String concatenation means joining pieces of text to make one string. In Java, start with the + operator for quick joins, use String.concat for two strings, prefer StringBuilder in loops for speed, and use String.format for readable templates.

1. + operator (simple and readable)

   Great for quick joins and short strings.

   String greeting = "Hello, " + name + "!";
   String fullName = first + " " + last;

2. String.concat (when you already have two strings)

   Useful for chaining two or more existing strings.

   String result = "Hello".concat(", ").concat(name);

3. StringBuilder (best inside loops)

   Avoids creating many temporary strings when appending repeatedly.

   StringBuilder sb = new StringBuilder();
   sb.append(first).append(" ").append(last);
   for (int i = 0; i < 3; i++) {
     sb.append("!");
   }
   String message = sb.toString();

4. String.format (clean templates)

   Readable formatting when mixing values and text.

   String msg = String.format("%s scored %d points", name, score);

5. String.join (combine with a delimiter)

   Great for building comma‑separated or other delimited strings.

   String csv = String.join(", ", "red", "green", "blue");