Java代写 | CS1027: Foundations of Computer Science II

本次Java代写是用数组和队列完成一个寻路算法
CS1027: Foundations of Computer Science II  
Assignment 4  
2.1 CellData.java  
This class represents the data items that will be stored in the circular array. Each object of this class stores  
two things: an identifier and a value. This class will be declared as follows:  
public class CellData<T>  
This class will have two instance variables:  
1
private T id. A reference to the identifier stored in this object/  
private int value. This is the value of the data item stored in this object.  
You need to implement the following methods in this class:  
public CellData(T theId, int theValue). Constructor for the class. Initializes id to theId and value  
to theValue.  
public int getValue(). Returns instance variable value.  
public T getId(). Returns instance variable id.  
public void setValue(int newValue). Assigns newValue to instance variable value.  
public void setId(T newId). Assigns the value of newId to instance variable id.  
1
Middlesex  
College cell  
Road crossing  
5
2
3
4
6
East road  
North road  
South road  
Destination  
Block of houses  
Park  
Figure 1.  
2.2 OrderedCircularArray.java  
This class implements an ordered list using a circular array. This class must implement the interface  
SortedListADT.java. The header of this class must be this:  
public class OrderedCircularArray<T> implements SortedListADT<T>  
You can download SortedListADT.java from the course’s website. This class must have the following  
private instance variables:  
private CellData[] list. This array will store the data items of the ordered list.  
private int front. This variable stores the position of the first data item in the list; this is the data  
item with the smallest value. In the constructor of this class this variable must be initialized to 1.  
2
Note that this is different from the way in which the variable front is initialized in the lecture  
notes.  
private int rear. This is the index of the last data item in the ordered list; this is the data item with  
the largest value. In the constructor for this class this variable must be initialized to 0. Again, note  
that this is different from the way in which variable rear is used in the lecture notes.  
private int count. The value of this variable is equal to the number of data items in the list.  
Circular array list stores objects of the class CellData. These objects are kept sorted in the array  
according to their integer value attributes. An example of the array list storing 3 CellData objects in  
order of their integer values is shown in the next page.  
This class needs to provide the following public methods.  
public OrderedCircularArray(). Initializes the instance variables as specified above. Array list  
must initially have length 5.  
Note that when initializing array list as list = new CellData<T>[5]; the compiler will issue an  
error message because type T is not known at compilation time. To solve the problem use casting  
(create an array of type CellData[] and then cast it to the type that you need).  
public void insert (T id, int value). Adds a new CellData object storing the given id and value to  
the ordered list. You must ensure that after adding this new CellData object the list remains  
sorted. Note that the value of front must not change when adding a new CellData object into  
the list.  
For example, if the array list is the following  
rear  
front  
“id 4”, 20  
id 1”, 3  
id 2”, 8  
“id 3”, 12  
list  
0
1
2
3
4
and we invoke insert(“id 4”, 1), then the new array must be as follows  
rear  
front  
“id 3”, 12  
“id 4”, 20  
id 4”, 1  
id 1”, 3  
“id 2”, 8  
list  
0
1
2
3
4
If the array is full when the insert method is invoked, a new array of size twice as large as the  
original one must be created, and the information from the old array must be copied there. You  
must ensure that the value of front does not change. So, if in the above array with 5 elements  
we invoke insert(“id 5”,2), the new array must be as follows  
id 4”,1  
“id 5”,2  
“id 1”,3  
“id 2”,8  
“id 3”,12  
“id 4”,20  
0
1
2
3
4
5
6
7
8
9
list  
3
public int getValue(T id) throws InvalidDataItemException. Returns the integer value of the  
CellData object with the specified id. An InvalidDataItemException is thrown if no CellData  
object with the given id is in the ordered list. In this assignment we will assume that no two  
CellData objects in the ordered list have the same id.  
Note that to check whether an object with the given id is in the ordered list you need to scan the  
ordered list using linear search. To check if the CellData object stored in some position of the  
ordered list has the same id attribute as id you must use the equals method, not the “==” operator.  
public void remove(T id) throws InvalidDataItemException. Removes from the ordered list the  
CellData object with the specified id. An InvalidDataItemException is thrown if no CellData  
object in the ordered list has the specified id. Note that the value of front must not change when  
removing a data item from the ordered list using this method.  
public void changeValue (T id, int newValue) throws InvalidDataItemException. Changes the  
value attribute of the CellData object with the given id to the specified newValue. An  
InvalidDataItemException is thrown if no object in the ordered list has the given id. After  
changing the value of the CellData obejct, you need to ensure that the list is still sorted by value,  
so you might have to reorder some of the information stored in the circular array.  
Hint. First remove the CellData object with the given id and then add a new CellData item with  
the given id and newValue.  
public T getSmallest() throws EmptyListException. Removes and returns the id or the CellData  
object in the ordered list with smallest associated value. Note that this is the CellData object  
stored in the position of the circular array specified by front. An EmptyListException is thrown if  
the list is empty. Note that for this method the value of front must change once the CellData  
object with smallest value has been removed.  
public boolean isEmpty(). Returns true if the ordered list is empty and it returns false otherwise.  
public int size(). Returns the number of data items in the ordered list.  
public int getFront(). Returns the value of instance variable front.  
public int getRear(). Returns the value of instance variable rear.  
front  
rear  
id 1”, 3  
id 2”, 8  
id 3”, 12  
list  
0
1
2
3
4
To make sure you understand the above methods, consider the above ordered list where each entry stores  
a CellData object in which the id attribute is of type String. Initially the list was empty, and the value of  
front was 1, then method insert was invoked 3 times to add the 3 CellData objects shown. Note how  
front did not change and the entries are sorted by the integer attribute value of the CellData objects. If  
we invoke on this list method getDataValue (“id 2”), this method should return the value 8, while  
invoking getDataValue(“id 4”) should throw an InvalidDataItemException. Invoking changeValue(“id  
2
”, 0) will set the integer value stored in list[2] to 0. Since now the CellData objects are not in increasing  
order of value, the CellData object “id 2”, 0 needs to move to the front of the list as shown below.  
front  
rear  
id 2”, 0  
id 1”, 3  
id 3”, 12  
list  
0
1
2
3
4
4
If now we invoke getSmallest(), this will return “id 2” (as this data item has the smallest value) and  
then the value of front will increase to 2.  
front  
rear  
id 2”, 0  
id 1”, 3  
id 3”, 12  
list  
0
1
2
3
4
You can implement other methods in this class, if you want to, but they must be declared as private.  
.3 ShortestPath.java  
2
This class will have an instance variable  
CityMap cityMap;  
This variable references the object representing the city map where your program will try to find a  
shortest path, a path with the minimum number of map cells, from the starting cell to the destination cell.  
Class CityMap, described below, is given to you. You must implement the following methods in this  
class:  
public ShortestPath (CityMap theMap). This is the constructor for the class. It receives as input  
a reference to a CityMap object representing the city map. In this method you must initialize  
instance variable cityMap: cityMap = theMap.  
public void findShortestPath(). This method will look for a path with the minimum number of  
map cells from the starting cell to the destination cell. If a path is found, this method must print a  
message indicating how many cells there were in the path (including the starting cell and the  
destination cell). If no path was found, this method must print the message “No path found”.  
You are encouraged to design your own algorithm to look for a shortest path from the starting  
cell to the destination. Read the description of the classes Map and MapCell below to see how to  
select the starting cell and the destination cell, and how to mark cells as visited. If you cannot  
figure out how to find a path, please read the description of an algorithm given in the next section.  
private MapCell nextCell(MapCell cell). The parameter of this method is the current map cell.  
This method returns the first unmarked neighboring map cell that can be used to continue the path  
from the current one. Use method isMarked from class MapCell (described below) to determine  
whether a neighboring map cell has been marked or not. This method is similar to the nextCell  
method from Assignment 2, except that now we do not prefer the destination or a road intersection  
cell over the other cells.  
If there are no unmarked cells adjacent to the current one that can be used to continue the path,  
this method must return null.  
Your program must catch any exceptions that might be thrown. For each exception caught an appropriate  
message must be printed. The message must explain what caused the exception to be thrown.  
You can write more methods in this class, if you want to, but they must be declared as private.  
5
3. An Algorithm for Computing a Shortest Path  
This section describes an algorithm for computing a shortest path between two map cells. You do not  
have to implement this algorithm if you do not want to. You are encouraged to design your own  
algorithm, but the algorithm must use an ordered list from class OrderedCircularArray.  
The algorithm starts at the starting cell and as it traverses the map it will store in the ordered list the map  
cells that it might visit next. Each entry of the circular array will store a CellData object containing  
a map cell and  
an integer value equal to the distance from that cell to the starting cell where the Middlesex  
College building is.  
As the algorithm looks for a shortest path to the destination cell it will keep track of the distance from  
the current cell to the destination cell. A description of the algorithm and an example of how the algorithm  
works are given below.  
1
2
. First, create an empty ordered list using class OrderedCircularArray.  
. Get the starting cell where the Middlesex College building is located by using method getStart()  
from class Map. Each map cell is represented with an object of the class MapCell, described in  
Section 5.  
3
. Insert the starting cell in the ordered list with a distance value of zero (as the distance from the  
starting cell to itself is zero). Mark this cell as in the ordered list (use method markInList() from  
class MapCell).  
4
. While the list is not empty, and the destination cell has not been reached, perform the following  
steps:  
o
Remove from the ordered list the map cell S with smallest distance value and mark it as  
out of the ordered list (to mark the cell use method markOutList from class MapCell).  
o
If S is the destination cell, then the algorithm exits the while loop.  
Otherwise, the algorithm considers each one of the neighbouring cells of S that  
is not null,  
is not a block of houses or a park,  
it has not been marked, and  
it can continue the path from S  
To find these cells you will use your method nextCell described above. For each one of  
these neighbouring cells C perform the following steps:  
Set D = 1 + distance from S to the starting cell.  
If the distance between C and the starting cell (to find this distance use method  
getDistanceToStart from class MapCell) is larger than D then:  
set the distance of C to the starting cell to D (to do this use method  
setDistanceToStart from class MapCell).  
Set S as the predecessor of C in the path to the starting cell (use method  
setPredecessor from class MapCell); this is necessary to allow the  
algorithm to reconstruct the path from the initial cell to the destination once  
the destination has been reached.  
Set P = distance from C to the starting cell.  
If C is marked as in the ordered list and P is smaller than the distance value stored  
in the entry of the circular array storing C (to find this value use method getValue  
6
from class CellData) then use the setValue method from class CellData to update  
the distance value of C to P.  
If C is not marked as in the ordered list, then add it to the ordered list with distance  
value equal to P. Then mark C as in the ordered list.  
Note. Recall that you are expected to use meaningful names for your variables when implementing this  
algorithm. So, do not use C, P and S as names for your variables; use more meaningful names.  
To understand how the algorithm works, consider the map given in page 2. The algorithm starts at cell 1  
and sets the distance from this cell to the starting cell to 0. Then it examines the neighboring cells; since  
the east and west neighbors are blocks of houses they are ignored. Cell 3 is added to the ordered list with  
a distance value of 1 and the predecessor of cell 3 is set to cell 1.  
In the next iteration of the while loop, the algorithm removes the map cell 3 from the ordered list. Then  
two CellData objects are added to the ordered list containing map cells 2 and 4; for each one of these  
map cells the associated distance value is 2. To understand why the distance value for map cell 2 is 2,  
note that the shortest path between map cells 2 and 1 is the path 1, 3, 2; this path has length 2 (the distance  
between 1 and 3 is 1 and the distance between 3 and 2 is 1. Similarly, the shortest path between map cells  
4
and 1 is the path 1, 3, 4, which also has length 2.  
Next the algorithm removes from the ordered list the CellData object with smallest value, namely cell 4  
because map cells 2 and 4 have the same distance value and map cell 4 was added to the ordered list  
(
before map cell 2). From here the algorithm will examine the neighboring unmarked cells of map cell 4,  
namely 5 and 6. CellData objects storing map cells 5 and 6 are added to the ordered list, each with  
associated distance value 3; map cell 4 is set as the predecessor of map cells 5 and 6. In the next iteration  
map cell 2 will be removed from the ordered list.  
The algorithm keeps examining cells in this manner until it reaches the destination cell, or it determines  
that the destination cannot be reached. When the destination cell is found, the code given to you will  
automatically highlight in red the path that your algorithm has found.  
4
. Command Line Arguments  
The main method is in the provided class CityMap. The program will read the name of the input map file  
from the command line. From the console you can run the program as follows:  
java CityMap name_of_map_file  
where name_of_map_fileis the name of the file containing the city map. To run the program from  
Eclipse you need to indicate what the name of the input file is. To do this, in Eclipse select “Run Run  
Configurations…”. Make it sure that “Java Application CityMap” is the active selection on the left-  
hand side. Select the “Arguments” tab. Enter the name of the file for the map in the “Program arguments”  
text box. For more details check the tutorial posted in the course’s website:  
http://www.csd.uwo.ca/courses/CS1027b/passingParameters.html  
5. Classes Provided  
You can download from the course’s webpage several java classes that allow your program to display  
the map on the screen. You are encouraged to study the given code to you learn how it works. Below is  
a description of some of these classes.  
7
5.1 Class CityMap.java  
This class represents the map of the city including the starting cell and the destination cell. The method  
that you might use from this class is the following:  
o public MapCell getStart(). Returns a MapCell object representing the starting cell where the  
Middlesex College Building is located.  
5.2 Class MapCell.java  
This class represents the cells of the map. Objects of this class are created inside the class CityMap when  
its constructor reads the map file. The methods that you might use form this class are the following:  
o public MapCell getNeighbour (int i) throws InvalidNeighbourIndexException. As explained  
above, each cell of the map has up to four neighbouring cells, indexed from 0 to 3. This method  
returns either a MapCell object representing the i-th neighbor of the current cell or null if such a  
neighbor does not exist. Remember that if a cell has fewer than 4 neighbouring cells, these  
neighbouring cells do not necessarily need to appear at consecutive index values. So, it might be  
for example, that this.getNeighbour(2) is null, but this.getNeighbour(i) for all other values of i  
are not null.  
An InvalidNeighbourIndexException is thrown if the value of the parameter i is negative or larger  
than 3.  
o public boolean methods: isBlock(), isIntersection(), isNorthRoad(), isEastRoad(),  
isSouthRoad(), isWestRoad(), isStart(), isDestination(), return true if this MapCell object  
represents a cell corresponding to a block of houses or a park (where a car cannot drive), a road  
intersection, a north road, an east road, a south road, a west road, the staring cell where the  
Middlesex College Building is located, or the destination cell where your house is located,  
respectively.  
o public boolean isMarkedInList() returns true if this MapCell object represents a cell that has been  
marked as in the ordered list.  
o public boolean isMarkedOutList() returns true if this MapCell object represents a cell that has  
been marked as out of the ordered list.  
o public boolean isMarked() returns true if this MapCell object represents a map cell that has been  
marked as in or out of the ordered list.  
o public void markInList() marks this MapCell object as in the ordered list.  
o public void markOutList() marks this MapCell object as out of the ordered list.  
o public int getDistanceToStart(). Returns the distance from the cell represented by this MapCell  
object to the starting map cell.  
o public void setDistanceToStart(int dist). Sets to the specified value dist the distance from the cell  
represented by this MapCell object to the starting map cell.  
o public void setPredecessor(MapCell pred). Sets the predecessor of this MapCell object to the  
specified value.  
o public Boolean equals(MapCell otherCell). Returns true if otherCell points to this MapCell  
object, i.e. if this object and otherCell have the same address; otherwise it returns false.  

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