What is String in Java - Java String Methods & Type (Examples)

A String in Java is an object that represents a sequence of characters, and it is one of the most frequently used classes in the entire language. Unlike primitive types, String is a class defined in java.lang, which means every string comes with a rich set of built-in methods for searching, comparing, and transforming text.

Why Strings Are Immutable

Once a String object is created in Java, its value can never be changed. Any operation that appears to "modify" a string, such as concatenation, actually creates a brand-new String object in memory. This immutability makes strings safe to share across threads and enables Java's String Pool, which reuses identical string literals to save memory.

Commonly Used String Methods

• length() — returns the number of characters in the string
• charAt(index) — returns the character at a specific position
• substring(start, end) — extracts a portion of the string
• equals() and equalsIgnoreCase() — compare string content
• toUpperCase() / toLowerCase() — changes letter casing
• trim() — removes leading and trailing whitespace
• split(regex) — breaks a string into an array based on a delimiter
• replace() — swaps characters or substrings with new ones

String vs StringBuilder vs StringBuffer

Because String objects are immutable, repeatedly modifying strings in a loop (like building a long piece of text) creates unnecessary objects and hurts performance. StringBuilder solves this by being mutable and unsynchronized, making it fast for single-threaded use. StringBuffer offers the same mutability but is synchronized, making it thread-safe at a small performance cost.