1. #1

   Juniorfundamentalsjvm

   What is the difference between the JVM, JRE, and JDK?

   The JVM runs bytecode, the JRE bundles the JVM plus core libraries to run apps, and the JDK adds compilers and tools to develop them.

2. #2

   Juniortypesfundamentals

   What is the difference between primitive types and wrapper objects in Java?

   Primitives (int, double) store raw values on the stack; wrappers (Integer, Double) are objects enabling use in collections and nullability, with autoboxing between them.

3. #3

   Mid-levelobjectscollections

   Why must you override equals() and hashCode() together?

   Hash-based collections rely on the contract that equal objects have equal hash codes; overriding one without the other breaks HashMap and HashSet behaviour.

4. #4

   Mid-leveloopdesign

   What is the difference between an interface and an abstract class?

   An interface defines a contract a class can implement (multiple allowed); an abstract class provides partial implementation and state via single inheritance.

5. #5

   Mid-levelcollectionsdata-structures

   What are the main Java collection types and when do you use each?

   List for ordered sequences (ArrayList/LinkedList), Set for uniqueness (HashSet/TreeSet), and Map for key-value lookups (HashMap/TreeMap).

6. #6

   Mid-levelexceptionserror-handling

   What is the difference between checked and unchecked exceptions?

   Checked exceptions must be declared or caught at compile time (recoverable conditions); unchecked (RuntimeException) signal programming bugs and need no declaration.

7. #7

   Mid-levelstringsmemory

   Why are Strings immutable in Java, and what is the String pool?

   Immutability makes Strings safe to share and cache; the String pool interns literals so identical strings reuse one instance, saving memory.

8. #8

   Seniormemorygc

   How does garbage collection work in the JVM?

   The JVM automatically reclaims unreachable objects using generational, mostly mark-and-sweep collectors (G1, ZGC) that minimise pause times.

9. #9

   Seniorconcurrencythreads

   What tools does Java provide for safe concurrency?

   synchronized and volatile for low-level control, plus java.util.concurrent — ExecutorService, concurrent collections, locks, and atomics — for higher-level safety.

10. #10

    Seniorfunctionalstreams

    What is the Java Stream API and how does it differ from collections?

    Streams express functional pipelines (map, filter, reduce) over data; they are lazy, single-use, and don’t store elements, unlike collections.