Practice · Sets and Maps

Q1

What does this code print?

Set<String> s = new HashSet<>();
boolean a = s.add("kiwi");
boolean b = s.add("kiwi");
boolean c = s.add("KIWI");
System.out.println(a + " " + b + " " + c + " " + s.size());

A true true true 3 B true false true 2 C true false false 1 D true false true 3

Q2

What does this code print?

Set<Integer> linked = new LinkedHashSet<>();
Set<Integer> tree = new TreeSet<>();
int[] data = {5, 1, 3, 1, 9, 5};
for (int d : data) { linked.add(d); tree.add(d); }
System.out.println(linked + " " + tree);

A [5, 1, 3, 9] [1, 3, 5, 9] B [5, 1, 3, 1, 9, 5] [1, 1, 3, 5, 5, 9] C [1, 3, 9, 5] [1, 3, 5, 9] D [1, 3, 5, 9] [1, 3, 5, 9]

Q3

What does this code print?

Set<String> s = new TreeSet<>();
s.add("pear"); s.add("Apple"); s.add("banana");
s.add("apple"); s.add("Banana");
System.out.println(s);

A [Apple, apple, Banana, banana, pear] B [apple, banana, pear] C [Apple, Banana, apple, banana, pear] D [pear, Apple, banana, apple, Banana]

Q4

Tag overrides hashCode but not equals. What does the program print?

class Tag {
    String name;
    Tag(String n) { name = n; }
    @Override public int hashCode() { return name.hashCode(); }
}
// in main:
Set<Tag> s = new HashSet<>();
s.add(new Tag("x"));
s.add(new Tag("x"));
System.out.println(s.size() + " " + s.contains(new Tag("x")));

A 1 true B 1 false C 2 true D 2 false

Q5

A class Point overrides equals (comparing x and y) but does not override hashCode. Which statement best describes a HashSet<Point>?

A It works correctly, because HashSet only ever calls equals. B contains(new Point(1, 2)) will usually return false even after add(new Point(1, 2)). C The code does not compile: a class that overrides equals must override hashCode. D Every add throws IllegalStateException because the contract is broken.

Q6

Which of these is not required by the equals/hashCode contract?

A If a.equals(b), then a.hashCode() == b.hashCode(). B If a.hashCode() == b.hashCode(), then a.equals(b). C hashCode() returns the same value on repeated calls while the fields used by equals are unchanged. D a.equals(a) is true for any non-null a.

Q7

What does this code print?

Set<String> s = new TreeSet<>(Comparator.comparingInt(String::length));
s.add("cat"); s.add("dog"); s.add("bird"); s.add("ox"); s.add("emu");
System.out.println(s.size() + " " + s);

A 5 [ox, cat, dog, emu, bird] B 3 [ox, cat, bird] C 3 [ox, emu, bird] D 5 [bird, cat, dog, emu, ox]

Q8

Box does not implement Comparable. What happens?

class Box {
    int w;
    Box(int w) { this.w = w; }
}
// in main:
Set<Box> s = new TreeSet<>();
s.add(new Box(1));
System.out.println(s.size());

A It does not compile: Box is not Comparable. B It prints 1. C It compiles, but add throws ClassCastException. D It compiles, but println throws NullPointerException.

Q9

What does this code print?

TreeSet<Integer> t = new TreeSet<>(Arrays.asList(10, 20, 30, 40));
System.out.println(t.floor(25) + " " + t.ceiling(25) + " "
        + t.higher(40) + " " + t.lower(10) + " " + t.floor(30));

A 20 30 null null 30 B 20 30 40 10 20 C 30 20 null null 30 D 20 30 null null 20

Q10

What does this code print?

TreeSet<Integer> t = new TreeSet<>(Arrays.asList(10, 20, 30, 40, 50));
System.out.println(t.headSet(30) + " " + t.tailSet(30) + " " + t.subSet(20, 40));

A [10, 20, 30] [30, 40, 50] [20, 30, 40] B [10, 20] [40, 50] [30] C [10, 20] [30, 40, 50] [20, 30] D [10, 20, 30] [40, 50] [20, 30, 40]

Q11

What does this code print?

Map<String, Integer> m = new HashMap<>();
m.put("a", 1);
Integer x = m.put("a", 2);
Integer y = m.put("b", 3);
System.out.println(x + " " + y + " " + m.get("a"));

A 1 null 2 B 2 3 2 C null null 2 D 1 0 1

Q12

What happens when this code runs?

Map<String, Integer> stock = new HashMap<>();
stock.put("pen", 4);
int n = stock.get("ink");
System.out.println(n);

A It prints 0. B It prints null. C It throws NullPointerException. D It does not compile.

Q13

What does this code print?

Map<String, Integer> freq = new TreeMap<>();
for (String w : "to be or not to be".split(" ")) {
    freq.merge(w, 1, Integer::sum);
}
System.out.println(freq);

A {to=2, be=2, or=1, not=1} B {be=2, not=1, or=1, to=2} C {be=1, not=1, or=1, to=1} D {be=2, or=1, not=1, to=2}

Q14

What does this code print?

Map<Integer, List<String>> groups = new TreeMap<>();
String[] words = {"ox", "cat", "ant", "be", "emu", "go"};
for (String w : words) {
    groups.computeIfAbsent(w.length(), k -> new ArrayList<>()).add(w);
}
System.out.println(groups);

A {2=[go], 3=[emu]} B {2=[be, go, ox], 3=[ant, cat, emu]} C {2=[ox, be, go], 3=[cat, ant, emu]} D {3=[cat, ant, emu], 2=[ox, be, go]}

Q15

Which numbered line throws an exception?

Map<String, Integer> h = new HashMap<>();
h.put(null, 1);          // line 1
h.put(null, 2);          // line 2
Map<String, Integer> t = new TreeMap<>();
t.put("a", null);        // line 3
t.put(null, 3);          // line 4

A None of them. B Line 2, because a HashMap allows the null key only once. C Line 3, because a TreeMap does not allow null values. D Line 4, because a TreeMap cannot compare a null key.

Q16

What does this code print?

Map<String, Integer> m = new TreeMap<>();
m.put("a", 1); m.put("b", 2); m.put("c", 3);
m.keySet().remove("b");
m.values().remove(3);
System.out.println(m);

A {a=1, b=2, c=3} B {a=1, c=3} C {a=1} D It throws UnsupportedOperationException.

Q17

What happens when this code runs?

Map<String, Integer> m = new TreeMap<>();
m.put("a", 1); m.put("b", 2); m.put("c", 3);
for (String k : m.keySet()) {
    if (k.equals("a")) m.remove(k);
}
System.out.println(m);

A It prints {b=2, c=3}. B It prints {a=1, b=2, c=3}. C It throws ConcurrentModificationException. D It does not compile: you cannot call remove inside a for-each loop.

Q18

What does this code print?

Set<Integer> a = new TreeSet<>(Arrays.asList(1, 2, 3, 4));
Set<Integer> b = new TreeSet<>(Arrays.asList(3, 4, 5));
Set<Integer> u = new TreeSet<>(a);
u.addAll(b);
boolean changed = a.retainAll(b);
System.out.println(u + " " + a + " " + changed);

A [1, 2, 3, 4, 5] [3, 4] true B [1, 2, 3, 4, 5] [1, 2, 3, 4] false C [1, 2, 3, 4, 3, 4, 5] [3, 4] true D [1, 2, 3, 4, 5] [3, 4] false

Q19

You store 1,000,000 student IDs and must answer "is this ID registered?" as fast as possible; the order is irrelevant. Which choice and expected cost per query are correct?

A TreeSet, O(1) B HashSet, O(1) expected C ArrayList with contains, O(log n) D TreeSet, O(n)

Q20

A program reads configuration lines key=value and must later write them back in the order they appeared, with later lines overriding earlier ones for the same key. Which map fits best?

A HashMap B TreeMap C LinkedHashMap D TreeMap with a reverse comparator

Q21

What does this code print?

TreeMap<Integer, String> g = new TreeMap<>();
g.put(90, "A"); g.put(80, "B"); g.put(70, "C"); g.put(60, "D"); g.put(0, "F");
System.out.println(g.floorEntry(85).getValue() + g.floorEntry(60).getValue()
        + g.ceilingKey(91) + " " + g.headMap(70));

A BD90 {0=F, 60=D} B BDnull {0=F, 60=D} C AC90 {0=F, 60=D, 70=C} D BDnull {0=F, 60=D, 70=C}

Q22

What does this code print?

Map<String, Integer> m = new LinkedHashMap<>();
m.put("z", 3); m.put("x", 1); m.put("y", 2);
int total = 0;
for (Map.Entry<String, Integer> e : m.entrySet()) {
    e.setValue(e.getValue() * 10);
    total += e.getValue();
}
System.out.println(m + " " + total);

A {x=10, y=20, z=30} 60 B {z=3, x=1, y=2} 60 C {z=30, x=10, y=20} 60 D It throws ConcurrentModificationException.

Q23

P overrides equals (comparing x and y) and hashCode (31 * x + y) correctly. What does this code print?

Set<P> s = new HashSet<>();
P p = new P(1, 2);
s.add(p);
p.x = 5;
System.out.println(s.contains(p) + " " + s.contains(new P(1, 2)) + " " + s.size());

A true false 1 B false true 1 C false false 1 D true true 1

Q24

What does this code print?

Set<String> s = new TreeSet<>(String.CASE_INSENSITIVE_ORDER);
s.add("Java"); s.add("JAVA"); s.add("java");
System.out.println(s + " " + s.contains("jAvA"));

A [JAVA, Java, java] false B [Java] true C [java] true D [Java] false

Q25

Does this code compile? If so, what does it print?

Set<String> s = new TreeSet<>();
s.add("b");
s.add("a");
System.out.println(s.first());

A It compiles and prints a. B It compiles and prints b. C It does not compile. D It compiles but throws NoSuchElementException.

Q26 Short answer

A class Isbn wraps a String code and overrides equals to compare codes, but not hashCode. A colleague says "it's fine, equals works". Explain concretely what goes wrong when Isbn objects are stored in a HashSet, and write a correct hashCode.

Q27 Short answer

Compare HashMap, LinkedHashMap and TreeMap under three headings: iteration order, cost of get/put, and null keys. Then give one situation in which each is the best choice.

Q28 Short answer

A programmer stores Student objects in new TreeSet<>(Comparator.comparingDouble(Student::getGpa)). After adding 40 students, size() is 31. Explain why, and show how to fix the comparator while still sorting by GPA.

Q29 Short answer

You need a Map<String, List<String>> that groups course codes by department. Explain why this loop body fails on the first course of a department, and rewrite it in one line.

byDept.get(dept).add(code);

Q30 Programming

Write a method

static Set<Integer> symmetricDifference(Set<Integer> a, Set<Integer> b)

that returns a new, sorted set containing the elements that are in exactly one of a and b. The arguments must not be modified. Example: {1, 2, 3, 4} and {3, 4, 5} → [1, 2, 5].

Q31 Programming

Write a method

static String firstRepeated(String[] words)

that returns the first word whose second occurrence comes earliest in the array, or null if no word repeats. Example: {"a", "b", "c", "b", "a"} → "b" (its repeat at index 3 comes before the repeat of "a" at index 4). It must run in O(n) expected time.