Exam 1 Review Problems solution

1. Suppose we have a class A which has a constructor that takes a single integer.
(a) After the following statements have been executed, how many A objects will exist
(not counting garbage objects) and which objects are they? Explain your answer
and include in your explanation a picture of Java’s memory.
A a = new A(100);
A b = new A(150);
A c = b;
b = a;
a = null;
(b) After the following statements have been executed, how many A objects will exist
(not counting garbage objects) and which objects are they? Explain your answer
and include in your explanation a picture of Java’s memory.
A a1 = new A(200);
A a2 = new A(250);
A a3 = a2;
a2 = a1;
a1 = null;
2. Here is a simple Point and Circle class.
class Point class Circle
{ private double x, y; { private Point c; // center
private double r; // radius
public Point(double x, double y)
{ this.x = x; public Circle(Point c, double r)
this.y = y; { this.r = r;
} this.c = c;
public double getX(){ return x; } }
public double getY(){ return y; } // more stuff
} }
(a) The constructor in Circle has a “privacy leak”. Explain why.
Hint: Consider the following code.
Point p = new Point(1,2);
Circle c = new Circle(p, 10);
p.setX(100);
(b) Rewrite the Circle constructor to fix this problem.
3. Consider this code that creates some Location objects:
Location a, b, c;
a = new Location(10,20);
b = new Location(10,20);
c = b;
After this code executes, what are the values of these boolean expressions?
a==b
a.equals(b)
a==c
a.equals(c)
b==c
b.equals(c)
Also, write two clear sentences that explain the difference between == and the equals()
method.
4. Consider this code that creates some Location objects:
Location a, b, c;
a = new Location(10,20);
b = (Location)a.clone( );
c = a;
c.shift(2,0);
After this code executes, what are the values of these boolean expressions?
a==b
a.equals(b)
a==c
a.equals(c)
b==c
b.equals(c)
5. What does the following program print out. Explain why.
class Thing
{ public int a;
public int b;
public Thing(int a, int b){this.a=a; this.b=b}
}
public class Test
{ public static void f(Thing x, int y)
{
x.a++;
y++;
}
public static void main(String[] args)
{
Thing x = new Thing(1,1);
int y = 1;
f(x, y);
System.out.println("x.a = " + x.a + " and x.b = " + x.b);
System.out.println(" y = " + y);
}
}
6. Suppose that we have classes A, B, C and D. Suppose that B is a subclass of A, that C is a
subclass of B, and D is a subclass of A. Suppose that we make the following declarations.
A a1 = new A();
A a2 = new C();
D d1 = new D();
For each part below, explain what, if any, errors would be caused by the statement in
that part. Be sure to consider both compile time and run time errors.
(a) A a3 = new B();
(b) B b1 = new A();
(c) B b2 = (B) a1;
(d) B b3 = (B) a2;
(e) B b4 = (B) d1;
(f) B b5 = (C)(A)new D();
7. Consider the following classes:
public class Organization extends Object {
Organization() { /* null constructor */ }
public void printMe() { System.out.println("Organize."); }
}
public class Company extends Organization {
Company() { /* null constructor */ }
public void printMe() { System.out.println("Be productive."); }
}
public class MidCap extends Company {
MidCap() { /* null constructor */ }
public void printMe() { System.out.println("Think big."); }
}
public class InternetCo extends MidCap {
InternetCo() { /* null constructor */ }
public void printMe() { System.out.println("Be cool."); }
public static void main(String[] args) {
Company mid = new MidCap();
MidCap netscape = new InternetCo();
Object obj = new Organization();
Organization startup = new Company();
mid.printMe();
netscape.printMe();
((Organization) obj).printMe();
obj = netscape;
((MidCap) obj).printMe();
obj = startup;
((Organization) obj).printMe();
}
}
What is the output from running the InternetCo class?
8. Suppose we implement the IntArrayBag class using two partially-filled, “parallel arrays”
instead of a single (partially-filled) array. The first array, data, holds the values of the
items in the bag and the second array, dataCounts, holds a count of the number of times
that the associated item is in the bag. In other words, data[i] is an integer in the bag,
and dataCounts[i] is the number of times that integer is in the bag.
We assume that there is an instance variable manyDataItems that tells us how many of
the entries from the partially-filled arrays data and dataCounts are used to hold items
from the bag (so manyDataItems <= data.length). The instance variable manyItems
is a count of how many items are in the bag.
We assume that the part of the array data that stores the bag does not have any duplicate
entries and we assume that each value in dataCounts is strictly greater than zero for all
elements with index less than manyDataItems.
(a) Describe what is meant by the “capacity” of a bag in this implementation.
(b) Describe an advantage that this implementation of IntArrayBag has over the single
array implementation from the textbook.
(c) Describe a disadvantage that this implementation of IntArrayBag has when com-
pared to the single array implementation from the textbook.
(d) Write an implementation for each of the add(int element) and remove(int target)
methods (see the next page).
public class IntArrayBag
{ // use two partially-filled "parallel arrays"
private int[ ] data; // the data items
private int[ ] dataCounts; // how many times each item is in the bag
private int manyDataItems; // number of elements in partially-filled array
private int manyItems; // total number of items in the bag
public IntArrayBag( )
{ final int INITIAL_CAPACITY = 10;
data = new int[INITIAL_CAPACITY];
dataCounts = new int[INITIAL_CAPACITY];
manyDataItems = 0;
manyItems = 0;
}
public IntArrayBag(int initialCapacity)
{ if (initialCapacity < 0) throw new IllegalArgumentException("Capacity<0.");
data = new int[initialCapacity];
dataCounts = new int[initialCapacity];
manyDataItems = 0;
manyItems = 0;
}
/** Add a new element to this bag. If the new element would take this
* bag beyond its current capacity, then the capacity is increased. **/
public void add(int element)
{
}//add()
/** Remove one copy of a specified element from this bag.
* If target was found in the bag, then one copy of target
* has been removed and the method returns true. Otherwise
* the bag remains unchanged and the method returns false. **/
public boolean remove(int target)
{
}//remove()
}//IntArrayBag
9. Here is part of the definition for a LinkedList class.
class LinkedList
{
private ListNode head;
private int size;
public LinkedList()
{
this.head = null;
this.size = 0;
}
// a private class
class ListNode
{
public int item; // An item in the list.
public ListNode next; // Reference to next item in the list.
}
// LinkedList methods...
}
(a) Write a method
public void add( int element )
that adds a new node at the head of the linked list. (Notice that the inner class
ListNode only has a default constructor.)
(b) Write a method
public int remove( )
that removes from the linked list the node at the head of the list and returns the
int that was stored in that node. Throw an exception if the linked list is empty.
(c) Explain how you would modify the add method so that the following two lines of
code will compile.
LinkedList list = new LinkedList();
list.add(3).add(2).add(5).add(0).add(8);
10. On the last page of these review problems is an implementation of the IntNode class.
(a) Write an implementation of the static method
public static int countZeros( IntNode node )
that will count the number of zeros that occur in the given linked list of ints.
(b) Write an implementation of a static method
public static String list2String( IntNode node )
that returns a String representation of the linked list referred to by the parameter
node. If the linked list is empty, the String representation should be "[]" (two
square brackets next to each other). If the linked list is not empty, the String
representation should look like this, "[ 3 52 0 2 -4 16 ]", with a space before
each entry of the list and a space before the closing bracket.
(c) Write a method
public static IntNode removeFirst( IntNode head )
that returns a reference to the second node from the linked list referred to by the
parameter head.
(d) Write a method
public static IntNode addFirst( int element, IntNode head )
that returns a reference to the new head of a linked list with a node containing
element followed by the list referred to by the parameter head.
(e) Write a method
public static void set( int element, int i )
that modifies the list referred to by the parameter head so that the i’th node in
the list has its data changed to element. If there is no i’th node in the list, then
the list is not modified.
11. Once again using the IntNode class, consider the following three lines of code.
IntNode head = new IntNode(4,new IntNode(7,new IntNode(5,new IntNode(3,null))));
IntNode ptr = head.getLink().getLink();
head.getLink().setLink( new IntNode(22, null) );
(a) Draw a picture of Java’s memory after the first line above has been executed. Be
sure to include what data is in each node.
(b) Draw a picture of Java’s memory after the first and second lines above have been
executed.
(c) Draw a picture of Java’s memory after all three lines above have been executed.
(d) What would be a String representation for the linked list referred to by head?
(e) What would be a String representation for the linked list referred to by ptr?
(f) What would be a String representation for the linked list referred to by ptr after
executing the following line (which would be executed after the above three lines)?
ptr.getLink().setLink( head.getLink() );
12. (a) In the class name IntArrayBag, explain the significance of each part of the name:
int, array, and bag.
(b) In the class name IntArraySeq, explain the significance of each part of the name:
int, array, and seq.
(c) In the class name DoubleLinkedBag, explain the significance of each part of the
name: double, linked, and bag.
13. Suppose that
x = y;
is a “widening” assignment.
(a) If x and y are primitive variables, explain why the assignment can also be referred
to as a “widening conversion”.
(b) If x and y are reference variables, how are the types of x and y related to each
other?
(c) If x and y are reference variables, explain why the assignment should not be referred
to as a “conversion”.
(d) If x and y are reference variables, you can even make a case that the assignment
should be called a “narrowing” assignment. Explain why. (Hint: What can you say
about the methods callable on x as compared to the methods callable on y?)
14. Let A be an array of size n  2 containing integers from 1 to n−1, inclusive, with exactly
one number repeated.
(a) Write a method
public static int findRepeatedNumber(int[] A)
that returns the value of the repeated number in the array A.
(b) Rewrite the method so that it uses just a sinlge loop. (Hint: Make use of another
array.)
15. Suppose that a Sequence ADT has the following interface.
public interface Sequence
{ public int size(); // Return number of elements in sequence.
public void addFirst(int e); // Insert e at the front of the sequence.
public void addLast(int e); // Insert e at the back of the sequence.
// Inserts an element e to be at index i.
public void add(int i, int e) throws IndexOutOfBoundsException;
// Returns the element at index i, without removing it.
public int get(int i) throws IndexOutOfBoundsException;
// Removes and returns the element at index i.
public int remove(int i) throws IndexOutOfBoundsException;
}
Starting with an empty sequence A, below each operation write down what the (cumula-
tive) contents of the list would be after performing the operation. (Write the contents of
the sequence as a horizontal, comma separated, list of numbers with the index 0 element
on the left.)
A.add(0, 4)
A.add(0, 3)
A.addFirst(2)
A.addLast(7)
A.add(2, 1)
A.add(1, 4)
A.add(1, 5)
A.add(3, 2)
16. Below is an outline of a class that implements a linked list of integer nodes with two
sentinel nodes (see pages 240–241 of the textbook).
(a) Draw a picture of the empty list created by the default constructor.
(b) Write an implementation for the addFirst() method.
(c) Write an implementation for the removeFirst() method that assumes the list is
not empty.
public class IntLinkedList
{ IntNode head;
IntNode tail;
int manyItems;
public IntLinkedList() // Create an empty list,
{ tail = new IntNode(0, null); // with two sentinel nodes.
head = new IntNode(0, tail);
manyItems = 0;
}
/** Add a new node to the beginning of the list. */
public void addFirst(int n)
{
}
/** Remove the first node from a nonempty list and return its data. */
public int removeFirst()
{
}
}//IntLinkedList
class IntNode
{
private int data;
private IntNode link;
public IntNode(int data, IntNode link)
{
this.data = data;
this.link = link;
}
public int getData( ) { return data; }
public IntNode getLink( ) { return link; }
public void setData(int data) { this.data = data; }
public void setLink(IntNode link) { this.link = link; }
}//IntNode
10-12-2015 at 01:22 h
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