Practice · ArrayList, StringBuilder, and Arrays of Objects

Q1

What does Student[] roll = new Student[3]; create?

A Three Student objects. B An array of three null references. C An empty array of length 0. D A compile-time error.

Q2

What happens when roll[0].getName() is called immediately after the previous declaration?

A It prints null B A NullPointerException is thrown C A compile-time error D It returns an empty string

Q3

Which import is required for ArrayList?

A java.io.ArrayList B java.util.ArrayList C java.lang.ArrayList D none

Q4

Which declaration is correct?

A ArrayList<int> a = new ArrayList<>(); B ArrayList<Integer> a = new ArrayList<>(); C ArrayList a = new ArrayList<int>(); D ArrayList[] a = new ArrayList[];

Q5

How is the number of elements of an ArrayList obtained?

A a.length B a.size() C a.length() D a.count()

Q6

What is the output?

ArrayList<String> list = new ArrayList<>();
list.add("a"); list.add("b"); list.add(0, "c");
System.out.print(list);

A [a, b, c] B [c, a, b] C [a, b] D [c]

Q7

What is the output?

ArrayList<String> m = new ArrayList<>();
m.add("Louvre"); m.add("Prado"); m.add("Uffizi"); m.add("Tate");
m.remove(2);
System.out.print(m);

A [Louvre, Prado, Tate] B [Louvre, Prado, Uffizi] C [Prado, Uffizi, Tate] D [Louvre, Uffizi, Tate]

Q8

For ArrayList<Integer> n containing [10, 20, 30], what does n.remove(2) remove?

A The value 2. B The element at index 2, namely 30. C Nothing, because 2 is not in the list. D It does not compile.

Q9

For the same list, which call removes the value 20?

A n.remove(20) B n.remove(Integer.valueOf(20)) C n.removeValue(20) D n.delete(20)

Q10

What is the output?

ArrayList<Integer> n = new ArrayList<>();
n.add(5); n.add(7);
int x = n.get(0) + n.get(1);
System.out.print(x);

A 57 B 12 C It does not compile. D 2

Q11

The automatic conversion that lets n.add(5) work on an ArrayList<Integer> is called:

A casting B autoboxing C overloading D widening

Q12

What does list.set(1, "X") do?

A Inserts "X" at index 1 and shifts the rest right. B Replaces the element at index 1 with "X". C Adds "X" at the end. D Removes the element at index 1.

Q13

What is the valid index range for get on a list of size 4?

A 1 to 4 B 0 to 4 C 0 to 3 D 0 to 5

Q14

What is the output?

ArrayList<String> a = new ArrayList<>();
a.add("x");
System.out.print(a.isEmpty() + " " + a.contains("x") + " " + a.indexOf("y"));

A false true -1 B true true 0 C false true 0 D false false -1

Q15

Which statement about ArrayList is false?

A It grows automatically as elements are added. B It can store primitive int values directly, without wrapping. C Its elements are accessed with get and set. D It is found in java.util.

Q16

Which loop prints every element of ArrayList<String> list?

A for (int i = 0; i <= list.size(); i++) System.out.println(list.get(i)); B for (String s : list) System.out.println(s); C for (String s = 0; s < list.size(); s++) ... D for (int i = 0; i < list.length; i++) ...

Q17

Why can removing elements inside a forward for loop skip items?

A Because remove is slow. B Because the remaining elements shift left while the index still moves right. C Because size() never changes. D It cannot; the loop is always correct.

Q18

Which is the main advantage of an array over an ArrayList?

A It can hold primitives directly and has a fixed, known size B It grows automatically C It has a contains method D It can hold objects

Q19

Why is StringBuilder preferred to repeated String concatenation in a loop?

A String cannot be used inside loops. B Each concatenation creates a new String object, while a builder modifies one object in place. C StringBuilder uses less memory per character. D String is not part of java.lang.

Q20

What is the output?

StringBuilder sb = new StringBuilder("Java");
sb.append(" SE").insert(0, "* ");
System.out.print(sb);

A Java SE B * Java SE C Java SE * D It does not compile.

Q21

What is the output?

StringBuilder sb = new StringBuilder("abcdef");
sb.delete(1, 3);
System.out.print(sb);

A adef B acdef C abef D aef

Q22

What is the output?

StringBuilder b = new StringBuilder("stressed");
System.out.print(b.reverse());

A stressed B desserts C Desserts D It does not compile.

Q23

What is the result of compiling the following?

String b = "stressed";
System.out.print(b.reverse());

A It prints desserts. B It prints stressed. C It does not compile, because String has no reverse method. D It throws an exception at run time.

Q24

What is the output?

StringBuilder sb = new StringBuilder();
for (int i = 1; i <= 3; i++) sb.append(i).append(",");
System.out.print(sb.length());

A 3 B 4 C 6 D 7

Q25

Which StringBuilder method returns an ordinary String?

A toString() B valueOf() C getString() D parse()

Q26

What is the output?

StringBuilder a = new StringBuilder("one");
StringBuilder b = a;
b.append(" two");
System.out.print(a);

A one B one two C two D It does not compile.

Q27

Why does the previous question behave differently from the same code written with String?

A Because StringBuilder copies itself on assignment. B Because a StringBuilder object is mutable, so both references see the change; a String method would have created a new object. C Because append is static. D Because String is not an object.

Q28

What is the output?

ArrayList<Student> c = new ArrayList<>();
c.add(new Student("Ali", 40));
c.add(new Student("Sara", 80));
int passed = 0;
for (Student s : c) if (s.isPassed()) passed++;
System.out.print(c.size() + " " + passed);

(assume isPassed() returns grade >= 50)

A 2 1 B 2 2 C 1 1 D 2 0

Q29

For contains to work as expected on an ArrayList of your own objects, the class must:

A be static B override equals C implement Comparable D override toString

Q30

Which statement correctly declares a list that can hold Student objects?

A ArrayList<Student> c = new ArrayList<Student>(); B ArrayList Student c; C Student<ArrayList> c; D ArrayList c = new Student[];

Q31

What is printed?

ArrayList<String> t = new ArrayList<>();
t.add("a"); t.add("b"); t.add("c");
for (int i = t.size() - 1; i >= 0; i--) System.out.print(t.get(i));

A abc B cba C aabbcc D An exception is thrown.

Q32

Which call empties a list completely?

A list.clear() B list.remove() C list = 0 D list.isEmpty()

Q33

What is the output?

ArrayList<Integer> v = new ArrayList<>();
for (int i = 0; i < 5; i++) v.add(i * i);
System.out.print(v.get(3) + " " + v.size());

A 9 5 B 3 5 C 16 4 D 9 4

Q34 Short answer

Explain, in terms of objects created in memory, the difference between building a 10,000-character report with String concatenation and with StringBuilder.

Q35 Short answer

A list ArrayList<Integer> scores contains [3, 3, 7, 9, 3]. Write the code that removes every occurrence of the value 3, and explain why the obvious forward loop fails.

Q36 Programming

Write a class Classroom that holds an ArrayList<Student> and offers addStudent(Student), removeStudent(int id), findStudent(int id) returning the object or null, averageGrade() and topStudent(). Write a driver that adds four students, removes one, and prints the average and the best student.

Q37 Programming

Write a method public static String compress(String s) that uses a StringBuilder to turn "aaabccddd" into "a3b1c2d3", and a main that tests it on at least three inputs including the empty string.