The Patterns Behind Everything ⭐

Master these five patterns and you'll solve most Java interview programs efficiently.

Pattern Code Pattern Common Problems Solved
Digit Extraction digit = n % 10; n = n / 10; Reverse number, palindrome, Armstrong, digit sum
Build Reverse reverse = reverse * 10 + digit; Reverse number, palindrome
Two Pointers start++; end--; Reverse array, reverse string, palindrome
HashSet Deduplication if (!set.add(value)) Duplicate detection, first duplicate, pair sum
HashMap Frequency Counting map.put(key, map.getOrDefault(key,0)+1) Frequency count, first non-repeated element, anagram

Array Programs (~50)

Basics & Calculations

Learn more: Array Basics & Calculations

Program Approach Time Complexity
Declare & Initialize Array int[] array = {...} —
Print Array Elements Loop or Arrays.toString() O(n)
Sum of Elements Accumulate values O(n)
Average Sum then divide using double O(n)
Largest / Smallest Element Single pass tracking maximum and minimum O(n)
Second Largest ⭐ Track largest and second largest simultaneously O(n)
Second Smallest Track smallest and second smallest O(n)
Maximum Difference Single traversal maintaining max and min O(n)

Sorting, Reverse & Duplicates

Learn more: Array Sorting, Reverse & Duplicates

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Program Approach Time Complexity
Sort Ascending Bubble Sort O(n²)
Sort Descending Reverse comparison O(n²)
Reverse Array Two-pointer swapping O(n)
Duplicate Detection (Brute Force) Nested loops O(n²)
Duplicate Detection (Optimal) ⭐ HashSet O(n)
Remove Duplicates ⭐ LinkedHashSet O(n)
Count Frequency visited[] or HashMap O(n)
Unique Elements Frequency equals one O(n)
First Duplicate First failed HashSet.add() O(n)

Rotation, Search & Patterns

Learn more: Array Rotation & Patterns

Program Approach Time Complexity
Left Rotation Shift left and append first element O(n)
Right Rotation Shift right and prepend last element O(n)
Missing Number ⭐ Expected sum − actual sum O(n)
Pair with Given Sum ⭐ HashSet complement lookup O(n)
Majority Element Moore's Voting Algorithm O(n)
Move Zeros to End Write-index approach O(n)
Count Even / Odd % 2 check O(n)
Count Positive / Negative Sign comparison O(n)
Merge Arrays Copy into new array O(n+m)
Common Elements HashSet lookup O(n)
Compare Arrays Arrays.equals() or loop O(n)
Array ↔ ArrayList Conversion add() / toArray() O(n)

Number Programs (~23)

Basics & Intermediate

Learn more: Number Basics

Program Recommended Approach
Even / Odd % 2 == 0
Positive / Negative Sign comparison
Swap Two Numbers Temporary variable or arithmetic swap
Reverse Number ⭐ Digit extraction + reverse construction
Count Digits Division by 10 or String.length()
Factorial Multiply values from 1 to n
Power Loop or Math.pow()
Leap Year Standard leap-year formula
Prime Number ⭐ Divisibility check up to √n
Palindrome Number ⭐ Reverse and compare
Armstrong Number Sum of digit cubes
Perfect Number Sum of proper divisors
Strong Number Sum of digit factorials
Fibonacci Series ⭐ Two running variables

Advanced & Interview-Oriented

Learn more: Advanced Number Programs

Program Recommended Approach
GCD / LCM ⭐ Euclidean Algorithm
Sum of Digits Digit extraction
Smallest / Largest Digit Track while scanning
Reverse Using Recursion Recursive digit processing
Print Numbers Recursively Recursive traversal
Decimal → Binary Divide by 2 repeatedly
Binary → Decimal Multiply accumulated result by 2
Neon Number Sum digits of square

String Programs (~23)

Basics

Learn more: String Basics

Program Recommended Approach
Reverse String ⭐ StringBuilder.reverse()
Palindrome Check ⭐ Two pointers or reverse comparison
Compare Strings .equals() / .equalsIgnoreCase()
Count Vowels & Consonants Character classification
Count Characters .length() or loop
Duplicate Characters ⭐ HashMap frequency
Remove Duplicate Characters ⭐ LinkedHashSet<Character>
Non-Repeated Characters Frequency count
First Non-Repeated Character ⭐ LinkedHashMap
Swap Characters Character array manipulation
Digits / Alphabets Only Character.isDigit() / isLetter()

Advanced (Word-Level)

Learn more: Advanced String Programs

Program Recommended Approach
Count Words split("\\s+")
Reverse Each Word ⭐ Split + StringBuilder.reverse()
Reverse Word Order Reverse words array
Longest / Shortest Word Track maximum and minimum length
Word Frequency HashMap counting
Anagram Check ⭐ Sort characters or compare frequencies
Sort Words Alphabetically Arrays.sort()
Character Frequency HashMap.getOrDefault()

⭐ = Most Frequently Asked Java Coding Programs

Coding Round Tips

  • Practice writing programs from memory without relying on an IDE.
  • Explain both the brute-force and optimized approaches whenever applicable.
  • Perform a dry run using sample input before finalizing the solution.
  • Always mention time and space complexity.
  • Watch for common mistakes such as integer division, using == instead of .equals() for strings, and inefficient string concatenation inside loops.

Go Deeper

Continue learning with:

  • Complete Java Coding Programs Guide
  • Top 30 SDET Coding Questions
  • Complete Java for Testers Guide
  • List vs Set vs Map