CpSc 215: Software Development Foundations Fall 2014 Closed Lab #8: The Flyweight Pattern 1 Overview There are four objectives to this closed lab. First, you will gain additional experience developing classes that define static methods, fields, and initializers. Second, you will gain additional experience working with the Java IO library. Third, you will be introduced to the Flyweight pattern, a cousin of the Singleton pattern discussed in class. Finally, you will gain some insight into the memory footprint associated with Java objects. 2 Flyweight Pattern In designing object-oriented systems, it is convenient to represent all data elements within a program as objects. This enables developers to adopt a uniform programming approach that is independent of the differences between scalar types and reference types. In some cases, however, the number of data elements that need to be managed is large, making it prohibitively expensive to use a single object per data element. Consider, for example, a word processing application in which each character is represented as an object. This could get very expensive1 ! The Flyweight pattern is a cousin of the Singleton pattern. Its intent is to enable object-based representations of fine-grained data elements through an efficient object sharing strategy. As you will see, the approach is very similar to that prescribed by the Singleton pattern. 3 Requirements You are required to develop the following classes. In doing so, it is up to you to decide on the most appropriate accessibility modifiers (e.g., public, private) to use when declaring your fields, methods, and constructors. Your TA will check to see that you have made the right choices. • CharacterA. This class will be used to represent a single character. More precisely, the class will serve as a basic wrapper for an element of type char. The class should provide a constructor that accepts a single argument — namely, the char value to be wrapped by the constructed object. The class should also provide a suitable implementation of toString(). • CharacterB. This class will begin as an exact copy of CharacterA. You will then apply the following changes: First, you should prevent instances of CharacterB from being constructed by clients. Next, you should define a static method, createCharacter(), to enable clients to retrieve a reference to a CharacterB object. The particular object returned should depend on the char argument passed to the method. So, for example, a call to createCharacter() with ‘a’ passed as argument should yield the CharacterB object that wraps ‘a’. should create each of the required Character wrappers at load-time, without an explicit call placed by the programmer. When createCharacter() is invoked, it should simply return a reference to the appropriate CharacterB object. CharacterB 1 This example comes from the Gang of Four’s famous text, Design Patterns: Elements of Reusable Object-Oriented Software. The Singleton and Flyweight patterns were originally described by these authors. 1 CpSc 215: Software Development Foundations Fall 2014 • MainDriverA. This class will implement a simple “echo” application, similar to the application you developed in a previous closed lab. It will begin by reading the contents of the text file passed as argument and storing each character that is read within an array of CharacterA objects. Next, it will iterate through the array and print out each object; the result should exactly match the contents of the input text file. Finally, the application will report the approximate amount of memory consumed by the objects within the CharacterA array. Note that MainDriverA should allocate an array (of CharacterA) that is exactly the right size to hold the contents of the text file – no larger, no smaller. (Hint: You can determine the number of bytes in a file using one of the methods provided by the java.io.File class.) • MainDriverB. This class will implement the same logic as MainDriverA. The only difference is that the implementation will use CharacterB instead of CharacterA. 4 Memory Usage To retrieve the amount of heap space available within the Java virtual machine, you should use the following code fragment: long freeMemory = Runtime.getRuntime().freeMemory(); Recall that Java is a garbage collected language. If you invoke this method at various points in your program, you might see surprising results if the garbage collector is invoked unexpectedly. 5 Grading To receive full credit for this lab, you must demonstrate that you have satisfied the above requirements. When you think you are ready, ask one of the TAs to check your solution. When they have checked your name off of their list, you are free to leave. If you are unable to complete the lab before the end of the period, but have made a reasonable attempt, you will receive 1/2 credit for the day. If the TA feels that you have not made a reasonable attempt, you will not receive any credit for your participation. 6 Collaboration You may work independently or with one partner. You must not discuss the problem or the solution with classmates outside of your group. Keep this in mind before you choose to work independently. 2