Iterator COMP 401 Prasun Dewan1 18 Iterator Scanning Streams Now that we understand some basic principles of creating and using objects let us revisit the scanning problem we saw earlier and redo it using objects In this process we will learn about an important kind of interface called an enumeration or iterator interface which applies to scanning and other kinds of problems We will motivate this interface using another scanning problem we will show that the solutions to the two problems can reuse code if such an interface is defined Previous Scanning Problem Recall the scanning problem we implemented in the introduction Figure 1 Finding Upper Case Letters The following is the code we wrote package warmup public class AnUpperCasePrinter public static void main String args if args length 1 System out println Illegal number of arguments args length Terminating program System exit 1 System out println Upper Case Letters 1 1 Copyright Prasun Dewan 2009 Iterator int index 0 while index args 0 length if Character isUpperCase args 0 charAt index System out print args 0 charAt index index System out println Figure 2 An UpperCasePrinter Scanning Problem Extension Consider an extension of the problem above in which we wante to print a string forwards and backwards as shown below Figure 3 Forward and Reverse Printing We can scan the string twice once forwards and once backwards to implement this user interface but that is inefficient So what we will do instead is store the upper case characters we scan into an array and then simply print the array backwards as shown below package warmup public class AReverseUpperCasePrinter static final int MAX CHARS 5 static char upperCaseLetters new char MAX CHARS static int numberOfUpperCaseLetters 0 public static void main String args if args length 1 System out println Illegal number of arguments args length Terminating program System exit 1 int index 0 System out println Upper Case Letters while index args 0 length 2 Iterator if Character isUpperCase args 0 charAt index System out print args 0 charAt index storeChar args 0 charAt index index System out println printReverse public static void storeChar char c if numberOfUpperCaseLetters MAX CHARS System out println Too many upper case letters Terminating program System exit 1 upperCaseLetters numberOfUpperCaseLetters c numberOfUpperCaseLetters public static void printReverse System out println Upper Case Letters in Reverse for int index numberOfUpperCaseLetters 1 index 0 index System out print upperCaseLetters index Figure 4 Code for Reverse and Forward Printing In the interest of modularity separate methods are defined for storing a character in the array and for printing them in reverse Comparison of two solutions As we can see by comparing Figures 1 and 2 the second user interface is an extension of the first one Yet the programs implementing the two user interfaces do not share any code A symptom of this problem is that the main class does all of the computation Ideally a main class should be skinny simply instantiating classes and invoking methods in the instantiated classses The real work should be carried out in the instantiated classes that is what makes the code object oriented and allows for code sharing Let us look at some concrete consequences of violating this principle Compare the two loops in the classes loop in AnUpperCasePrinter while index args 0 length if Chracater isUpperCase args 0 charAt index System out print args 0 charAt index 3 Iterator index loop in AReverseUpperCasePrinter while index args 0 length if isUpperCase args 0 charAt index System out print args 0 charAt index storeChar args 0 charAt index index The only difference in the two is that the latter does an extra step in the loop body of storing a character in the array Yet we do not share the code If we followed our principle of creating an extra class for the scanning part as shown in Figure 5 then we should be able to share code Scanner class that produces tokens Instance of Scanner object Intantiates Uses Invokes methods in Main Class that prints and or stores tokens Figure 5 Creating a separate scanner class Index based Scanner Interface It is not easy to figure out the nature of the interface of such a class One approach is to create one that exports an array of scanned tokens public interface IndexBasedScanner public String getTokenArray public int getNumberOfTokens Here is an outline of the implementation of such an interface 4 Iterator public class AnIndexBasedScanner implements IndexBasedScanner static final int MAX CHARS 5 static char upperCaseLetters new char MAX CHARS static int numberOfUpperCaseLetters 0 public AnIndexBasedScanner String theScannedString scanAndStore theScannerString void scanAndStore String theScannedString store all scanned tokens in upperCaseLetters public String getTokenArray return upperCaseLetters public int getNumberOfTokens return numberOfUpperCaseLetters The idea here is to extract and store all tokens when the scanner is instantiated A user of the class simply accesses the stored values which are exposed by two properties NumberOfTokens which stores the number of scanned tokens and TokenArray which stores the tokens in its first NumberOfTokens slots The getter methods for these are trivial simply returning values of associated instance variables The only non trivial method is scan However it is a straightforward variation of the code we saw above scanning all tokens in a single loop and storing each of them in the array In comparison to the implementations we see below this implementation is the simplest one However it has the serious disadvantage that it scans and stores all of the tokens even if the user of the object needs only the first few tokens and does no need these tokens to be stored To help us determine an interface let us look at a real world problem involving radio scanning and at Java input libraries that do scanning Radio Scanning Figure 6 shows how two parties can be involved in a scanning problem one doing the scanning and the other using the results of the scanning 5 Iterator Figure 6 Analagy with Radio Scanning Here the two parties are the human and a radio providing an automatic scan operation Each time the human invokes this operation the radio automatically tunes to the next station As the figure shows we have to worry about invoking the operation when we reach the station broadcasting at the highest frequency Modern radios simply restart the scan tuning