DEV Community

Cover image for How to Concatenate Strings in Java like a Pro
Joshua Austin
Joshua Austin

Posted on

How to Concatenate Strings in Java like a Pro

#java #programming #tutorial

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();
Enter fullscreen mode Exit fullscreen mode

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;
Enter fullscreen mode Exit fullscreen mode

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();
Enter fullscreen mode Exit fullscreen mode

...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";
}
Enter fullscreen mode Exit fullscreen mode

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();
}
Enter fullscreen mode Exit fullscreen mode

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();
Enter fullscreen mode Exit fullscreen mode

In conclusion:

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

Top comments (1)

Collapse
 
raspberrycode profile image
RaspberryCode •

Declarative approach:

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

Read next

mdarifulhaque profile image

3163. String Compression III

MD ARIFUL HAQUE -

gaundergod profile image

My minimal VS Code setup for Vue, Typescript, Tailwind CSS, etc

Gleb Kotovsky -

arshisaxena26 profile image

Exploring Nuances of the Java Scanner Class

Arshi Saxena -

anwaar profile image

Multithreading Concepts Part 3 : Deadlock

Anwar -