Java String Tutorial

A String in Java is an immutable sequence of characters used to represent text. It’s one of the most fundamental classes, powering everything from user input to data processing.

🔑 Key Features:

• Immutable: Once created, its value cannot change (all modifications create new objects).
• String Pool: Java optimizes memory by reusing literals in a shared pool.
• Rich API: Methods like substring(), length(), equals(), and replace() simplify text manipulation.
• Unicode Support: Handles global languages and symbols seamlessly.

Introduction to Strings in Java

• What is a String? (Object vs. primitive)
• Creating Strings: Literals vs. new keyword
• The String Pool (interning) and memory management

String Immutability

• Why Strings are immutable in Java
• Pros/cons of immutability
• Implications for concatenation and performance

Common String Operations

• Key methods: length(), charAt(), substring(), indexOf(), contains(), etc.
• Concatenation: + operator vs. concat()
• Case conversion: toUpperCase(), toLowerCase()
• Trimming and whitespace: trim(), strip() (Java 11+)

String Comparison

• equals() vs. ==
• equalsIgnoreCase()
• compareTo() and lexicographical order

String Formatting

• String.format() and System.out.printf()
• Format specifiers: %s, %d, %f, etc.
• Formatting dates/numbers with Formatter

String Performance & Optimization

• Avoiding costly operations in loops
• StringBuilder vs. StringBuffer (mutable alternatives)
• Pre-sized StringBuilder and capacity management

Manipulation & Transformation

• Replacing text: replace(), replaceAll(), replaceFirst()
• Splitting strings: split() and regex
• Joining strings: String.join() (Java 8+)
• Converting to/from char[], byte[], and other types

Regular Expressions (Regex)

• Pattern and Matcher classes
• Common regex patterns (email, phone numbers)
• Methods: matches(), split(), replaceAll()

Special Characters & Escaping

• Escape sequences: \n, \t, \", etc.
• Unicode characters: \uXXXX
• Text blocks (Java 15+): Multi-line strings with """

Security Considerations

• Why char[] is safer for passwords than String
• Avoiding SQL injection with parameterized queries
• Validating and sanitizing user input

Java 8+ String Enhancements

• String.join(), chars(), codePoints()
• Java 11 methods: isBlank(), lines(), repeat(), stripLeading(), stripTrailing()

Internationalization (i18n)

• Locale-specific formatting
• Resource bundles for multilingual apps
• Collation and sorting localized text

Advanced Topics

• Compact Strings (Java 9+ memory optimization)
• String interning with intern()
• Custom string-backed data structures

Best Practices & Pitfalls

• When to use StringBuilder
• Avoiding NullPointerException with Objects.requireNonNull()
• Logging sensitive data safely
• Common mistakes (e.g., == comparisons, inefficient loops)