DemoProgramsJava

A curated collection of Core Java programs and practical examples designed to strengthen Java fundamentals, improve problem-solving skills, and support technical interview preparation.

This repository demonstrates real-world Java concepts including:

• Collections Framework
• Java 8 Stream API
• Multithreading
• Functional Interfaces
• Comparator & Comparable
• Generics
• Reflection API
• Recursion
• Data Structures
• Common coding patterns

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Why This Repository?

Many developers understand Java syntax but struggle with applying concepts in practical scenarios.

This repository helps by providing:

✅ Small focused examples
✅ Interview-oriented coding problems
✅ Clean Java implementations
✅ Topic-wise organization
✅ Easy-to-understand logic

It can be used as a:

• Java revision guide
• Interview preparation resource
• Learning reference
• Mentoring support repository

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Project Structure

DemoProgramsJava/
│── collections/
│── streams/
│── multithreading/
│── recursion/
│── generics/
│── comparator/
│── reflection/
│── datastructures/
└── README.md

In this repository all Core Java related programs are added, which are mostly tricky one.
Topics: Core java, Collection, Multi threading, Data Structure, Generics, Java8 features, Streams, Functional Interfaces, Reflection, Recursion, Comparator related interview programs. Here all related codes are places in respective packages listed below.

Topics Covered per package

• API call
  • Rest API consumer
  • Whitelist api endpoints
• ci (Coding Interview)
  • Maximum Profit
  • Roman to Integer
  • Shift to Zeros to left or right in array
• Combination
  • Array of Combination (Number)
  • String Combination
• Comparator
  • Comparator Demo
  • Comparator on Multiple Fields (Java 7)
  • Comparator on Multiple Fields (Java 8)
  • HashMap Sorting
  • TreeMap reverse operations
• Conversions
• Copy
  • Deep Copy
  • Deep Copy using CLone method
  • Shallow Copy
• Data Structure
  • Searching
    • BFS (Breath First Search) Graph
    • BFS (Breath First Search) Shortest Path Graph
    • DFS (Depth First Search) Graph
  • Sorting
    • Bubble Sort
    • Insertion Sort
    • Selection Sort
  • Binary Tree Depth
  • Detect Loop in Linked List
  • Detect and Remove Loop in Linked List
  • LRU (Last Recently Used) Cache
  • Find Value in Binary Tree
  • Traverse through Weighted Graph
• Elevator program
• File
  • Remove matching files in java8
  • Rename files from directory in java8
• Finally Block
• Generics
  • Lower Bounded Wildcards
  • Unbounded Wildcards
  • Upper Bounded Wildcards
• Hash and Equals
• Image Compression
• Immutable
• Interface - Functional Interface
• Java 8 Features
  • Design pattern using java8
    • Builder Design pattern using java8
  • Advanced Optional Demo
  • Advanced Stream operations on Department
  • Functional Interfaces
  • BiConsumer
  • BiFunction
  • BiPredicate
  • Collectors utility class
  • Comparator
  • Consumer
  • Custom Collector class
  • Custom Functional Interface with default & static methods
  • Functions
  • Map Reduce
  • Method Reference
  • Optionals
  • Predicates
  • Sequential and Parallel Streams
  • Streams
  • Suppliers
• Leet code
  • Array operations
    • Average salary excluding min and max salary
    • Find duplicate elements in array
    • Find the pair of K difference
    • Find missing element from array
    • Jump by value
    • Merge 2 sorted arrays
    • Move all Zeros at end
    • Number of sub arrays multiplication less than K
    • Number of Sequential digits between given range of numbers
    • Number of paise which sum of pairs equals to K
  • Number operations
    • Number Palindrome
    • X power of N
  • String operations
    • Anagram string
    • Reverse string
• List Operations
  • Find Missing Elements from Duplicate List
  • Remove Duplicate Elements from List
• Map Operations
  • Count Chars and Print in Order as per Count
  • Iterate Maps using Java 7 and Java 8
  • Max Repetitive Letter from String
• Method Calls
  • Method Calls on Overloaded Classes with Extending
  • Integer wrapper and primitive method calls
  • Static method calls by Extending
• Numbers Operations
  • N Prime Numbers
  • Sum of Numbers
• Overriding
  • Overriding methods
  • Overriding Rules while Extending Classes
• Palindrome
  • Array of Numbers Palindrome
  • String Palindrome with Multiple Techniques
• Pattern programs
  • Christmas Tree using Stars (*)
• Permutation
  • Permutation using Recursion and Swapping
  • Permutation using Substring
• Recursion
  • Biggest in Array
  • Factorial
  • Fibonacci Series
  • Reverse String
• Reflection
  • Get Class's Public and Private Fields
  • Get Class Information
  • Get Class's Public and Private Methods
• Runtime Package
  • Garbage Collection
  • Log Monitor
  • Run Commands using Java
• Singleton Prevention
  • Restrict Singleton from Cloning
  • Restrict Singleton from Reflection
  • Restrict Singleton from Serialization
• Store SQL
  • Run SQL Script on Oracle DB
• Multi-threading
  • Blocking Queue
  • Completable Future
  • Countdown Latch
  • Cyclic Barrier
  • Hello World Printing(100 times) from 2 Different Threads
  • Java Atomic Integer
  • Join in Thread
  • Hello World Printing from 2 Different Threads
  • Producer and Consumer using Synchronized Blocks
  • Producer and Consumer using Locks
  • Return/Get Value from Thread
  • Return/Get Value from Thread using Callable Interface
  • Sum of Numbers Range using Executor Framework
  • Synchronized method test
  • Thread Groups
  • Wait and Notify

More topics coming soon.