Chapter 9 Collections and Generics

Question 1

1. Suppose you need to display a collection of products for sale, which may contain duplicates. Additionally, you have a collection of sales that you need to track, sorted by the natural order of the sale ID, and you need to retrieve the text of each. Which two of the following from the java.util package best suit your needs for this scenario? (Choose two.)
A. ArrayList
B. HashMap
C. HashSet
D. LinkedList
E. TreeMap
F. TreeSet

Question 2

2. Which of the following are true? (Choose all that apply.)

12: List<?> q = List.of("mouse", "parrot");
13: var v = List.of("mouse", "parrot");
14:
15: q.removeIf(String::isEmpty);
16: q.removeIf(s -> s.length() == 4);
17: v.removeIf(String::isEmpty);
18: v.removeIf(s -> s.length() == 4);

A. This code compiles and runs without error.
B. Exactly one of these lines contains a compiler error.
C. Exactly two of these lines contain a compiler error.
D. Exactly three of these lines contain a compiler error.
E. Exactly four of these lines contain a compiler error.
F. If any lines with compiler errors are removed, this code runs without throwing an exception.
G. If any lines with compiler errors are removed, this code throws an exception.

Question 3

3. What is the result of the following statements?

3: var greetings = new ArrayDeque<String>();
4: greetings.offerLast("hello");
5: greetings.offerLast("hi");
6: greetings.offerFirst("ola");
7: greetings.pop();
8: greetings.peek();
9: while (greetings.peek() != null)
10: System.out.print(greetings.pop());

A. hello
B. hellohi
C. hellohiola
D. hiola
E. The code does not compile.
F. An exception is thrown.

Question 4

4. Which of these statements compile? (Choose all that apply.)
A. HashSet<Number> hs = new HashSet<Integer>();
B. HashSet<? super ClassCastException> set = new HashSet<Exception>();
C. List<> list = new ArrayList<String>();
D. List<Object> values = new HashSet<Object>();
E. List<Object> objects = new ArrayList<? extends Object>();
F. Map<String, ? extends Number> hm = new HashMap<String, Integer>();

Question 5

5. What is the result of the following code?

1: public record Hello<T>(T t) {
2: public Hello(T t) { this.t = t; }
3: private <T> void println(T message) {
4: System.out.print(t + "-" + message);
5: }
6: public static void main(String[] args) {
7: new Hello<String>("hi").println(1);
8: new Hello("hola").println(true);
9: } }

A. hi followed by a runtime exception
B. hi-1hola-true
C. The first compiler error is on line 1.
D. The first compiler error is on line 3.
E. The first compiler error is on line 8.
F. The first compiler error is on another line.

Question 6

6. Which of the following can fill in the blank to print [7, 5, 3]? (Choose all that apply.)

8: public record Platypus(String name, int beakLength) {
9: @Override public String toString() {return "" + beakLength;}
10:
11: public static void main(String[] args) {
12: Platypus p1 = new Platypus("Paula", 3);
13: Platypus p2 = new Platypus("Peter", 5);
14: Platypus p3 = new Platypus("Peter", 7);
15:
16: List<Platypus> list = Arrays.asList(p1, p2, p3);
17:
18: Collections.sort(list, Comparator.comparing\_\_\_\_\_\_);
19:
20: System.out.println(list);
21: }
22: }

(Platypus::beakLength)
A.
(Platypus::beakLength).reversed()
B.
(Platypus::name)
.thenComparing(Platypus::beakLength)
C.
(Platypus::name)
.thenComparing(
Comparator.comparing(Platypus::beakLength)
.reversed())
D.
(Platypus::name)
.thenComparingNumber(Platypus::beakLength)
.reversed()
E.
(Platypus::name)
.thenComparingInt(Platypus::beakLength)
.reversed()
F.
G. None of the above

Question 7

7. Which of the following method signatures are valid overrides of the hairy() method in the Alpaca class? (Choose all that apply.)

import java.util.*;
public class Alpaca {
public List<String> hairy(List<String> list) { return null; }
}

public List<String> hairy(List<CharSequence> list) { return null; }

A.
~~~
public List<String> hairy(ArrayList<String> list) { return null; }
~~~
B.
~~~
public List<String> hairy(List<Integer> list) { return null; }
~~~
C.
~~~
public List<CharSequence> hairy(List<String> list) { return null; }
~~~
D.
~~~
public Object hairy(List<String> list) { return null; }
~~~
E.
~~~
public ArrayList<String> hairy(List<String> list) { return null;}
~~~
F.</String></String></String></String></CharSequence></Integer></String></String></String>

Question 8

8. What is the result of the following program?

3: public class MyComparator implements Comparator<String> {
4: public int compare(String a, String b) {
5: return b.toLowerCase().compareTo(a.toLowerCase());
6: }
7: public static void main(String[] args) {
8: String[] values = { "123", "Abb", "aab" };
9: Arrays.sort(values, new MyComparator());
10: for (var s: values)
11: System.out.print(s + " ");
12: }
13: }

A. Abb aab 123
B. aab Abb 123
C. 123 Abb aab
D. 123 aab Abb
E. The code does not compile.
F. A runtime exception is thrown.

Question 9

9. Which of these statements can fill in the blank so that the Helper class compiles successfully? (Choose all that apply.)

2: public class Helper {
3: public static <U extends Exception>
4: void printException(U u) {
5:
6: System.out.println(u.getMessage());
7: }
8: public static void main(String[] args) {
9: Helper.\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_;
10: } }

A. printException(new FileNotFoundException(“A”))
B. printException(new Exception(“B”))
C. <Throwable>printException(new Exception("C"))
D. <NullPointerException>printException(new NullPointerException ("D"))
E. printException(new Throwable(“E”))

Question 10

10. Which of the following will compile when filling in the blank? (Choose all that apply.)

var list = List.of(1, 2, 3);
var set = Set.of(1, 2, 3);
var map = Map.of(1, 2, 3, 4);
\_\_\_\_\_\_\_.forEach(System.out::println);

A. list
B. set
C. map
D. map.keys()
E. map.keySet()
F. map.values()
G. map.valueSet()

Question 11

11. Which of these statements can fill in the blank so that the Wildcard class compiles successfully? (Choose all that apply.)

3: public class Wildcard {
4: public void showSize(List<?> list) {
5: System.out.println(list.size());
6: }
7: public static void main(String[] args) {
8: Wildcard card = new Wildcard();
9: \_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_;
10: card.showSize(list);
11: } }

A. List<?> list = new HashSet <String>()
B. ArrayList<? super Date> list = new ArrayList<Date>()
C. List<?> list = new ArrayList<?>()
D. List<Exception> list = new LinkedList<java.io.IOException>()
E. ArrayList <? extends Number> list = new ArrayList <Integer>()
F. None of the above

Question 12

12. What is the result of the following program?

3: public record Sorted(int num, String text)
4: implements Comparable<Sorted>, Comparator<Sorted> {
5:
6: public String toString() { return "" + num; }
7: public int compareTo(Sorted s) {
8: return text.compareTo(s.text);
9: }
10: public int compare(Sorted s1, Sorted s2) {
11: return s1.num - s2.num;
12: }
13: public static void main(String[] args) {
14: var s1 = new Sorted(88, "a");
15: var s2 = new Sorted(55, "b");
16: var t1 = new TreeSet<Sorted>();
17: t1.add(s1); t1.add(s2);
18: var t2 = new TreeSet<Sorted>(s1);
19: t2.add(s1); t2.add(s2);
20: System.out.println(t1 + " " + t2);
21: } }

A. [55, 88] [55, 88]
B. [55, 88] [88, 55]
C. [88, 55] [55, 88]
D. [88, 55] [88, 55]
E. The code does not compile.
F. A runtime exception is thrown.

Question 13

13. What is the result of the following code? (Choose all that apply.)

Comparator<Integer> c1 = (o1, o2) -> o2 - o1;
Comparator<Integer> c2 = Comparator.naturalOrder();
Comparator<Integer> c3 = Comparator.reverseOrder();
var list = Arrays.asList(5, 4, 7, 2);
Collections.sort(list,\_\_\_\_\_\_\_\_\_);
Collections.reverse(list);
Collections.reverse(list);
System.out.println(Collections.binarySearch(list, 2));

A. One or more of the comparators can fill in the blank so that the code prints 0.
B. One or more of the comparators can fill in the blank so that the code prints 1.
C. One or more of the comparators can fill in the blank so that the code prints 2.
D. The result is undefined regardless of which comparator is used.
E. A runtime exception is thrown regardless of which comparator is used.
F. The code does not compile.

Question 14

14. Which of the following lines can be inserted to make the code compile? (Choose all that apply.)

class W {}
class X extends W {}
class Y extends X {}
class Z<Y> {
// INSERT CODE HERE
}

A. W w1 = new W();
B. W w2 = new X();
C. W w3 = new Y();
D. Y y1 = new W();
E. Y y2 = new X();
F. Y y1 = new Y();

Question 15

15. Which options are true of the following code? (Choose all that apply.)

3:\_\_\_\_\_\_\_\_\_\_\_\_\_\_ q = new LinkedList<>();
4: q.add(10);
5: q.add(12);
6: q.remove(1);
7: System.out.print(q);

A. If we fill in the blank with List<Integer>, the output is [10].
B. If we fill in the blank with Queue<Integer>, the output is [10].
C. If we fill in the blank with var, the output is [10].
D. One or more of the scenarios does not compile.
E. One or more of the scenarios throws a runtime exception.</Integer></Integer>

Question 16

16. What is the result of the following code?

4: Map m = new HashMap();
5: m.put(123, "456");
6: m.put("abc", "def");
7: System.out.println(m.contains("123"));

A. false
B. true
C. Compiler error on line 4
D. Compiler error on line 5
E. Compiler error on line 7
F. A runtime exception is thrown.

Question 17

17. What is the result of the following code? (Choose all that apply.)

48: var map = Map.of(1,2, 3, 6);
49: var list = List.copyOf(map.entrySet());
50:
51: List<Integer> one = List.of(8, 16, 2);
52: var copy = List.copyOf(one);
53: var copyOfCopy = List.copyOf(copy);
54: var thirdCopy = new ArrayList<>(copyOfCopy);
55:
56: list.replaceAll(x -> x * 2);
57: one.replaceAll(x -> x * 2);
58: thirdCopy.replaceAll(x -> x * 2);
59:
60: System.out.println(thirdCopy);

A. One line fails to compile.
B. Two lines fail to compile.
C. Three lines fail to compile.
D. The code compiles but throws an exception at runtime.
E. If any lines with compiler errors are removed, the code throws an
exception at runtime.
F. If any lines with compiler errors are removed, the code prints [16, 32,
4].
G. The code compiles and prints [16, 32, 4] without any changes.

Question 18

18. What code change is needed to make the method compile, assuming there is no class named T?

public static T identity(T t) {
return t;
}

A. Add <T> after the public keyword.
B. Add <T> after the static keyword.
C. Add <T> after T.
D. Add <?> after the public keyword.
E. Add <?> after the static keyword.
F. No change is required. The code already compiles.</T></T></T>

Question 19

19. What is the result of the following?

var map = new HashMap<Integer, Integer>();
map.put(1, 10);
map.put(2, 20);
map.put(3, null);
map.merge(1, 3, (a,b) -> a + b);
map.merge(3, 3, (a,b) -> a + b);
System.out.println(map);

A. {1=10, 2=20}
B. {1=10, 2=20, 3=null}
C. {1=10, 2=20, 3=3}
D. {1=13, 2=20}
E. {1=13, 2=20, 3=null}
F. {1=13, 2=20, 3=3}
G. The code does not compile.
H. An exception is thrown.

Question 20

20. Which of the following statements are true? (Choose all that apply.)
A. Comparable is in the java.util package.
B. Comparator is in the java.util package.
C. compare() is in the Comparable interface.
D. compare() is in the Comparator interface.
E. compare() takes one method parameter.
F. compare() takes two method parameters.