Top 50 Java Interview Questions (2026)
Ranked by how often they come up. Every question has a complete worked answer, runnable code and follow-ups — free to read.
50 questions17 Medium7 Easy14 Hard12 Tricky
- 1
String vs StringBuilder vs StringBuffer in Java
Medium - 2
Explain JVM architecture.
Medium - 3
Difference between JDK, JRE, and JVM.
Easy - 4
Explain the Class Loading Mechanism.
Hard - 5
What are Bootstrap, Platform, and Application ClassLoaders?
Hard - 6
What happens when a Java program starts?
Medium - 7
Explain Heap, Stack, Metaspace, PC Register, and Native Method Stack.
Medium - 8
How is an object created in Java?
Medium - 9
What is StackOverflowError?
Easy - 10
What is OutOfMemoryError?
Easy - 11
How do you identify a memory leak?
Hard - 12
Explain Garbage Collection.
Medium - 13
How does G1 GC work?
Hard - 14
Serial GC vs Parallel GC vs CMS vs G1.
Hard - 15
Young Generation vs Old Generation.
Medium - 16
Minor GC vs Major GC vs Full GC.
Tricky - 17
Explain Encapsulation with a real-world example.
Easy - 18
Explain Inheritance.
Easy - 19
Explain Polymorphism.
Medium - 20
Explain Abstraction.
Easy - 21
Interface vs Abstract Class.
Medium - 22
Why is String immutable?
Medium - 23
Explain String Pool.
Medium - 24
String vs StringBuilder vs StringBuffer.
Medium - 25
How does intern() work?
Hard - 26
How is String concatenation optimized?
Tricky - 27
Explain Collection Framework.
Easy - 28
List vs Set vs Queue.
Medium - 29
ArrayList vs LinkedList.
Medium - 30
HashSet vs TreeSet.
Medium - 31
HashMap internal working.
Hard - 32
Explain map.put() internal flow.
Hard - 33
Hash Collision in HashMap.
Hard - 34
Why Red-Black Tree in Java 8 HashMap?
Hard - 35
Load Factor and Initial Capacity.
Medium - 36
ConcurrentHashMap internals.
Hard - 37
Hashtable vs ConcurrentHashMap.
Tricky - 38
TreeMap internal working.
Hard - 39
PriorityQueue internal implementation.
Hard - 40
Comparable vs Comparator.
Tricky - 41
CopyOnWriteArrayList.
Hard - 42
Difference between == and equals().
Tricky - 43
Contract between equals() and hashCode().
Tricky - 44
What happens if hashCode() is not overridden?
Tricky - 45
Clone() vs Copy Constructor.
Tricky - 46
What methods are available in Object class?
Tricky - 47
Checked vs Unchecked Exception.
Tricky - 48
throw vs throws.
Tricky - 49
finally block behavior.
Tricky - 50
Custom Exception.
Medium
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