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Java Interview Production Ready

Table of Contents

Java Basics

⚡ Difference between JDK, JRE, and JVM

  • JVM runs bytecode, JRE provides JVM + libraries, JDK provides JRE + dev tools.

⚡ Difference between == and .equals()

  • == compares references, .equals() compares object content.

Primitive data types

  • byte, short, int, long, float, double, char, boolean.

⚡ Difference between final, finally, and finalize()

  • final: constants or prevent inheritance/overriding.
  • finally: block executed after try-catch.
  • finalize(): called before GC destroys an object.

⚡ String vs StringBuilder vs StringBuffer

  • String: immutable.
  • StringBuilder: mutable, fast, not thread-safe.
  • StringBuffer: mutable, thread-safe, slower.

⚡ String immutability reason

  • Security, thread-safety, caching in String pool.

How concatenation works with Strings

  • Compiler uses StringBuilder internally for + operations.
OOP Concepts

⚡ Polymorphism

  • One object can take many forms; includes overloading (compile-time) and overriding (runtime).

Abstract class vs Interface

  • Abstract class: methods with/without implementation, constructors, fields.
  • Interface: method declarations (Java 8+ allows default/static methods), supports multiple inheritance.

Encapsulation

  • Wrapping data and methods in a class; control access with getters/setters.

Inheritance

  • Mechanism to derive a class from another to reuse code.

⚡ Method Overloading vs Overriding

  • Overloading: same name, different params, compile-time.
  • Overriding: subclass changes parent method behavior, runtime.
Collections

List vs Set vs Map

  • List: ordered, allows duplicates.
  • Set: unordered, no duplicates.
  • Map: key-value, unique keys.

⚡ HashMap vs Hashtable vs ConcurrentHashMap

  • HashMap: not synchronized, allows null keys/values.
  • Hashtable: synchronized, no null keys/values.
  • ConcurrentHashMap: thread-safe, efficient concurrency.

ArrayList vs LinkedList

  • ArrayList: index-based, fast random access.
  • LinkedList: node-based, fast insert/remove.

Fail-fast vs Fail-safe iterators

  • Fail-fast: throws ConcurrentModificationException (ArrayList).
  • Fail-safe: works on clone, no exception (ConcurrentHashMap).
Exception Handling

Checked vs Unchecked Exceptions

  • Checked: compiler-enforced (IOException).
  • Unchecked: runtime errors (NullPointerException).

Custom exceptions

  • Extend Exception for checked, RuntimeException for unchecked.

try-with-resources

  • Auto-closes resources like files or streams.

throw vs throws

  • throw: explicitly throw exception.
  • throws: declare exceptions a method may throw.
Multithreading & Concurrency

⚡ synchronized vs volatile

  • synchronized: mutual exclusion + visibility.
  • volatile: visibility only, no locking.

ExecutorService

  • Manages thread pool and task submission efficiently.

Runnable vs Callable

  • Runnable: no return, cannot throw checked exceptions.
  • Callable: returns result, can throw checked exceptions.

Avoid deadlocks

  • Acquire locks in consistent order, minimize scope, use tryLock.

ThreadLocal

  • Thread-local variable; each thread has its own copy.
Streams & Lambda Expressions

⚡ Stream

  • Functional processing of collections; supports map, filter, reduce.

map vs flatMap

  • map: one-to-one mapping.
  • flatMap: flattens nested collections.

Optional

  • Container that may/may not hold a value; avoids NullPointerException.

Lambda syntax

(int a, int b) -> a + b
Java 8/11 Features Default vs static methods

  • Default: method with implementation in interface.

  • Static: belongs to interface, not instance.

Functional interface

  • Interface with single abstract method (Runnable, Callable, Comparator).

LocalDate vs Date

  • LocalDate: immutable, thread-safe.

  • Date: mutable, older API.

parallelStream

  • Allows multi-core processing automatically.
JVM & Memory Management JVM memory areas
  • Heap, Stack, Metaspace, Code Cache, PC Registers.

Stack vs Heap

  • Stack: method calls, primitives, short-lived.

  • Heap: objects, garbage collected.

ClassLoader

  • Loads classes into JVM at runtime (Bootstrap, Extension, Application).
Garbage Collection How GC works
  • Frees memory of unreachable objects automatically.

GC algorithms

  • Serial, Parallel, CMS, G1.

Forcing GC

  • System.gc() requests GC (not guaranteed immediately).

⚡ Memory leaks

  • Objects unintentionally referenced prevent GC.
JDBC Connecting to DB
  • DriverManager.getConnection(url, user, pass) or DataSource.

Statement vs PreparedStatement vs CallableStatement

  • Statement: simple SQL, prone to injection.

  • PreparedStatement: precompiled, safer.

  • CallableStatement: calls stored procedures.

Transaction management

  • Use commit() and rollback() for atomicity.
Production-Level Questions ⚡ Detect memory leaks
  • Use heap dump analysis with VisualVM/Eclipse MAT.

Performance improvement with collections

  • Choose proper data structures, avoid unnecessary synchronization.

Concurrency issues

  • Use Concurrent collections, locks, atomic operations.

GC tuning

  • Pick GC type based on workload (G1: low-latency, Parallel: throughput).

⚡ Debug slow apps

  • Profile, analyze thread dumps, GC logs, monitor performance.
Best Practices

  • Prefer immutability (final) for thread safety.

  • Use StringBuilder for heavy string ops.

  • Always close resources (try-with-resources).

  • Avoid unnecessary object creation in loops.

  • Use Concurrent collections for multi-threaded apps.

  • Handle exceptions gracefully.

  • Write unit tests.

  • Follow proper naming conventions and document code.

Java Commands / Tools Cheat Sheet

  • javac FileName.java → Compile Java file.

  • java ClassName → Run compiled class.

  • jar cf app.jar *.class → Create JAR file.

  • jar xf app.jar → Extract JAR.

  • javadoc FileName.java → Generate documentation.

  • jdb ClassName → Debug Java app.

  • java -Xmx512m ClassName → Set max heap size.

  • java -version → Check Java version.