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Part I. Multiple Choice Questions
For each multiple choice question circle only one answer. Note. In all questions that refer to singly linked
lists or doubly linked lists, methods getNext, getPrevious, setNext and setPrevious are getter and setter
methods to obtain and to set the next or previous node in the list.
1. (1 mark) Consider this class definition: public class A implements B. Class A must provide
code to implement every method in the interface B.
(A) True (B) False
2. (1 mark) An abstract data type (ADT) specifies a set of operations and one specific way to implement
them
(A) True (B) False
3. (1 mark) The following statement: String[] s; creates an array of String objects all of whose entries
are null.
(A) True (B) False
4. (1 mark) Consider the following Java code
private int m(int x, int y) {
if (x != 0) return x;
else return y/x;
}
Which of the following statements regarding this code is correct?
(A) The code has compilation errors. (B) The code could cause runtime errors.
(C) The code has no errors.
5. (1 mark) Consider the following Java statement
String s = new String("hello");
What value is stored in s?
(A) "hello" (B) The object String("hello") (C) The address of object String("hello")
6. (3 marks) Consider the following code fragment
int len = 3, sum = 0, i = 3;
int[] arr = new int[len];
try {
while (i >= 0) {i = i - 1;
arr[i] = i;sum = sum + arr[i];}}
catch (ArrayIndexOutOfBoundsException e) {sum = sum + 1;}
catch (NullPointerException e) {sum = sum - 1;}
catch (Exception e) {sum = 2 * sum;}
What value does sum have at the end of the execution of the above code?
(A) 2 (B) 3 (C) 4 (D) 6 (E) 12
7. (2 marks) Consider the following code fragment
int[] a = new int[2]; int[] b = new int[2];
for (int i = 0; i < 2; ++i) { a[i] = b[i] = i;}
if (a == b) System.out.println("equal"); // Line 4
else System.out.println("different");
What is the output produced by the above code fragment?
(A) "equal" (B) "different" (C) Nothing. Line 4 of the code produces a compilation error.
2
8. (3 marks) Consider the following Java interface for the queue ADT
public interface QueueADT {
public void enqueue(T element);
public T dequeue();
}
and the following Java code fragment
QueueADT s = new QueueADT();
QueueADT q = new QueueADT();
if (s.equals(q)) System.out.println("equal");
else System.out.println("different");
What is printed by this above code?
(A) "equal" (B) "different" (C) Nothing. The above code has compilation errors.
9. (3 marks) Consider the following code fragment
public class MyClass {
private static int value;
public MyClass (int v) {value = v;}
public static void main(String[] args) {
value = 1;
MyClass v1 = new MyClass(2);
MyClass v2 = new MyClass(4);
System.out.println(v1.value + "," + v2.value);}}
What is printed when the above code is executed?
(A) 1,1 (B) 2,2 (C) 2,4 (D) 4,4
10. (3 marks) Consider the following singly linked list, where each node stores an integer value.
front 2 −5 3
The following code fragment is executed on the above list. Method getValue() returns the value
stored in a node.
LinearNode p = front;
int[] arr = new int[3];
int count = 3;
try {while (count > 0) {
if (p.getValue() > 0) {
arr[count-1] = p.getValue();
count = count -1;}p = p.getNext();}
System.out.println("Finish");}
catch (NullPointerException e) {System.out.println("Null pointer");}
catch (IndexOutOfBoundsException e) {System.out.println("Invalid index");}
catch (Exception e) {System.out.println("Error");}
What is printed by the above code?
(A) "Finish" (B) "Null pointer" (C) "Invalid index" (D) "Error"
(E) "Null pointer" and "Invalid index" (F) "Finish" and "Invalid index"
3
11. (2 marks) Consider the following two Java classes.
public class A {
public void m() {System.out.println("Class A");}
public A() { }
}
public class B extends A {
public void m() {System.out.println("Class B");}
public B() { }
}
Consider now the following code fragment
A var1 = new B(); // Line 1
B var2 = new A(); // Line 2
Which line(s) cause compilation errors?
(A) Line 1 (B) Line 2 (C) Lines 1 and 2 (D) None
12. (4 marks) Consider the two classes from the previous question and the following class
public class C extends B {
public void m() {System.out.println("Class C");}
public void foo() {System.out.println("Method foo");}
public C() { }
}
Consider the following code fragment
C var3 = (C) new A();
var3.foo();
What does this code fragment print when it is executed?
(A) "Method foo" (B) Nothing. The code has compilation errors.
(C) Nothing. The code produces a runtime error.
13. (3 marks) Consider the same 3 classes from the above two questions and the following code fragment
int[] arr = new int[3];
arr[0] = 10;
A var4;
if (arr.length == 1) var4 = new C();
else var4 = new B();
var4.m();
What does this fragment print when it is executed?
(A) "Class A" (B) "Class B" (C) "Class C"
(D) Nothing. The code has compilation errors. (E) Nothing. The code produces a runtime error.
14. (2 marks) What is the output produced by the following program?
public class C {
public static String s = "hey";
public static void changeString(String s) {
s = "hi";
}
public static void main (String[] args) {
String s = "hello";
changeString(s);
System.out.println(s);
}
}
(A) "hey" (B) "hi" (C) "hello"
4
15. (2 marks) Consider the following code fragment
public class A {
public int i = 1;
private j = 2;
public static void main (String[] args) {
//Switch i and j
int k = i; // Line 1
i = j; // Line 2
j = k; // Line 3
}
}
Which line(s) have compilation errors?
(A) Only line 1 (B) Only lines 2 and 3 (C) Lines 1, 2, and 3 (D) None
16. (4 marks) Consider the following code fragment
curr = front;
next = curr.getNext();
prev = null;
while (curr != null) {
(*)
}
front = prev;
Which code must be added in the part marked (*) so the above code correctly inverts a non-empty
singly linked list? See the figure to understand what ”invert” means.
invert
front 3 7 6 3 7 6 front
list
(A) next.setNext(prev); prev = curr; curr = next; if (next != null) next = next.getNext();
(B) curr.setNext(prev); prev = curr; curr = next; if (next != null) next = next.getNext();
(C) next.setNext(curr); prev = curr; curr = next; if (next != null) next = next.getNext();
(D) prev = curr; curr = next; curr.setNext(prev); if (next != null) next = next.getNext();
17. (1 mark) Does the code fragment from the previous question also work correctly if the list is empty?
(A) Yes (B) No
18. (3 marks) A double stack is a variation of a stack that allows access to the top and to the bottom of
the stack. A double stack ADT provides operations pushTop(T x) that adds x to the top of the stack,
pushBottom(T x) that adds x to the bottom of the stack, popTop() that removes the element at the
top of the stack, and popBottom() that removes the element at the bottom of the stack.
Consider a double stack ds implemented using an array as shown in the following figure.
bottom = 2
2 3 1 5 7 4
0 1 2 3 4 5 6 7 8 9 10
top = 7
When pushTop(T x) (popTop()) is executed the value of top is increased (decreased); similarly when
pushBottom(T x) (popBottom()) is executed the value of bottom is decreased (increased). Consider
the following code fragment.
for (int i = 0; i < 3; ++i) ds.pushTop(ds.popBottom());
for (int i = 0; i < 2; ++i) x = ds.popTop();
ds.pushBottom(x);
After this code fragment is executed on the double stack ds represented by the array in the above
figure which are the final values for top and bottom?
(A) top=8, bottom=4 (B) top=9, bottom=5 (C) top=7, bottom=3 (D) top=10, bottom=3
5
Part II. Written Answers
19. (6 marks) Consider a stack implemented using a singly linked list where top and count are instance
variables pointing to the first node of the list and giving the number of data items in the stack,
respectively. The following Java code implements the push operation.
private void push (T newValue) {
LinearNode newNode = new LinearNode(newValue);
top = newNode;
newNode.setNext(top.getNext());
++count;
}
• Is this method correct (circle one)? Yes No
• If the method is wrong, give an example showing that the code does not produce the correct
result.
Draw initial stack in your example (show top and count):
Draw the stack after performing push(x) (show top and count):
20. (4 marks) Consider the following linked list, and node r
r
front a b c d e
p
Consider the following code intended to add node r to the end of the list.
successor = p.getNext();
successor = r;
Draw the linked list resulting after the above code is executed and indicate whether the code is correct
or not.
6
21. (6 marks) Consider the following code.
public class C {
private int i;
public C() {i = 0;}
private void method1 (int i) throws Exception2 {
try {
if (i == 0) throw new Exception2();
method2 (i);
this.i = 7;
} catch (Exception1 e) {
this.i = 5;
System.out.println("Exception 1 caught in method1");
}
}
private void method2 (int i) throws Exception1 {
try {
if (i > 0) throw new Exception1();
else throw new Exception2();
}
catch (Exception2 e) {System.out.println("Exception 2 caught in method 2");}
}
public static void main (String[] args) {
C objectC = new C();
try {
objectC.method1(4);
}
catch (Exception2 e) {System.out.println("Exception 2 caught in main");}
System.out.println("i = " + objectC.i);
}
}
Exception1 and Exception2 are not parent/child classes of each other. Write all the output produced
when this program is executed.
7
22. The following code is intended to remove a node p from a doubly linked list. Assume that we know
that p is in the list, so the list is not empty.
prev = p.getPrevious();
succ = p.getNext();
if (p == front) {
front = front.getNext();
if (front == null) rear = null;
else front.setPrevious(null);
}
else prev.setNext(succ);
if (p == rear) {
rear = rear.getPrevious();
rear.setNext(null);
}
else succ.setPrevious(prev);
Consider the following three doubly linked lists
(4 marks) If the above code can be executed on list (a) of the figure without crashing, draw the doubly
linked list (including front and rear) resulting after it is executed; otherwise explain why the code
would crash and which statement would throw an exception.
(4 marks) If the above code can be executed on list (b) of the figure without crashing, draw the doubly
linked list (including front and rear) resulting after it is executed; otherwise explain why the code
would crash and which statement would throw an exception.
(4 marks) If the above code can be executed on list (b) of the figure without crashing, draw the doubly
linked list (including front and rear) resulting after it is executed; otherwise explain why the code
would crash and which statement would throw an exception.
8
For the following two questions write algorithms in Java or in detailed Java-like pseudocode like the
one used in the lecture notes.
23. (16 marks) Consider an array A storing n > 1 different integer values sorted in increasing order.
At least one value in A is negative and at least one of them is positive. Write an algorithm
invertPositive(A,n) that changes the positions of the positive values stored in A so the positive
values appear in decreasing order. So, for example, if the array A is as in the figure on the left, the
modified array must be as in the figure on the right.
A A 7
0 1 2 3 4 5 6 7 8 0 1 2 3 4 5 6 7 8
−7 −5 −3 −1 3 4 6 7 9 −7 −5 −3 −1 9 6 4 3
You can either use a stack or a queue in your solution, but not both; you need to decide which of
these auxiliary data structures you need to use to answer this question. You cannot use any other
additional data structures.
The only operations that you can perform on the stack are push, pop, peek, and isEmpty; you
can write auxStack = new stack to create an empty stack. If you decide to use a queue the only
operations that you can perform on the queue are enqueue, dequeue, first, and isEmpty; you can
write auxQueue = new queue to create an empty queue. You CANNOT assume that the stack or
the queue are implemented using an array, singly linked list, doubly linked list, or other data structure.
The only way to manipulate the stack or the queue is through the use of the above operations.
To manipulate the array you must use detailed pseudocode. For example, you can write A[i] = x or
s.push(A[i]), but you cannot write statements like “add x to the array A”, or “remove element x
from A”.
9
24. (16 marks) Given a stack s storing n integer values and an integer value x, write an algorithm
inStack(x) that returns true if x is in the stack and it returns false otherwise. After the execution
of the algorithm s must be exactly as it was before the algorithm was executed, i.e. it must
have the same values and in the same positions.
Your algorithm can use one additional stack as auxiliary data structure. You cannot use any other
additional data structures. Write auxStack = new stack to create an empty stack, and use methods
push, pop, and isEmpty to manipulate a stack. You CANNOT assume that the stack is implemented
using an array, singly linked list, doubly linked list, or other data structure. The only way to manipulate
the stacks is through the use of the above operations.
Assume that s is an instance variable.
Hint. Move the values in s to the auxiliary stack so you can compare them with x. Remember that
before the algorithm terminates all values must be put back in s.
10
Western University
Department of Computer Science
CS1027b Foundations of Computer Science II
Midterm Exam
March 9, 2019
Last Name:
First Name:
Student Number:
Section Number (1- Kontogiannis 2 - Solis-Oba):
Total
Instructions
• Fill in your name, student number, and section.
• The exam is 2 hours long and it has a total of 100 marks.
• The exam has 10 pages and 24 questions.
• The first part of the exam consist of multiple choice questions. For each question circle only one
answer.
• For the second part of the exam, answer each question only in the space provided.
• When you are done, raise your hand and one of the TA’s will collect your exam.


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