Question 1

What is the difference between JDK, JRE and JVM?

Accepted Answer

The JVM executes bytecode. The JRE is the JVM plus the standard class library needed to run apps. The JDK is the JRE plus development tools such as javac, jar and jshell.

Question 2

Why is Java called platform independent?

Accepted Answer

The compiler produces bytecode, not machine code. Each operating system has its own JVM that runs the same bytecode, so a .class file runs unchanged everywhere. The JVM itself is platform specific.

Question 3

What is autoboxing and what can go wrong?

Accepted Answer

Automatic conversion between primitives and their wrappers. Pitfalls: NullPointerException when unboxing null, hidden allocations in hot loops, and == comparing references instead of values.

Question 4

Is var dynamic typing?

Accepted Answer

No. var is local type inference at compile time. The variable's type is fixed once inferred, and var only works for local variables that have an initializer.

Question 5

What is the difference between i++ and ++i?

Accepted Answer

i++ returns the old value and then increments; ++i increments first and returns the new value. On a line of their own they behave the same.

Question 6

What does short-circuit evaluation mean?

Accepted Answer

With && the right side is skipped if the left side is false; with || it's skipped if the left side is true. It saves work and avoids NullPointerExceptions.

Question 7

What is the difference between a switch statement and a switch expression?

Accepted Answer

An expression produces a value and must be exhaustive; its arrow labels don't fall through. A statement just runs code and can fall through when it uses colon labels.

Question 8

What does yield do?

Accepted Answer

It returns a value from a block arm of a switch expression. It's a contextual keyword, so you can still use yield as a variable name elsewhere.

Question 9

Can you change an array's size?

Accepted Answer

No, the length is fixed. Create a new array with Arrays.copyOf, or use ArrayList, which does this for you and grows by about 50% when full.

Question 10

Array or ArrayList?

Accepted Answer

Arrays: fixed size, can hold primitives, slightly faster. ArrayList: resizable, objects only (so primitives get boxed), a rich API, and works with generics and streams.

Question 11

Is Java pass-by-reference for objects?

Accepted Answer

No. It passes a copy of the reference. You can change the object through that copy, but reassigning the parameter never affects the caller's variable.

Question 12

Overloading vs overriding?

Accepted Answer

Overloading: same name, different parameters, chosen at compile time. Overriding: a subclass redefines an inherited method with the same signature, chosen at runtime.

Question 13

Why is String immutable?

Accepted Answer

Security (class names, URLs and file paths can't change after being checked), thread safety without locks, safe use as HashMap keys with a cached hashCode, and string pooling.

Question 14

String vs StringBuilder vs StringBuffer?

Accepted Answer

String is immutable. StringBuilder is mutable and not synchronized; use it by default. StringBuffer is the older synchronized version and is rarely needed.

Question 15

Why does nextLine() return an empty string after nextInt()?

Accepted Answer

nextInt() reads only the digits and leaves the line break; the next nextLine() reads the rest of that same line, which is empty.

Question 16

printf vs String.format?

Accepted Answer

printf writes formatted text to the output stream. String.format (or formatted()) returns the formatted String without printing it.

Question 17

Why not use double for money?

Accepted Answer

double stores binary fractions, so values like 0.1 can't be represented exactly and rounding errors add up. BigDecimal, or whole paise in a long, is exact.

Question 18

Random vs SecureRandom?

Accepted Answer

Random is fast but predictable from its seed. SecureRandom uses a cryptographically strong source; use it for tokens, passwords and OTPs.

Question 19

What causes StackOverflowError?

Accepted Answer

Too many nested method calls, usually infinite or very deep recursion. Each call uses a stack frame and the thread's stack runs out.

Question 20

Recursion or iteration?

Accepted Answer

Recursion is clearer for tree-shaped problems; iteration uses constant stack space and is safer for deep inputs, since Java has no tail-call optimisation.

Question 21

Can a constructor be private?

Accepted Answer

Yes. It's used for singletons, static factory methods such as List.of, and utility classes that should never be instantiated.

Question 22

What is constructor chaining?

Accepted Answer

Calling one constructor from another, with this(...) in the same class or super(...) for the parent, so shared setup code lives in one place.

Question 23

Can a static method access instance variables?

Accepted Answer

Not directly: there's no this in a static context. It needs an object passed in.

Question 24

final, finally and finalize?

Accepted Answer

final prevents reassignment, overriding or extension. finally is the block that always runs after try. finalize was an object clean-up hook, deprecated for removal since Java 18.

Question 25

Why use packages?

Accepted Answer

To group related classes, avoid name clashes, and control visibility with package-private access.

Question 26

What is package-private access?

Accepted Answer

The default when no modifier is written: the class or member is visible only inside the same package.

Question 27

What are the four pillars of OOP?

Accepted Answer

Encapsulation, abstraction, inheritance and polymorphism.

Question 28

Why doesn't Java allow multiple inheritance of classes?

Accepted Answer

To avoid the diamond problem, where two parents define the same method and state. A class can implement several interfaces; if two default methods clash, it must override the method and choose explicitly.

Question 29

When would you pick an abstract class over an interface today?

Accepted Answer

When subclasses need shared instance state, constructors or non-public members. Otherwise interfaces with default methods are more flexible, because a class can implement many of them.

Question 30

Can an interface have a constructor?

Accepted Answer

No. Interfaces cannot be instantiated and have no instance state to initialize.

Question 31

Static nested vs inner class?

Accepted Answer

A static nested class has no link to an outer instance. An inner class holds a reference to its outer object, can use its fields, and needs an outer instance to be created.

Question 32

When is an anonymous class still useful instead of a lambda?

Accepted Answer

When the type has several abstract methods, is an abstract class, or you need fields or a this that refers to the object itself.

Question 33

Can enums have constructors?

Accepted Answer

Yes, and they're always private. They run once per constant when the enum class loads.

Question 34

Why are enums good singletons?

Accepted Answer

The JVM guarantees one instance per constant, including across serialization and against reflection, with no extra code.

Question 35

What does a record generate automatically?

Accepted Answer

A canonical constructor, private final fields, public accessor methods named after the components (with no get prefix), equals, hashCode and toString.

Question 36

Why use sealed classes?

Accepted Answer

To restrict a hierarchy to known subtypes, so switches and pattern matching can be checked for exhaustiveness and the domain model can't be extended in unexpected ways.

Question 37

What happens if you override equals without hashCode?

Accepted Answer

Hash-based collections break: equal objects can get different hash codes, land in different buckets, and lookups fail or a HashSet stores duplicates.

Question 38

Can two unequal objects have the same hashCode?

Accepted Answer

Yes. That's a collision and it's allowed; it only costs some performance. The reverse, equal objects with different hash codes, is forbidden.

Question 39

How do you make a class immutable?

Accepted Answer

Private final fields set in the constructor, no setters, a final class, defensive copies of mutable fields going in and out, and methods that return new instances for changes.

Question 40

unmodifiableList vs List.copyOf?

Accepted Answer

unmodifiableList wraps the original, so changes to it still show through. List.copyOf creates an independent unmodifiable copy and rejects nulls.

Question 41

Checked vs unchecked exceptions?

Accepted Answer

Checked exceptions extend Exception (but not RuntimeException) and must be caught or declared; they model recoverable conditions like I/O failures. Unchecked exceptions extend RuntimeException, need no declaration, and usually signal bugs.

Question 42

Does finally always run?

Accepted Answer

Almost always: after a return, a break or an exception. It doesn't run if the JVM exits (System.exit), crashes, or the thread is killed.

Question 43

ArrayList vs LinkedList?

Accepted Answer

ArrayList is backed by an array: fast random access and cache friendly, with amortized resizing. LinkedList has O(n) access and more memory per element, and only wins for frequent inserts and removals through an iterator, which is rare in practice.

Question 44

HashSet vs LinkedHashSet vs TreeSet?

Accepted Answer

HashSet: O(1), no order. LinkedHashSet: O(1), keeps insertion order. TreeSet: O(log n), sorted by natural order or a Comparator.

Question 45

How does PriorityQueue order its elements?

Accepted Answer

It's a binary min-heap ordered by natural order or a Comparator. peek is O(1), offer and poll are O(log n), and iteration order is not sorted.

Question 46

Queue vs Deque?

Accepted Answer

A Queue adds at the tail and removes from the head (FIFO). A Deque works at both ends, so it can act as a queue or a stack.

Question 47

Comparable vs Comparator?

Accepted Answer

Comparable defines one natural order inside the class (compareTo). Comparator defines any number of external orders (compare) and can be combined with thenComparing and reversed.

Question 48

What happens if compareTo is inconsistent with equals?

Accepted Answer

Sorted collections such as TreeSet use compareTo to decide equality, so they may drop elements that equals considers different.

Question 49

What happens when two keys have the same hashCode?

Accepted Answer

They go into the same bucket, stored as a linked list, or a tree once there are 8 or more entries in Java 8+. get() walks the bucket and uses equals() to find the matching key.

Question 50

Why is the default load factor 0.75?

Accepted Answer

It balances memory against collisions. A higher value saves space but makes chains longer; a lower one wastes space. 0.75 keeps average chains very short.