Why is String immutable, and what does string pooling change?

Immutability prevents one caller from changing text that another caller assumes is stable, which matters for class names, file paths, URLs, security tokens, and map keys. Repeated concatenation in a loop can create many intermediate strings, so StringBuilder is usually clearer and cheaper for incremental construction. A literal may reuse a pooled instance, while new String("x") creates a distinct object. Calling intern() can obtain the pooled representative, but ordinary application code rarely needs to manage interning explicitly.