Lecture 32: Implementing the Composite Pattern

1. Computer Science 313 – Advanced Programming Topics Lecture 32:Implementing theComposite Pattern

2. Pimp my File Manager • Writing a Java-based file manager • Starts at drive, but drill-down into directories • Considers only directories & files (for now) • Named required for all directory entries • Print out the names of directories & files • Print out files sizes as we traverse directories • Allow user to create & delete files • Remain true to our basic nature

3. Pimp my File Manager • Writing a Java-based file manager • Starts at drive, but drill-down into directories • Considers only directories & files (for now) • Named required for all directory entries • Print out the names of directories & files • Print out files sizes as we traverse directories • Allow user to create & delete files • Remain true to our basic nature

4. Composite Pattern Classes • Create abstract superclass for the pattern • This class normally used by client code • Common methods & fields defined in superclass • Should also declare any abstract methods needed • Other classes subclass of abstract superclass • Decorator-like parallel hierarchies not required • Subinterfaces not required & almost never used • Could create hierarchies as needed • But consider composition vs. inheritence

5. Composite Code Inside public abstract class OSEntry {public String name;public abstract String toString();public void createFile(String fileName) throws UnsupportedOperationException { throw new UnsupportedOperationException();}public void createDir(String dirName) throws UnsupportedOperationException { throw new UnsupportedOperationException();} }

6. Directories Contain…? • Currently, each directory has lots of files • Will need a field to store these references • Could use an array or some type of List • Question is: what type should List hold? • List could simply store references to File • Unable to hold new types of entries we create • Class is forever closed to any type of extension • Containers must refer to abstract superclass

7. Woot! Pimped-Out Recursion! public class Directory extends OSEntry {public List<OSEntry> contents;public String toString() { String retVal = getName(); for (OSEntry entry : contents) {retVal += entry.toString(); } return retVal;}

8. Container Code public class Directory extends OSEntry {public List<OSEntry> contents;public void createFile(String fName) { String newFile = new File(getName(), fName);contents.add(newFile);}public void createDir(String dName) { String dir = new Directory(getName(), dName);contents.add(dir);}

9. File Methods… • Files should not be able to create files & dirs • Methods declared previously, so that is good • Even better, already throw documented exception • Useful trick when option may/may not be possible • Should be careful about too many exceptions • Should be able to get size of a file • Most classes should not provide this method • Instead, we will not declare this in the superclass • Clients must have a File before calling method

10. File Code public class File extends OSEntry {public java.util.FilefileInOS;public String toString() { return getName();}public long getSize() { return fileInOS.getLength();} }

11. Client Code OSEntry entry; System.out.println(entry.toString()); if (entry instanceof File) {System.out.println(“Size is :” + ((File)entry).getSize()); } else {System.out.println(“No size, bozo!”); }

12. For Next Class • Lab available on the web • Lab will be due 2 weeks from Friday • No class this Friday • Some percentage of class with me in Hartford, CT • Use time wisely – work on lab, study for the test #2 • Test #2 in class on Monday • Can include any and all material since last test • Patterns & optimizations fair game to ask about • Open-note, open-book, open-template, but closed slide