String Handling

Last updated    - V7 (14-Dec-2025)

Key Concepts

1	Introduction to Strings	Definition, String vs character array, Memory concept, Immutable concept, String literal vs object
2	String Creation	Literal, new keyword, Intern pool, String constants, equals() behavior
3	Basic Operations	Concatenation, length(), substring(), charAt(), indexOf()
4	Equality & Comparison	equals(), == operator, compareTo(), equalsIgnoreCase(), best practices
5	String Pool	String intern(), memory saving, literal reuse, JVM mechanism, debugging pool
6	StringBuilder	Mutable concept, append(), insert(), reverse(), capacity()
7	StringBuffer	Thread safety, synchronization, performance, use case, compare with StringBuilder
8	Conversion	toCharArray(), getBytes(), valueOf(), parse methods, format() output
9	Useful Methods	trim(), replace(), split(), startsWith(), endsWith()
10	Regex Basics	Pattern, Matcher, meta characters, match(), replaceAll()
11	Advanced Regex	Groups, quantifiers, lookahead, lookbehind, capturing
12	Tokenization	StringTokenizer, Scanner use, split vs tokenizer, stream tokenization, parsing
13	String Formatting	printf(), format(), placeholders, locales, formatting numbers
14	Case Handling	toLowerCase(), toUpperCase(), locale support, Unicode cases, normalization
15	Substring & Indexing	Extract substring, boundaries, index operations, find substring, custom search
16	Searching Algorithms	Brute force search, KMP logic, pattern matching, rolling hash basics, use cases
17	Performance Optimization	Cost of concatenation, StringBuilder usage, profiling, memory leaks, repeated operations
18	Mutable vs Immutable	Deep dive, advantages, thread safety, identity hash code, security
19	Memory Management	Heap allocation, String pool garbage collection, intern optimization, examples
20	Unicode & Encoding	UTF-8, UTF-16, char vs byte, surrogate pairs, emoji handling
21	Character Handling	Character class, codePoint(), isDigit(), isLetter(), conversion
22	File & IO Strings	Reading text, StringBuilder for building, buffer read, split lines, writing
23	Stream API with Strings	map(), filter(), joining(), flatMap(), collectors
24	Security Aspects	Password handling, String vs char[], secure erase, log sanitization, risks
25	Localization	Locale, Collator, Comparator, region-based formatting, internationalization
26	Complex Parsing	JSON parsing basics, CSV parsing, escaping quotes, splitting rules, custom delimiters
27	Design Patterns	String pool pattern, builder pattern, flyweight pattern, factory use, internalization
28	Testing Strings	JUnit tests, assertions, regex tests, corner cases, mock inputs
29	Enterprise Use Cases	Logging, request handling, templates, localization, user input validation
30	Interview Prep	Top 20 questions, coding challenges, regex tasks, performance traps, custom implementation

Interview question

Basic Level

1. What is a String in Java?
2. How do you create a String object?
3. Difference between String and char[].
4. Why are Strings immutable in Java?
5. What is the String constant pool?
6. Difference between == and equals() in String comparison.
7. What does length() method return in a String?
8. How do you find a character at a specific index in a String?
9. Difference between charAt() and substring().
10. How do you check if a String is empty?
11. How do you concatenate two Strings?
12. What is the difference between + operator and concat() method?
13. How do you convert a String to lowercase or uppercase?
14. How do you trim whitespace from a String?
15. How do you compare two Strings ignoring case?
16. How do you find the index of a character in a String?
17. What does lastIndexOf() method do?
18. How do you check if a String contains another String?
19. What is startsWith() and endsWith() used for?
20. How do you replace characters in a String?
21. Difference between replace() and replaceAll().
22. How do you split a String by a delimiter?
23. How do you join Strings in Java 8?
24. How do you convert a String to a char[]?
25. How do you convert a char[] back to a String?

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Intermediate Level

1. What is String interning in Java?
2. How does the intern() method work?
3. What is the difference between heap Strings and pool Strings?
4. How do you check if a String is a palindrome?
5. How do you reverse a String in Java?
6. What happens when you use + operator inside a loop with Strings?
7. Why is StringBuilder faster than String for concatenation?
8. Difference between StringBuffer and StringBuilder.
9. What is the substring() method used for?
10. How does substring() implementation differ in Java 6 vs Java 7?
11. How do you count the number of words in a String?
12. How do you count the number of occurrences of a character?
13. How do you extract only digits from a String?
14. How do you check if a String contains only alphabets?
15. How do you validate an email using Regex with Strings?
16. Difference between matches() and contains().
17. How do you remove special characters from a String?
18. How do you check if a String is numeric?
19. How do you find all substrings of a String?
20. How do you find duplicate characters in a String?
21. How do you sort the characters in a String?
22. How do you remove duplicate characters from a String?
23. How do you convert String to int and double?
24. How do you convert numbers to Strings?
25. How do you use String.format()?

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Advanced Level

1. Explain memory allocation of Strings in Java.
2. How does garbage collection work with String literals?
3. Why are Strings immutable from a security perspective?
4. Why is char[] better than String for storing passwords?
5. How do you compare two Strings lexicographically?
6. How do you handle Unicode in Java Strings?
7. How do you normalize Unicode Strings?
8. How does encoding affect Strings in Java?
9. How do you convert a String to bytes?
10. How do you convert bytes back to a String?
11. How do you handle special characters in JSON Strings?
12. Explain the role of CharSequence interface.
13. What is StringJoiner introduced in Java 8?
14. How does StringJoiner work internally?
15. How do you join multiple Strings using Streams?
16. How do you split a multiline String into lines?
17. How do you check if a String is an anagram?
18. How do you implement case-insensitive search?
19. How do you replace multiple substrings at once?
20. How do you tokenize a String in Java?
21. What are Pattern and Matcher classes used for?
22. How does replaceAll() use Regex internally?
23. How do you extract a substring using Regex?
24. How do you efficiently join large Strings?
25. How do you check if two Strings are rotations of each other?

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Expert Level

1. How does JVM optimize String usage internally?
2. How does String constant pool improve performance?
3. What are the performance implications of String immutability?
4. Explain lazy loading of Strings.
5. How does String concatenation compile into StringBuilder?
6. How do Streams improve String processing in Java 8+?
7. What are best practices for handling Strings in enterprise apps?
8. How do you handle memory leaks with Strings?
9. How does String interning affect garbage collection?
10. How do you measure performance of String operations?
11. What is the impact of substring() holding references to large arrays?
12. How was this fixed in Java 7 update?
13. How do you create a custom immutable String-like class?
14. How do you secure sensitive data stored in Strings?
15. What are pitfalls of using == in String comparison?
16. How do you handle locale-sensitive String operations?
17. What is collation in String comparison?
18. How do you optimize String-heavy applications?
19. How does StringBuilder grow internally?
20. What is the difference in thread safety between StringBuffer and StringBuilder?
21. How does String deduplication work in JVM?
22. How do you debug encoding issues with Strings?
23. How do you use Charset class with Strings?
24. How do you build efficient String utilities?
25. What is the future of String handling in modern Java versions?

Related Topics

StringBuilder

Java String - Sample Coding