How to Concatenate Strings in Java like a Pro

String concatenation in Java can be initially tricky, especially if you come from other programming languages. When Java was first implemented, the decision was made to render the String class immutable. So what if you have to concatenate two or more strings? I remember being instructed to rely on the StringBuilder class like so:

StringBuilder sb = new StringBuilder();
sb.append("Gandalf");
sb.append(" ");
sb.append("The");
sb.append(" ");
sb.append("Grey");

String gandalf = sb.toString();

Prior to Java 8, this was more efficient. However, now the compiler handles that for you by making + operations into StringBuilder operations at compile time.

However, this implementation is very naïve.

If this code

String firstName = "Frodo";
String lastName = "Baggins";
String fullName = firstName + " " + lastName;

translates to this equivalent at compile time...

String firstName = "Frodo";
String lastName = "Baggins";
StringBuilder sb = new StringBuilder();
sb.append(firstName);
sb.append(" ");
sb.append(lastName);
String fullName = sb.toString();

...what happens during a loop?

Unfortunately, this code

List<String> hobbits = List.of("Frodo", "Samwise", "Merry", "Pippin");
String greetAllHobbits = "";
for (String hobbit : hobbits) {
  greetAllHobbits += "Greetings, " + hobbit + "!\n";
}

translates to roughly this equivalent at compile time

List<String> hobbits = List.of("Frodo", "Samwise", "Merry", "Pippin");
String greetAllHobbits = "";
for (String hobbit : hobbits) {
  StringBuilder sb = new StringBuilder(greetAllHobbits);
  sb.append("Greetings, ");
  sb.append(hobbit);
  sb.append("!\n");
  greetAllHobbits = sb.toString();
}

A separate StringBuilder instance is created during each iteration, which can defeat the purpose of being efficient!

In the above case, you'll need to create a StringBuilder outside the loop so you aren't creating an unnecessary new object for each iteration.

List<String> hobbits = List.of("Frodo", "Samwise", "Merry", "Pippin");
StringBuilder sb = new StringBuilder();
for (String hobbit : hobbits) {
  sb.append("Greetings, ");
  sb.append(hobbit);
  sb.append("!\n");
}
String greetAllHobbits = sb.toString();

In conclusion:

• Use + when concatenating ad hoc
• Use StringBuilder when concatenating with a loop

Comments

[RaspberryCode]

Declarative approach:

StringBuilder greetAllHobbits = hobbits.stream()
                .reduce(new StringBuilder(),
                        (acc, b) -> acc.append("Greetings, %s!%n".formatted(b)),
                        StringBuilder::append);