✅ 100+ Must-Know Java Concepts for Interviews ☕💡

📍 Java Basics

1.What is Java?
2.JVM, JRE, JDK
3.Data Types & Variables
4.Operators
5.Type Casting
6.Wrapper Classes
7.Autoboxing & Unboxing
8.Enum Types
9.Varargs (...)
10.Final Keyword

📍 Control Flow

11.if–else
12.switch–case
13.for, while, do-while loops
14.break & continue
15.Ternary operator

📍 Object-Oriented Programming (OOP)

16.Class & Object
17.Inheritance
18.Polymorphism (compile-time & runtime)
19.Abstraction
20.Encapsulation
21.Inner & Nested Classes
22.Anonymous Classes
23.Packages & Access Modifiers

📍 Core OOP Concepts

24.Method Overloading
25.Method Overriding
26.Constructors & Constructor Overloading
27.this & super
28.Static Keyword
29.Object Class Methods (equals, hashCode, toString)
30.Creating Immutable Classes

📍 Exception Handling

31.try–catch–finally
32.throw vs throws
33.Checked vs Unchecked Exceptions
34.Custom Exceptions
35.Common Exceptions

📍 Collections Framework

36.List, Set, Map
37.ArrayList vs LinkedList
38.HashSet vs TreeSet
39.HashMap vs TreeMap
40.LinkedHashSet & LinkedHashMap
41.Iterator & Enhanced for-loop
42.PriorityQueue
43.Generics
44.Comparable vs Comparator
45.Collections Utility Class
46.Fail-Fast vs Fail-Safe Iterators
47.ConcurrentHashMap

📍 Strings & Arrays

48.String vs StringBuilder vs StringBuffer
49.Common String Methods
50.1D & 2D Arrays
51.Array vs ArrayList
52.String Immutability

📍 Advanced Java Concepts

53.Interfaces vs Abstract Classes
54.Lambda Expressions
55.Functional Interfaces
56.Streams API
57.Map–Filter–Reduce
58.Optional Class
59.Method References
60.Parallel Streams
61.File Handling
62.Serialization & Deserialization
63.transient Keyword
64.NIO & NIO2
65.Working with Paths & Files

📍 Java Memory & JVM Internals

66.Heap vs Stack
67.Java Memory Model
68.Classloader Mechanism
69.Garbage Collection Basics
70.GC Algorithms
71.Memory Leaks
72.Strong, Weak, Soft & Phantom References

📍 Multithreading & Concurrency

73.Thread vs Runnable
74.Thread Lifecycle
75.Synchronization
76.wait(), notify(), notifyAll()
77.Thread Pools
78.Callable & Future
79.Executor Framework
80.Deadlock, Livelock, Starvation
81.ReentrantLock
82.Volatile Keyword
83.Atomic Classes
84.Concurrent Package Basics

📍 Java 8+ Features

85.LocalDate, LocalTime, LocalDateTime
86.Default & Static Methods in Interfaces
87.Collectors Class
88.var Keyword
89.Records
90.Sealed Classes

📍 JDBC & Databases

91.JDBC Architecture
92.Connection, Statement, PreparedStatement
93.ResultSet Handling
94.SQL Injection Prevention
95.Connection Pooling

📍 Spring & Enterprise Basics

96.Spring Core
97.Spring Boot Basics
98.REST API Basics
99.Dependency Injection
100.Microservices Overview

📍 Best Practices & Tools

101.Writing Clean Code
102.Unit Testing (JUnit)
103.Git & GitHub Basics