JAVA homework please help me out, thank you 3. Background: There are two sensibl

Question

JAVA homework please help me out, thank you

3. Background: There are two sensible options that a constructor or factory method could use to report failure. It could throw an exception, or itn the case of a factory method, it could return null. Look at the code distributed with the Sept. 14 lecture notes, in the readNotwork method. This code contains the line inters. add new Intersection sc )): What we want, with either solution, is to not-only not create a new intersection, but not add anything to the list inters. a) Rewrite this line so that it handles the Interectiou. ConstructorFailuw exception thrown by the constructor. (0.5 points) b) Rewrite this line so that it handles a return value of null from a call to a factory method that replaces new Intersection0. 0.5 points)

Explanation / Answer

The code so far modified

import java.util.LinkedList;
import java.io.File;
import java.io.FileNotFoundException;
import java.util.Scanner;

/** Error reporting package
* provides standard prefix and behavior for messages
*/
class Errors {

/** Report nonfatal errors, output a message and return
* @arg message the message to output
*/
public static void warn( String message ) {
System.err.println( "RoadNetwork: " + message );
}

/** Report fatal errors, output a message and exit, never to return
* @arg message the message to output
*/
public static void fatal( String message ) {
warn( message );
System.exit( 1 );
}
}

/** Roads are joined by Intersections and driven over by Vehicles
* @see Intersection
*/
class Road {
float travelTime; // measured in seconds
Intersection destination; // where this road goes
Intersection source; // where this road comes from
// name of a road is source-destination

/** construct a new road by scanning its description from the source file
*/
Road( Scanner sc ) {
// Bug: Can we prevent creation of a faulty road?
String sourceName = sc.next();
String dstName = sc.next();
source = RoadNetwork.findIntersection( sourceName );
destination = RoadNetwork.findIntersection( dstName );
if (source == null) {
Errors.warn( "No such source intersection: Road "
+ sourceName + dstName
);
// Bug: Can we prevent creation of a faulty road?
}
if (destination == null) {
Errors.warn( "No such destination intersection: Road "
+ sourceName + dstName
);
// Bug: Can we prevent creation of a faulty road?
}

if (sc.hasNextFloat()) {
travelTime = sc.nextFloat();
// Bug: Ought to complain about negative tavel times
} else {
Errors.warn( "Floating point argument expected: Road "
+ sourceName + " " + dstName
);
// Bug: An obviously wrong value?
travelTime = 99999.0F;
}
String skip = sc.nextLine();
}

/** output the road in a form like that used for input
*/
public String toString() {
// Bug: Can source be null for a badly constructed road?
// Bug: Can destination be null for a badly constructed road?
// Bug: Can the name fields be null for either intersection?
return "road "
+ source.name + " "
+ destination.name + " "
+ travelTime;
}
}

/** Intersections pass Vehicles between Roads
* @see Road
*/
class Intersection {
public String name; // textual name of intersection, never null!
LinkedList <Road> outgoing; // set of all roads out of this intersection
LinkedList <Road> incoming; // set of all roads in to this intersection
// Bug: Is incoming really needed?
// Bug: When are the above ever set to anything?

Intersection( Scanner sc ) {
name = sc.next();
// Bug: what if name = null because there was no next
if (RoadNetwork.findIntersection( name ) != null) {
Errors.warn( "Intersection redefined: " + name );
// Bug: Can we prevent creation of a faulty intersection?
}
// Bug: Finish scan of description of an intersection and create it
}

/** output the intersection in a form like that used for input
*/
public String toString() {
// Bug: Can name be null because of a bad definition?
return "intersection "
+ name;
}
}

/** Vehicles drive over Roads between Intersections
* @see Road
* @see Intersection
*/
class Vehicle {
// Bug: what are the attributes of a vehicle?
// Bug: do vehicles need to know where they are?
}

public class RoadNetwork {

/* the sets of all roads and all intersections */
private static LinkedList <Road> roads
= new LinkedList <Road> ();
private static LinkedList <Intersection> inters
= new LinkedList <Intersection> ();

/** Find an intersection by textual name in the set inters
* @param s name of an intersection
* @return the intersection named s or null if none
*/
public static Intersection findIntersection( String s ) {
// quick and dirty implementation
for (Intersection i: inters) {
if (i.name.equals( s )) {
return i;
}
}
return null;
}

/** Initialize this road network by scanning its description
*/
private static void readNetwork( Scanner sc ) {
while (sc.hasNext()) {
String command = sc.next();
if ("intersection".equals( command )) {
inters.add( new Intersection( sc ) );
} else if ("road".equals( command )) {
roads.add( new Road( sc ) );
} else if ("--".equals( command )) {
sc.nextLine();
} else {
Errors.warn( "unknown command: " + command );
sc.nextLine();
}
}
}

/** Print out the road network to system.out
*/
private static void printNetwork() {
for (Intersection i: inters) {
System.out.println( i.toString() );
}
for (Road r: roads) {
System.out.println( r.toString() );
}
}

/** Main program
*/
public static void main( String[] args ) {
if (args.length < 1) {
Errors.fatal( "Missing file name argument" );
} else if (args.length > 1) {
Errors.fatal( "Too many arguments" );
} else try {
readNetwork( new Scanner( new File( args[0] ) ) );
printNetwork();
} catch (FileNotFoundException e) {
Errors.fatal( "Can't open the file" );
}
}
}

Related Questions

Navigate

• Browse (All)
• Browse J
• Subjects

---

---

Integrity-first tutoring: explanations and feedback only — we do not complete graded work. Learn more.