Test your Core Java knowledge with topic-wise Java MCQs covering Strings, Exception Handling, Collections, Multithreading, Memory Management, JVM, OOPs, and more. Ideal for interviews and practice.
Sharpen your Core Java fundamentals with carefully curated multiple-choice questions covering Strings, Exception Handling, Collections Framework, Multithreading, JVM internals, Memory Management, OOP concepts, and more.
Practice Java MCQs with Detailed Explanations
Answer at least
12
questions to submit.
31.
Given:
3. class A { }
4. class B extends A { }
5. public class ComingThru {
6. static String s = "-";
7. public static void main(String[] args) {
8. A[] aa = new A[2];
9. B[] ba = new B[2];
10. sifter(aa);
11. sifter(ba);
12. sifter(7);
13. System.out.println(s);
14. }
15. static void sifter(A[]... a2) { s += "1"; }
16. static void sifter(B[]... b1) { s += "2"; }
17. static void sifter(B[] b1) { s += "3"; }
18. static void sifter(Object o) { s += "4"; }
19. }
What is the result?
Correct answer:-434
In general, overloaded var-args methods are chosen last. Remember that arrays are objects. Finally, an int can be boxed to an Integer and then "widened" to an Object
32.
Given:
3. import java.io.*;
4. class Vehicle { }
5. class Wheels { }
6. class Car extends Vehicle implements Serializable { }
7. class Ford extends Car { }
8. class Dodge extends Car {
9. Wheels w = new Wheels();
10. }
Instances of which class(es) can be serialized? (Choose all that apply.)
A. Car
B. Ford
C. Dodge
D. Wheels
E. Vehicle
Correct answer:A, B
Dodge instances cannot be serialized because they "have" an instance of Wheels, which is not serializable. Vehicle instances cannot be serialized even though the
subclass Car can be.
33.
Given:
3. import java.util.regex.*;
4. public class Archie {
5. public static void main(String[] args) {
6. Pattern p = Pattern.compile(args[0]);
7. Matcher m = p.matcher(args[1]);
8. int count = 0;
9. while(m.find())
10. count++;
11. System.out.print(count);
12. }
13. }
And given the command line invocation:
java Archie "\d+" ab2c4d67
What is the result?
Correct answer:3
The "\d" metacharacter looks for digits, and the + quantifier says look for "one or more" occurrences. The find() method will find three sets of one or more consecutive
digits: 2, 4, and 67.
34.
Given:
3. import java.util.*;
4. public class Looking {
5. public static void main(String[] args) {
6. String input = "1 2 a 3 45 6";
7. Scanner sc = new Scanner(input);
8. int x = 0;
9. do {
10. x = sc.nextInt();
11. System.out.print(x + " ");
12. } while (x!=0);
13. }
14. }
What is the result?
Correct answer:1 2 followed by an exception
The nextXxx() methods are typically invoked after a call to a hasNextXxx(), which determines whether the next token is of the correct type.
35.
Given the proper import statement(s), and
13. PriorityQueue<String> pq = new PriorityQueue<String>();
14. pq.add("2");
15. pq.add("4");
16. System.out.print(pq.peek() + " ");
17. pq.offer("1");
18. pq.add("3");
19. pq.remove("1");
20. System.out.print(pq.poll() + " ");
21. if(pq.remove("2")) System.out.print(pq.poll() + " ");
22. System.out.println(pq.poll() + " " + pq.peek());
What is the result?
Correct answer:2 2 3 4
For the sake of the exam, add() and offer() both add to (in this case), naturally sorted queues. The calls to poll() both return and then remove the first item
from the queue, so the if test fails.
36.
Given:
3. import java.util.*;
4. public class Mixup {
5. public static void main(String[] args) {
6. Object o = new Object();
7. // insert code here
8. s.add("o");
9. s.add(o);
10. }
11. }
And these three fragments:
I. Set s = new HashSet();
II. TreeSet s = new TreeSet();
III. LinkedHashSet s = new LinkedHashSet();
When fragments I, II, or III are inserted, independently, at line 7, which are true?
(Choose all that apply.)
A. Fragment I compiles
B. Fragment II compiles
C. Fragment III compiles
D. Fragment I executes without exception
E. Fragment II executes without exception
F. Fragment III executes without exception
Correct answer:A, B, C, D ,F
Only E is incorrect. Elements of a TreeSet must in some way implement Comparable
37.
Given:
3. import java.util.*;
4. class Turtle {
5. int size;
6. public Turtle(int s) { size = s; }
7. public boolean equals(Object o) { return (this.size == ((Turtle)o).size); }
8. // insert code here
9. }
10. public class TurtleTest {
11. public static void main(String[] args) {
12. LinkedHashSet<Turtle> t = new LinkedHashSet<Turtle>();
13. t.add(new Turtle(1)); t.add(new Turtle(2)); t.add(new Turtle(1));
14. System.out.println(t.size());
15. }
16. }
And these two fragments:
I. public int hashCode() { return size/5; }
II. // no hashCode method declared
If fragment I or II is inserted, independently, at line 8, which are true? (Choose all that apply.)
A. If fragment I is inserted, the output is 2
B. If fragment I is inserted, the output is 3
C. If fragment II is inserted, the output is 2
D. If fragment II is inserted, the output is 3
E. If fragment I is inserted, compilation fails
F. If fragment II is inserted, compilation fails
Correct answer:A, D
While fragment II wouldn’t fulfill the hashCode() contract (as you can see by the results), it is legal Java. For the purpose of the exam, if you don’t override
hashCode(), every object will have a unique hashcode.
38.
Given the proper import statement(s), and:
13. TreeSet<String> s = new TreeSet<String>();
14. TreeSet<String> subs = new TreeSet<String>();
15. s.add("a"); s.add("b"); s.add("c"); s.add("d"); s.add("e");
16.
17. subs = (TreeSet)s.subSet("b", true, "d", true);
18. s.add("g");
19. s.pollFirst();
20. s.pollFirst();
21. s.add("c2");
22. System.out.println(s.size() +" "+ subs.size());
Which are true? (Choose all that apply.)
A. The size of s is 4
B. The size of s is 5
C. The size of s is 7
D. The size of subs is 1
E. The size of subs is 2
F. The size of subs is 3
G. The size of subs is 4
H. An exception is thrown at runtime
Correct answer:B, F
After "g" is added, TreeSet s contains six elements and TreeSet subs contains three (b, c, d), because "g" is out of the range of subs. The first pollFirst()
finds and removes only the "a". The second pollFirst() finds and removes the "b" from both TreeSets (remember they are backed). The final add() is in range of both TreeSets.
The final contents are [c,c2,d,e,g] and [c,c2,d].
39.
Given:
3. import java.util.*;
4. public class Magellan {
5. public static void main(String[] args) {
6. TreeMap<String, String> myMap = new TreeMap<String, String>();
7. myMap.put("a", "apple"); myMap.put("d", "date");
8. myMap.put("f", "fig"); myMap.put("p", "pear");
9. System.out.println("1st after mango: " + // sop 1
10. myMap.higherKey("f"));
11. System.out.println("1st after mango: " + // sop 2
12. myMap.ceilingKey("f"));
13. System.out.println("1st after mango: " + // sop 3
14. myMap.floorKey("f"));
15. SortedMap<String, String> sub = new TreeMap<String, String>();
16. sub = myMap.tailMap("f");
17. System.out.println("1st after mango: " + // sop 4
18. sub.firstKey());
19. }
20. }
Which of the System.out.println statements will produce the output 1st after mango: p?
Correct answer:sop 1
The ceilingKey() method's argument is inclusive. The floorKey() method would be used to find keys before the specified key. The firstKey() method's argument is
also inclusive.
40.
Which are true about a static nested class?
A. You must have a reference to an instance of the enclosing class in order to instantiate it
B. It does not have access to non-static members of the enclosing class
C. Its variables and methods must be static
D. If the outer class is named MyOuter, and the nested class is named MyInner, it can be
instantiated using new MyOuter.MyInner();
E. It must extend the enclosing class
Correct answer:B, D
B is correct because a static nested class is not tied to an instance of the enclosing class, and thus can't access the non-static members of the class (just as a static method can't access non-static members of a class). D uses the correct syntax for instantiating a static nested class.
41.
Given:
class Boo {
Boo(String s) { }
Boo() { }
}
class Bar extends Boo {
Bar() { }
Bar(String s) {super(s);}
void zoo() {
// insert code here
}
}
Which create an anonymous inner class from within class Bar?
A. Boo f = new Boo(24) { };
B. Boo f = new Bar() { };
C. Boo f = new Boo() {String s; };
D. Bar f = new Boo(String s) { };
E. Boo f = new Boo.Bar(String s) { };
Correct answer:B, C
B is correct because anonymous inner classes are no different from any other class when it comes to polymorphism. That means you are always allowed to declare a reference variable of the superclass type and have that reference variable refer to an instance of a subclass type, which in this case is an anonymous subclass of Bar. Since Bar
is a subclass of Boo, it all works. C uses correct syntax for creating an instance of Boo.
42.
Given:
3. import java.util.*;
4. class Business { }
5. class Hotel extends Business { }
6. class Inn extends Hotel { }
7. public class Travel {
8. ArrayList<Hotel> go() {
9. // insert code here
10. }
11. }
Which, inserted independently at line 9, will compile?
Correct answer:return new ArrayList<Hotel>();
A is incorrect because polymorphic assignments don't apply to generic type parameters. C and D are incorrect because they don't follow basic polymorphism rules.
43.
Given:
3. import java.util.*;
4. class Dog { int size; Dog(int s) { size = s; } }
5. public class FirstGrade {
6. public static void main(String[] args) {
7. TreeSet<Integer> i = new TreeSet<Integer>();
8. TreeSet<Dog> d = new TreeSet<Dog>();
9.
10. d.add(new Dog(1)); d.add(new Dog(2)); d.add(new Dog(1));
11. i.add(1); i.add(2); i.add(1);
12. System.out.println(d.size() + " " + i.size());
13. }
14. }
What is the result?
Correct answer:An exception is thrown at runtime
Class Dog needs to implement Comparable in order for a TreeSet (which keeps its elements sorted) to be able to contain Dog objects.
44.
Which are true about a method-local inner class?
A. It must be marked final
B. It can be marked abstract
C. It can be marked public
D. It can be marked static
E. It can access private members of the enclosing class
Correct answer:B, E
B is correct because a method-local inner class can be abstract, although itmeans a subclass of the inner class must be created if the abstract class is to be used (so
an abstract method-local inner class is probably not useful). E is correct because amethod-local inner class works like any other inner class—it has a special relationship to
an instance of the enclosing class, thus it can access all members of the enclosing class.
45.
Given:
1. public class TestObj {
2. public static void main(String[] args) {
3. Object o = new Object() {
4. public boolean equals(Object obj) {
5. return true;
6. }
7. }
8. System.out.println(o.equals("Fred"));
9. }
10. }
What is the result?
Correct answer:Compilation fails because of an error on a line other than 3, 4, or 8
This code would be legal if line 7 ended with a semicolon. Remember that line 3 is a statement that doesn't end until line 7, and a statement needs a closing semicolon!
