String concatenation (+)

The + operator can concatenate strings. When used with other types, Java automatically converts non‑string operands to strings.

Basic concatenation:

String firstName = "John";
String lastName = "Doe";
String fullName = firstName + " " + lastName;
System.out.println(fullName); // John Doe

Concatenation with other types:

int age = 30;
String message = "I am " + age + " years old.";
System.out.println(message); // I am 30 years old.

Order matters: evaluation is left‑to‑right.

System.out.println(10 + 20 + " is sum"); // 30 is sum
System.out.println("Sum is " + 10 + 20); // Sum is 1020

Using += with strings:

String result = "";
result += "Hello";
result += " ";
result += "World";
System.out.println(result); // Hello World

Performance: For many concatenations (especially in loops), prefer StringBuilder or StringBuffer to avoid creating many intermediate String objects.

StringBuilder sb = new StringBuilder();
for (int i = 0; i < 100; i++) {
    sb.append(i);
}
String result = sb.toString();

Note: The + operator for strings is implemented internally using StringBuilder or StringBuffer by the compiler (except in loops where it may create multiple objects).