String & String Pool

String is immutable for security (credentials/URLs cannot be tampered), String Pool sharing (one object for identical literals), thread safety (no locks needed), and HashMap key stability.

The String Pool is a heap area where identical String literals are stored once and shared. Literals are auto-pooled. new String() bypasses the pool. intern() adds manually.

intern() adds a String to the pool and returns the pooled reference. After interning, == works because both references point to the same object. But always use .equals() in practice.

Simple + expressions are optimized by the compiler (Java 9+ invokedynamic). But += in loops creates a new String every iteration – use StringBuilder for loops.

String is immutable (new object per change). StringBuilder is mutable and fast. StringBuffer is mutable and thread-safe (synchronized). Use StringBuilder by default.

StringBuffer's methods are synchronized. Only one thread can modify it at a time. StringBuilder is identical but unsynchronized and faster. Use StringBuilder by default.

String creates a new object for every change. StringBuilder/Buffer modifies a resizable internal array in place, avoiding repeated allocation.

Almost never. Use StringBuilder by default. StringBuffer is only needed when multiple threads share the same builder instance, which is rare and usually a design smell.

String is immutable because the class is final, the internal array is private final, and no method modifies it. Every modification method returns a new String.

Strings can be created from literals (pooled), new String() (heap), valueOf(), StringBuilder.toString(), char arrays, String.format(), and text blocks.

Essential String methods: length, charAt, substring, equals, contains, trim/strip, replace, split, toUpperCase, isEmpty/isBlank. All return new Strings (immutable).

split() takes a regex, not a literal. "." matches anything. Trailing empties are removed. Use Pattern.quote() for literal delimiters.