Java Quiz

1. What does the javac tool do?

• Runs a Java program
• Compiles Java source code into bytecode
• Formats Java code
• Downloads libraries

The JVM then runs the bytecode (lesson 01).

2. Which file name must contain public class Invoice?

• invoice.java
• Invoice.java
• Main.java
• Any name ending in .java

A public class must be in a file with the same name (lesson 03).

3. What does System.out.println("3 + 4 = " + 3 + 4) print?

• 3 + 4 = 7
• 3 + 4 = 34
• 7
• A compile error

Once a string is involved, + concatenates from left to right (lesson 03).

4. What is the value of 7 / 2 in Java?

• 3.5
• 3
• 4
• 3.0

Dividing two ints gives an int (lesson 07).

5. What is (int) 9.99?

• 9
• 10
• 9.99
• A compile error

Casting truncates; use Math.round to round (lesson 07).

6. What does Integer.MAX_VALUE + 1 give?

• An exception
• 2147483648
• Integer.MIN_VALUE (−2147483648)
• 0

Integer overflow wraps around silently (lesson 06).

7. How should you compare the contents of two strings?

• a == b
• a.equals(b)
• a.compare(b)
• a = b

== compares references (lesson 09).

8. What happens to s after s.toUpperCase(); (result not stored)?

• Nothing — strings are immutable
• It becomes uppercase
• It becomes null
• A compile error occurs

String methods return new strings (lesson 09).

9. Which loop always runs its body at least once?

• while
• for
• do…while
• for-each

The condition is checked after the body (lesson 15).

10. What is the last valid index of an array with length 5?

• 5
• 4
• 6
• 0

Indexes start at 0 (lesson 18).

11. Why doesn't the arrow form of switch need break?

• It always runs every case
• Arrow cases never fall through to the next case
• It only works with enums
• It needs yield instead

Only the matching case runs (lesson 14).

12. A method receives an int and changes it. What happens to the caller's variable?

• Nothing — Java passes a copy of the value
• It changes too
• It becomes 0
• A compile error occurs

Java is always pass-by-value (lesson 21).

13. What does a recursive method need to stop?

• A break statement
• A base case that returns without recursing
• A static modifier
• A loop

Without a reachable base case you get a StackOverflowError (lesson 22).

14. What does static mean on a field?

• It can't change
• One copy is shared by all objects of the class
• It's private
• It's created lazily

Use final for "can't change" (lesson 24).

15. What happens to the free default constructor when you write your own constructor?

• Nothing
• It's no longer provided
• It becomes private
• It calls yours automatically

Declare a no-argument constructor yourself if you need one (lesson 25).

16. Which access modifier hides a member from every other class?

• protected
• (no modifier)
• private
• public

(lesson 26)

17. How many classes can a Java class extend?

• One
• Two
• Any number
• None

But it can implement many interfaces (lessons 28 and 31).

18. Animal a = new Cat(); — which speak() runs for a.speak()?

• Animal's, because of the variable's type
• Cat's, because of the object's actual type
• Both
• Neither — it doesn't compile

Dynamic dispatch uses the runtime type (lesson 29).

19. Can you create an object of an abstract class with new?

• Yes, always
• No — only of its concrete subclasses
• Only if it has a constructor
• Only inside the same package

(lesson 30)

20. What does a record automatically generate?

• Setters for every field
• A constructor, accessors, equals, hashCode and toString
• A main method
• A database table

Records are immutable data classes (lesson 32).

21. If you override equals, what else must you override?

• toString
• hashCode
• compareTo
• clone

Otherwise hash-based collections lose objects (lesson 35).

22. What's the difference between checked and unchecked exceptions?

• Checked exceptions must be caught or declared with throws
• Unchecked exceptions can't be caught
• Checked exceptions only happen at compile time
• There's no difference

Unchecked exceptions extend RuntimeException (lesson 36).

23. What does try-with-resources guarantee?

• That no exception is thrown
• That the declared resources are closed when the block ends
• That the code runs on a separate thread
• That the file exists

Resources must implement AutoCloseable (lesson 37).

24. Why does Integer c = 1000, d = 1000; c == d give false?

• 1000 is too large for an Integer
• == compares references, and only small values are cached
• Autoboxing is switched off
• It actually gives true

Compare wrappers with equals (lesson 38).

25. Which collection keeps elements unique and sorted?

• ArrayList
• HashSet
• TreeSet
• ArrayDeque

(lesson 41)

26. What does map.merge(word, 1, Integer::sum) do?

• Replaces the value with 1
• Puts 1 for a new key, or adds 1 to the existing value
• Removes the key
• Throws if the key exists

The idiomatic way to count (lesson 42).

27. Which is a valid lambda for Predicate<String>?

• () -> true
• s -> s.isEmpty()
• (a, b) -> a.equals(b)
• s -> s.length()

A predicate takes one value and returns a boolean (lesson 45).

28. When does a stream pipeline actually process elements?

• When stream() is called
• After each filter
• When a terminal operation such as toList() runs
• Never; streams are just descriptions

Intermediate operations are lazy (lesson 46).

29. What do sealed interfaces add to pattern-matching switches?

• Faster execution
• The compiler knows every subtype, so it can check the switch is exhaustive
• Automatic toString
• Thread safety

No default needed, and new subtypes cause compile errors (lesson 49).

30. Why does count++ from several threads lose updates?

• Threads can't share variables
• It's a read–modify–write sequence that threads can interleave
• int is too small
• The JVM ignores some increments on purpose

Use AtomicInteger, synchronized or locks (lesson 52).