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Strings

String Fundamentals

Master string internal representations, immutability vs mutability, ASCII/Unicode encoding, and memory pool allocations.

Last Updated: August 2, 2026 • 20 min read

1. Introduction

What is a String?

A String is a sequence of characters stored in memory. In computer memory, every character is represented by an integer code point according to an encoding standard like ASCII or UTF-8/UTF-16.

Why is it Important?

Strings are the primary format for human-readable data (text, JSON, URLs, user input). Because strings are immutable in languages like Java and Python, modifying a string repeatedly in a loop creates N temporary copies, turning an O(N) task into a slow O(N²) operation.

Where is it Used?

  • Web Browsers & Parsers: Parsing HTML tags, URLs, and JSON payloads.
  • Search Engines: Indexing millions of document text streams.

    • 2. Mental Model

    Imagine a string as an Inscribed Stone Tablet.

    Once carved into stone, you cannot change a single letter in place. To change "Hello" to "Helps", you must carve a brand new stone tablet containing all 5 letters! In languages like Java and Python, strings behave exactly like these stone tablets (immutable). In C++, strings are mutable like whiteboards.

    3. Concept: Immutability, Encodings & Memory

    String Immutability

    In Java and Python, String objects are immutable for security, thread safety, and memory caching (String Constant Pool).

    Character Encoding: ASCII vs Unicode

  • ASCII: 7-bit encoding (128 characters: A-Z, a-z, 0-9, punctuation).
    • - 'A' = 65, 'a' = 97, '0' = 48. - Uppercase to lowercase conversion: 'a' = 'A' + 32.
  • Unicode / UTF-8: Variable length (1 to 4 bytes per character) supporting global alphabets, emojis, and symbols.

    • 4. Visuals

    Memory Allocation: String Constant Pool (Java)

    Character Encoding Summary

    EncodingBits per CharCharacter CountUse Case
    ASCII7 / 8 bits (1 byte)128 (0 - 127)Standard English text
    UTF-88 to 32 bits (1 - 4 bytes)1,114,112Modern Web, Multi-lingual, Emojis
    UTF-1616 or 32 bits (2 or 4 bytes)1,114,112Java / C# internal char storage

    5. Real-World Examples

  • String Caching & Security: Database connection URLs and credentials are stored as immutable strings in Java so they cannot be altered maliciously by un-trusted code modules.
  • High-Performance Parsing: Protocol buffers and JSON parsers convert UTF-8 byte streams directly into character arrays without intermediate string object allocations.

    • 6. Interview Perspective

    How Interviewers Ask This Topic

    Interviewers check whether you know how to convert characters to zero-indexed frequency array slots: int index = c - 'a'; (for lower-case English letters 'a' through 'z').

    Common Mistakes

    Warning: 1. Repeated String Concatenation (+=) in Loops: Performing s += char inside an N-iteration loop creates N intermediate string objects, taking O(N²) time! Use StringBuilder in Java or ''.join(list) in Python.
    > 2. Comparing Strings with == in Java: == checks memory address reference equality, NOT content equality! Always use .equals() in Java.

    7. Summary

  • Strings are Character Arrays under the hood.
  • Immutability: String objects cannot be modified after creation in Java and Python.
  • Concatenation Cost: Repeated += inside loops is an O(N²) antipattern. Use StringBuilder / array join.
  • Character Arithmetic: Indexing letters 'a'..'z' using c - 'a' enables O(1) lookup in a size-26 frequency array.

    • 8. Quiz

    Question 1: What is the ASCII integer code for uppercase 'A' and lowercase 'a'? Answer: 'A' = 65, 'a' = 97 (difference of 32).
    Question 2: What is the time complexity of building a string of length N by executing s += 'a' inside an N-step loop in Python? Answer: O(N²), because each += creates a new copy of the string of length 1, 2, 3 ... N. Total work = \sum_{i=1}^N i = O(N²).
    Question 3: Why does String immutability make multi-threaded programs safer? Answer: Because immutable string instances cannot be modified by any thread, eliminating data races and thread synchronization locks during concurrent access.
    Question 4: How do you map a lowercase character c ('a'-'z') to an integer index between 0 and 25? Answer: int index = c - 'a';
    Question 5: In Java, what is the difference between String a = "test" and String b = new String("test")? Answer: "test" is stored once in the String Constant Pool (shared reference). new String("test") explicitly creates a brand new object on the heap outside the pool.