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String.join() is a static method in the Java String class (since Java 8).

It is used to combine multiple strings into one, with a delimiter between them.

The method helps in creating readable output like CSVs, file paths, or joined names.

Syntax: String.join(CharSequence delimiter, CharSequence... elements)

The delimiter is placed between each string in the sequence.

Example: String.join("-", "2025", "06", "06") returns "2025-06-06"

You can pass an array, list, or varargs of strings as the elements.

With collections: String.join(", ", listOfNames) joins all names with commas.

It returns a new string; original strings remain unchanged (immutability).

It throws NullPointerException if the elements or delimiter is null.

It’s cleaner and more readable than using manual string concatenation or loops.

Common use cases: logging, formatting output, or generating CSV rows.