☕ 30 Days Java Interview Preparation Challenge

Welcome to the 30 Days Java Interview Preparation Challenge!
This plan is designed to take you from Java basics refresher to interview-ready in one month, combining Core Java, Data Structures & Algorithms (DSA), problem solving, and mock interview practice.

📅 Challenge Structure

The challenge is divided into 4 weeks, each focusing on a key area:

1. Week 1 – Core Java Fundamentals
2. Week 2 – Collections & Java 8+ Features
3. Week 3 – Data Structures & Algorithms
4. Week 4 – Advanced Java & Mock Interviews

📌 Daily Breakdown

Week 1 – Core Java Fundamentals

Day	Topics	Practice	Code
1	OOP principles, Access Modifiers	Implement a simple Employee class with encapsulation & inheritance	Day_01.java
2	static, final, this, super, method overloading/overriding	Create a parent-child class demo	Day_02.java
3	Interfaces, Abstract classes, Functional interfaces (Java 8)	Implement a lambda expression for sorting	Day_03.java
4	Strings & StringBuilder vs StringBuffer	Reverse words in a sentence	Day_04.java
5	Arrays (1D, 2D) and Utility methods	Rotate an array by K steps	Day_05.java
6	Exception Handling, Custom Exceptions	Create custom exception for invalid age	Day_06.java
7	Java Collections Intro (List, Set, Map)	Count word frequency using HashMap	Day_07.java

Week 2 – Collections & Java 8+ Features

Day	Topics	Practice	Code
8	ArrayList vs LinkedList	Implement custom LinkedList	Day_08.java
9	HashSet, TreeSet, LinkedHashSet	Remove duplicates from a list	Day_09.java
10	HashMap, TreeMap, LinkedHashMap	Sort a Map by values	Day_10.java
11	Iterators, forEach, Streams	Filter even numbers from a list	Day_11.java
12	Stream API – map, filter, reduce	Find sum of salaries using reduce	Day_12.java
13	Optional class, Method references	Avoid NullPointerException using Optional	Day_13.java
14	Comparable vs Comparator	Sort custom objects by multiple fields	Day_14.java

Week 3 – Data Structures & Algorithms

Day	Topics	Practice	Code
15	Recursion basics	Factorial, Fibonacci	Day_15.java
16	Sorting algorithms	QuickSort, MergeSort	Day_16.java
17	Searching algorithms	Binary Search variations	Day_17.java
18	Linked List	Reverse, Detect cycle	Day_18.java
19	Stack & Queue	Balanced parentheses checker	Day_19.java
20	Trees (BST)	Inorder, Preorder, Postorder traversal	Day_20.java
21	Graphs (BFS, DFS)	Count connected components	Day_21.java

Week 4 – Advanced Java & Mock Interviews

Day	Topics	Practice
22	Multithreading, Synchronization	Producer-Consumer problem
23	Executors, Callable, Future	Run tasks in parallel
24	JVM Internals, Garbage Collection	Write code to simulate memory leaks
25	Design Patterns (Singleton, Factory)	Implement thread-safe Singleton
26	LRU Cache implementation	Using LinkedHashMap
27	Real-world coding problems	Merge intervals, Longest Substring
28	System design basics	Library management system
29	Full mock interview (Core Java + DSA)	Solve 3 coding + 5 theory Qs
30	Review & Weak areas	Revise mistakes & reattempt hard problems

🛠 How to Use This Plan

• Daily Practice: Spend 1-2 hours revising the topic and 1-2 hours coding problems.
• Track Progress: Check off each day when completed.
• Solve Problems: Use LeetCode, HackerRank, or GeeksforGeeks.
• Revision: Keep a running note of mistakes and patterns.
• Mock Interviews: Simulate real interview scenarios in Week 4.

📚 Recommended Resources

• Books:
  • Effective Java – Joshua Bloch
  • Java: The Complete Reference – Herbert Schildt
• Practice Platforms:
  • LeetCode
  • HackerRank
  • GeeksforGeeks

✅ Goal

By the end of these 30 days, you will:

• Have a solid grasp of Core Java concepts
• Be comfortable with DSA in Java
• Be ready for technical interviews at top companies

Happy Coding! 🚀