Going further Enumerated types Recursion Collections

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

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

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

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

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

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

Recursion

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

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

Recursive Definitions The recursive part of the LIST definition is used several times, terminating with the non-recursive part: number comma LIST 24 , 88, 40, 37 88 , 40, 37 40 , 37 number 37

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

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

Recursive Definitions 5! 5 * 4! 4 * 3! 3 * 2! 2 * 1! 1 2 6 24 120

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

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

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

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

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

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

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

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

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

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

Collections

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

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

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

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

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

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

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

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

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

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

The Java Collections API

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

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

Beyond java

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

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

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

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

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

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

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

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

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