Object Oriented Programming Lab Laboratory Exercise Object Oriented Programming in Java Supplemental Material for Baldwin and Scragg, Algorithms and Data Structures: The Science of Computing; Charles River Media, 2004 (Now published by Cengage Learning) Site Index Purpose This lab reinforces understanding of basic object oriented programming concepts (objects, classes and subclasses, methods) and their expression in Java. It also provides practice using non-object aspects of Java (loops, conditionals, etc.) Prerequisites Understanding of Chapter 2 of Algorithms and Data Structures: The Science of Computing. Understanding of basic Java syntax, as described in the appendix to Algorithms and Data Structures: The Science of Computing. Background This lab exercises many of the object oriented programming concepts discussed in Chapter 2 of Algorithms and Data Structures: The Science of Computing (for instance, objects, messages, methods, classes and subclasses). The lab does this by asking students to extend the abilities of the robots introduced in Chapter 2, and then use the extended robots to solve a certain problem. Robots This lab involves defining a subclass of the Robot class from Chapter 2 of Algorithms and Data Structures: The Science of Computing. Programs that use the Robot class need to include two Java files: Robot.java and RobotRoom.java. The “Final Details” section of this document explains how to find these files and their documentation. Any Java source file that refers to the Robot or RobotRoom classes should “import” those classes, via the statement import geneseo.cs.sc.*; at the beginning of the file. Constructors in Subclasses A constructor is basically a method that initializes a new object (see Sections 3.4.2 and A.4.4 of Algorithms and Data Structures: The Science of Computing for more on constructors). In Java, constructors have the same name as the class they initialize — for example, the constructors for Robot objects are named Robot, the constructors for instances of a hypothetical ExtendedRobot subclass of Robot would be named ExtendedRobot, and so forth. Note that subclasses don’t inherit constructors from their superclass the way they inherit other methods — for example, even if a constructor for Robot logically does everything necessary to initialize instances of an ExtendedRobot subclass, there is no way to automatically apply this constructor to ExtendedRobot objects. Even though Java doesn’t do it automatically, one often wants to initialize instances of a subclass by just calling a superclass’s constructor. This will probably be the case for the subclass of Robot defined in this lab. To do this, define constructors for the subclass that do nothing but call the corresponding superclass constructor. Within a constructor, the word super can be used to call a superclass constructor. For example, to allow instances of an ExtendedRobot subclass of Robot to be initialized with their position, heading, and room (just like the four-parameter constructor for Robot does), include the following constructor in ExtendedRobot: // Within the ExtendedRobot class...
public ExtendedRobot( int column, int row, int heading, RobotRoom room ) {
super( column, row, heading, room );
} A statement such as the following implicitly uses this constructor to initialize an extended robot: ExtendedRobot r = new ExtendedRobot( 1, 3, Robot.NORTH, myRoom ); Exercise Do the following… Part 1 Write a subclass of Robot that handles the following messages. Don’t forget to test the subclass, to make sure that each method works as intended: void travel( int n ) This message causes a robot to move forward n tiles. The message has a precondition that there are no obstructions within n tiles in front of the robot. void travel() This message causes a robot to move as far forward as it can, stopping when it comes to an obstacle. (Note that even though this message has the same name as the above “travel” message, it is distinguishable because it has a different set of parameters — no parameters in the case of this message, versus one integer parameter in the case of the previous “travel”. Many modern programming languages distinguish between messages based on their parameters as well as their names in this manner.) void face( int direction ) This message causes a robot to face in the direction specified by direction. The message has a precondition that direction is one of the direction constants defined in the Robot class (i.e., Robot.NORTH, Robot.EAST, Robot.SOUTH, Robot.WEST). void safeMove() This message causes a robot to move one tile forward if there is no obstruction immediately in front of the robot. Otherwise the robot stays where it is. void changeColor( java.awt.Color c ) This message causes a robot to paint the tile underneath itself color c, if the tile is not already that color. If the tile is already color c, the robot paints the tile white. This message has a precondition that c is not white. Part 2 Write a main program that creates two instances of the subclass from Part 1, positioned arbitrarily in a room, and then makes those two robots move until they are next to each other. More precisely, the code that moves the robots until they are next to each other should have preconditions There are two robots in the room There are no interior walls in the room This code should establish the postcondition The robots are on adjacent tiles (Notice that what these pre- and postconditions do not say is just as important as what they do say: for example, the preconditions do not allow assuming that the robots start in specific places, or with specific headings. The postconditions do not require the robots to be on particular tiles, or have particular headings.) Some, but not necessarily all, of the messages handled by the new subclass will probably be helpful in making the robots stand next to each other. Final Details The Robot Class Students can download both Robot.java and RobotRoom.java from the Web. Documentation on both classes is also available on the Web. The main documentation page is an index to documentation for all the Java classes written for use with Algorithms and Data Structures: The Science of Computing. To see the documentation for a specific class, click on that class’s name in the left-hand panel of the page. Submitting Your Work Turn in your work as directed by your instructor. Copyright © 2004 Charles River Media. All rights reserved. Revised Aug. 8, 2005 by Doug Baldwin Site Index
