attempt_3

Chapter 3: Review Questions Attempt 3

Results:

Date: 22/07/2023 Score: 13/25

Question #	Correct
1	✅
2	❌
3	✅
4	✅
5	✅
6	❌
7	✅
8	❌
9	✅
10	❌
11	❌
12	❌
13	❌
14	✅
15	✅
16	✅
17	✅
18	❌
19	❌
20	❌
21	✅
22	❌
23	✅
24	❌
25	✅

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Question 1

❓ Suppose you have a collection of products for sale in a database and you need to display those products. The products are not unique.

Which of the following collections classes in the java.util package best suit your need in this scenario? ❓🎃

A. Arrays 🎃
B. ArrayList 🎃
C. HashMap 🎃
D. HashSet 🎃
E. LinkedList 🎃

My Answer:

• B ✅ - Arrays is a utility class not a Collection
• We need anything other than Set, so D is out.
• An ArrayList would be best

Question 2:

❓Suppose that you need to work with a collection of elements that need to be sorted in their natural order, and each element has a unique string associated with its value.

Which of the following collections classes in the java.util package best suit your need in this scenario? ❓

A. ArrayList 🎃
B. HashMap 🎃
C. HashSet 🎃
D. TreeMap 🎃
E. TreeSet 🎃
F. Vector 🎃

My answer:

• A ❌ - correct answer: D. We need a map which is in its natural order so HashMap is invalid

• We can not use maps - so B and D are out

• We need sorting so C is out

• ArrayList would be best

Question 3:

❓ What is the result of the following statements? ❓

    List list = new ArrayList();
    list.add("one");
    list.add("two");
    list.add(7);            // LINE 6
    for (String s: list)    // LINE 7
    System.out.println(s);

A. `onetwo` 🎃
B. `oneTwo7` 🎃
C. `onetwo` followed by an exception 🎃
D. Compiler error on line 6 🎃
E. Compiler error on line 7 🎃

My answer:

• E ✅ - line 7 SHOULD loop though objects not strings
• Compiler error on line 7 from trying to cast Object to String

Question 4:

❓ What is the result of the following? ❓

    ArrayDeque<String> greetings = new ArrayDeque<String>();
    greetings.push("hello");
    greetings.push("hi");
    greetings.push("ola");
    greetings.pop();
    greetings.peek();
    while(greetings.peek() != null)
        System.out.println(greetings.pop);

A. `hello` 🎃
B. `hellohi` 🎃
C. `hellohiola` 🎃
D. `hi` 🎃
E. `hihello` 🎃
F. The code does not compile 🎃
G. An exception is thrown 🎃

My answer:

• E ✅ - since we are using push, the elements are added to the front.
• [Hello, hi, ola]
• [Hello, hi]
• hihello is printed

Question 5:

❓ Which of the statements compile? (Choose all that apply) ❓

A. 🎃

HashSet<Number> hs = new HashSet<Integer>();

B. 🎃

HashSet<? super ClassCastException> set = new HashSet<Exception>()

C. 🎃

List<String> list = new Vector<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>();

My Answer:

• B,C,F ✅
• A does not
• B DOES
• C DOES
• D does not compile
• E does not compile
• F DOES

Question 6:

❓ What is the result of the following code: ❓

    public class Hello<T> {
        T t;
        public Hello(T t) { this. t = t };
        public String toString() { return t.toString() };   // LINE 4
        public static void main(String[] args){
            System.out.println(new Hello<String>("hi"));    // LINE 6
            System.out.println(new Hello("there"));         // LINE 7
        }
    }

A. hi 🎃

B. hi followed by a runtime exception 🎃

C. hithere 🎃

D. Compiler error on line 4 🎃

E. Compiler error on line 6 🎃

F. Compiler error on line 🎃

My answer:

• F ❌ - The correct answer is C, the code DOES compile but there will be a compiler warning about not using generic! Line 7 just creates the class with type Object
• Compiler error on line 7, we need the generic specified!

---

Question 7:

❓ Which of the following statements are true for the following code: ❓

    Set<Number> numbers = new HashSet<>();  // LINE 3
    numbers.add(new Integer(86));
    numbers.add(75);
    numbers.add(new Integer(86));
    numbers.add(null);
    numbers.add(309L);
    Iterator iter = numbers.iterator();     // LINE 9
    while (iter.hasNext())
        System.out.print(iter.next());   

A. The code compiles successfully 🎃

B. Output is 8675null309 🎃

C. Output is 56758null309 🎃

D. The output is intermediate 🎃

E. There's a compiler error on line 3 🎃

F. Compiler error on line 9 🎃

G. An exception is thrown 🎃

My answer:

• A,D ✅
• HashSet does not preserve insertion order, so output is not defined.

---

Question 8:

❓ What is the result of the following code? ❓

TreeSet<String> tree = new TreeSet<String>();
tree.add("one");
tree.add("One");
tree.add("ONE");
System.out.println(tree.ceiling("On"));

A. On 🎃

B. one 🎃

C. One 🎃

D. ONE 🎃

E. The code does not compile 🎃

F. An exception is thrown 🎃

My answer:

• B ❌ - we need the lowest element which is greater than "On" which is "One"
• highest element which is lower than "On" - "one"

Question 9:

❓ Which of the answer choices are valid given the following❓

Map<String, Double> map = new HashMap<>();

A. 🎃

map.add("pi", 3.14159);

B. 🎃

map.add("e", 2L);

C. 🎃

map.add("log(1)", new Double(0.0));

D. 🎃

map.add('x', new Double(123.4));

E. None of the above 🎃

My answer:

• E ✅ - add() is not a method for maps!

Question 10:

❓ What is the result of the following: ❓

import java.util.*;

public class MyComparator implements Comparator<String> {
    public int compare(String a, String b){
        return b.toLowerCase().compareTo(a.toLowerCase());
    }

    public void static main(String[] args){
        String[] values = { "123", "Abb", "aab" };
        Arrays.sort(values, new MyComarator());
        for (String s:values)
            System.out.print(s + " ");
    }
}

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 🎃

My answer:

• B ❌ - answer is A! the sorting is done in a case insensitive fashion so abb > aab > 123
• reversed sorting. so aab > Abb > 123

Question 11:

❓ What is the result of the following: ❓

    Map<Integer, Integer> map = new HashMap<>(10);  // LINE 3
    for (int i = 1; i <=10; i++) {                  
        map.put(i, i*i);                            // LINE 5
    }                           
    System.out.println(map.get(4));                 // LINE 7

A. 16 🎃

B. 25 🎃

C. Compiler error on line 3 🎃

D. Compiler error on line 5 🎃

E. Compiler error on line 7 🎃

F. A runtime exception is thrown 🎃

My Answer:

• B ❌ - the get() method on maps uses an object so 4 is casted to Integer and 4 is mapped to 16!
• Gets element at index 4, so 25

Question 12:

❓ Which of the following statements fill in the blank so Helper compiles successfully ❓

public class Helper {
    public static <U extends Exception> void printException(U u){
        System.out.println(u.getMessage());
    }
    public static void main(String[] args){
        ______________________
    }
}

A. 🎃

Helper.printException(new FileNotFoundException("A"))

B. 🎃

Helper.printException(new Exception("B"))

C. 🎃

Helper.<Throwable>printException(new Exception("C"))

D. 🎃

Helper.<NullPointerException>printException(new NullPointerException("D"))

E. 🎃

Helper.printException(new Throwable("E"))

My answer:

• A,B ❌ Correct answer is A,B,D 0 D us explicitly listing the type
• A - works
• B - works

Question 13:

❓ Which of these statements can fill in the blank so that the WildCard class compiles successfully ❓

import java.util.*;

public class WildCard {
    public void showSize(List<?> list) {
        System.out.println(list.size());
    }
    public static void main(String[] args){
        Wildcard card = new WildCard();
        ________________
        card.showSize(list);
    }
}

A. 🎃

ArrayDeque<?> list = new ArrayDeque<String>();

B. 🎃

ArrayList<? super Date> list = new ArrayList<Date>();

C. 🎃

List<?> list = new ArrayList<?>();

D. 🎃

List<Exception> list = new LinkedList<java.io.IOException>();

E. 🎃

Vector<? extends Number> list = new Vector<Integer>();

F. None of the above 🎃

My answer:

• A,B,E❌ - correct anwser B, E. A is a queue and not a list so does not have size()
• A - compiles
• B - compiles
• C - does not compile
• D - does not compile
• E - compiles
• F - no

Question 14:

❓ What is the result of the following program ❓

    import java.util.*;
    public class Sorted implements Comparable<Sorted>, Comparator<Sorted> {
        private int num;
        private String text;

        Sorted(int n, String t) {
            this.num = n;
            this.text = t;
        }
        public String toString() { return ""+num; }
        public int compareTo(Sorted s){ return text.compareTo(s.text); }
        public int compare(Sorted s1, Sorted s2) { return s1.num - s2.num; }
        public static void main(String[] args) {
            Sorted s1 = new Sorted(88, "a");
            Sorted s2 = new Sorted(55, "b");
            TreeSet<Sorted> t1 = new TreeSet<>();
            t1.add(s1); t1.add(s2);
            TreeSet<Sorted> t2 = new TreeSet<>(s1);
            t2.add(s1); t2.add(s2);
            System.out.println(t1 + " " + t2);

        }
    }

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 🎃

My Answer:

• C✅
• T1 is naturally sorted by the text. so a < b,. so [88, 55]
• T2 is sorted by number so b < a. So [55, 88]

Question 15:

❓ What is the result of the following code? ❓

    Comparatator<Integer> c = (o1, o2) -> o2-o1;
    List<Integer> list = Arrays.asList(5, 4, 7, 1);
    Collections.sort(list, c);
    System.out.println(Collections.binarySearch(list, 1))

A. 0 🎃

B. 1 🎃

C. 2 🎃

D. The result is undefined 🎃

E. The code does not compile 🎃

F. A runtime exception is thrown 🎃

My answer:

• D✅ - gets sorted in descending order!

Question 16:

❓ Which of the following statements are true? ❓

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 🎃

My answer:

• B, D, F ✅
• Comparator is in java.util, Comparable is in java.lang.

Question 17:

❓ Which two options can fill in the blanks so the code compiles ❓

    public class Generic_____ {                     // LINE 1
        public static void main(String[] args){
            Generic<String> g = new Generic_____();  // LINE 3
            Generic<Object> g2 = new Generic();
        }
    }

A. Fill Line 1 with <> 🎃

B. Fill Line 1 with <T> 🎃

C. Fill Line 1 with <?> 🎃

D. Fill Line 3 with <> 🎃

E. Fill Line 3 with <T> 🎃

F. Fill Line 3 with <?> 🎃

My answer:

• B, D ✅

Question 18:

❓ Which of the following line can be inserted to make code compile? ❓

class A {}
class B extends A {}
class C extends B {}

class D<C> {
    // INSERT CODE HERE
}

A. A a1 = new A(); 🎃

B. A a2 = new B(); 🎃

C. A a3 = new C(); 🎃

D. C c1 = new A(); 🎃

E. C c2 = new B(); 🎃

F. C c1 = new C(); 🎃

My answer:

• *A,B,C ❌ - C no longer means class C so only A and B are valid!
• A - valid
• B - valid
• C - valid
• D - invalid
• E - invalid
• F - valid

Question 19:

❓ Which options are true of the following code? ❓

    _______<Integer> q = new LinkedList<>();
    q.add(10);
    q.add(12);
    q.remove(1);
    System.out.println(q);

A. If we fill in the blank with List, the output is [10] 🎃

B. If we fill in the blank with List, the output is [10, 12] 🎃

C. If we fill in the blank with Queue, the output is [10] 🎃

D. If we fill in the blank with Queue, the output is [10, 12] 🎃

E. The code does not compile in either scenario 🎃

F. A runtime exception is thrown 🎃

My answer:

• C ❌ - correct answer is A and D! Queues have a remove() method which takes an object not index!
• A - false
• B - false
• C - true
• D - false
• E - false
• F false

Question 20:

❓ What is the result of the following code? ❓

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

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 🎃

My answer:

• A❌ - Maps do not have a contains() method!!!

Question 21:

❓ Fill in the blanks to make this code compile and print 123❓

    List<String> list = Arrays.asList("1", "2", "3");
    Iterator iter = list.iterator();
    while (iter.______())
        System.out.println(iter._____());

A. On line 6, fill blank with hasNext() 🎃

B. On line 6, fill blank with isNext() 🎃

C. On line 6, fill blank with next() 🎃

D. On line 7, fill blank with getNext() 🎃

E. On line 7, fill blank with hasNext() 🎃

F. On line 7, fill blank with next() 🎃

My answer:

• A, F✅

Question 22:

❓ What code change is needed to make the method compile? ❓

    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 required the code already compiles 🎃

My answer:

• A, B❌ - correct answer is B - the generic specification goes before return type

Question 23:

❓ Which of the answer make sense to implement with lambda (choose all which apply) ❓

A. Comparable interface 🎃

B. Comparator interface 🎃

C. remove() method on Collection 🎃

D. removeAll() method on a Collection 🎃

E. removeIf() method on a Collection 🎃

My answer:

• B,E✅
• B is valid
• C - makes no sense
• D - makes no sense
• E is valid

Question 24:

❓ Which of the following compiles and prints out the entire set?❓

    Set<String> s = new HashSet<>();
    s.add("lion");
    s.add("tiger");
    s.add("bear");
    s.forEach(________);

A. 🎃

() -> System.out.println(s)

B. 🎃

s -> System.out.println(s)

C. 🎃

(s) -> System.out.println(s)

D. 🎃

System.out.println(s)

E. 🎃

System::out::println

F. 🎃

System.out::println

My Answer:

• B,C,F❌ B and C are wrong because they use s in the lambda! Only F is valid!!!
• A-invalid, B-valid,C-valid, F-valid

Question 25:

❓ What is the result of the following? ❓

    Map<Integer, Integer> map = new HashMap<>();
    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 🎃

My answer:

• F✅
• 1 is mapped to 13
• 3 is mapped to 3
• 2 sticks to 20