Java Knowledge

Question 1

Interface vs. Abstract Class

Answer 1

Definition:
An interface is a contract of methods (implicitly abstract), while an abstract class can have both abstract and concrete methods.

• Interfaces in Java 8+ can have default and static methods.
• An abstract class can maintain state (fields) and shared behavior.

Question 2

ArrayList vs. LinkedList

Answer 2

Definition:
ArrayList is backed by a dynamic array; LinkedList is a doubly linked list.

• ArrayList provides faster random access (O(1) for get).
• LinkedList offers faster insertion/deletion at known positions.

Question 3

final, finally, finalize

Answer 3

Definition:
- final: keyword for constants or methods that cannot be overridden.
- finally: a block that always executes after try-catch.
- finalize(): (deprecated) called by GC before object removal.

• finalize() is seldom used; modern Java uses other cleanup strategies.

Question 4

== vs. equals()

Answer 4

Definition:
“==” checks reference equality for objects, while equals() checks logical (content) equality.

• For primitives, == compares values directly.
• For objects, == compares references unless overridden by equals().

Question 5

Java Streams

Answer 5

Definition:
A sequence of elements supporting functional-style operations (map, filter, reduce).

• Can be sequential or parallel.
• Encourages a more declarative approach to iteration.

Question 6

ExecutorService

Answer 6

Definition:
Manages a pool of threads, simplifying concurrent execution of tasks.

• Allows creation of thread pools, scheduling, and concurrency control.
• Must be shut down to release resources.

Question 7

JVM Memory Areas

Answer 7

Definition:
Java divides memory into multiple areas (Heap, Stack, Metaspace, etc.).

• Objects and class-level structures go on the heap/metaspace.
• Local variables and function calls use the stack.

Question 8

Garbage Collection in Java

Answer 8

Definition:
Automatic memory management that frees unused objects.

• Various GC algorithms exist (Serial, Parallel, CMS, G1, ZGC…).
• Tuning GC can greatly impact performance.