or more files
A file can contain one or more classes and/or namespaces
name of file is not tied to name of class
At least one class must contain Main
There are several allowed signatures
return type is either int or void
either no parameter or String array
Missing main is compile-time error

43. First Program namespace FirstProgram { class First {
static void Main() {
System.Console.WriteLine(
"Welcome to C#!"); }
Notice different conventions for capitalizaion
namespace is optional; kind of like a package

44. Console I/O System.Console.WriteLine( arg) System.Console.ReadLine()
argument can be any type
for objects, ToString is invoked
System.Console.ReadLine()
returns a String
System is a namespace
using System; allows you to omit the namespace when calling the method

45. C# Types Value types Reference types - objects
simple types: primitive types from Java plus unsigned types and decimal
Reference types - objects
Coercion rules similar to those of Java
autoboxing

46. Operators
Has same operators as Java with similar precedence and associativity
== compares values for strings and simple types, addresses for all other objects

47. C# on onyx
mono is an open-source project that provides facilities for running C# programs under Linux
Compile a program by typing
mcs First.cs
Run a program by typing
mono First.exe

48. C# using VMWare
There is a Windows virtual machine on the onyx workstations
vmware &
C# Express is installed on the virtual machine
You can download the .NET development environment to your own Windows machine for free using your MSDN account