chapter10-string_formatting.md

String formatting

There are several ways to concatenate/format objects to strings in Java, mostly depending if there are a fixed number of values or if the values are in a list or any other data structures.

Let say we have some friends

record Friend(String name) {}
var bob = new Friend("bob");
var ana = new Friend("ana");
var jul = new Friend("jul");

With a fixed number of values

If there is a fixed number of value, the concatenation using '+' is the most readable (ok, when your are used to) and the fastest

Concatenation with +

Just do a '+' between the different values, this code is heavily optimized and will allocate only one String

System.out.println(bob.name() + ", " + ana.name() + ", " + jul.name());

Concatenation with String.format()

If you want more control on the formatting, you can use String.format that reuse the C formatting style But the method format() is quite slow.

System.out.println(String.format("%s, %s, %s", bob, ana, jul));
System.out.printf("%s, %s, %s\n", bob, ana, jul);

with a variable number of values

If there is a variable numbers of values, you have two cases, depending if it's a collection of String or not

var strings = List.of("bob", "ana", "jul");
var friends = List.of(bob, ana, jul);

Concatenation with a +

Never use '+' in this case, because the compiler is not smart enough to reuse the same buffer of characters for the whole loop, so it will create a new String for each loop trip.

String concatenate(List<?> list) {
  var string = "";
  var separator = "";
  for(var item: list) {
    string = string + separator + item;  // creates two many strings, ahhhh
    separator = ", ";
  } 
  return string;
}
System.out.println(concatenate(strings));
System.out.println(concatenate(friends));

Concatenation with a StringBuilder

A StringBuilder is a modifiable version of String with an expandable buffer of characters. There is no notion of separators

String concatenate(List<?> list) {
  var builder = new StringBuilder();
  var separator = "";
  for(var item: list) {
    builder.append(separator).append(item);
    separator = ", ";
  } 
  return builder.toString();
}
System.out.println(concatenate(strings));
System.out.println(concatenate(friends));

Don't use '+' inside a call to append(), you already have a StringBuilder, so use append() instead

Concatenation with String.join()

If you have an array of strings or a collection of strings, String.join is the simplest way to concatenate the items with a separator

System.out.println(String.join(", ", strings));

Concatenation with a StringJoiner

If you don't have a list of strings by a list of objects, you can use the StringJoiner which let you specify a separator and is implemented using expandable buffer of strings (StringJoiner.add only accepts strings).

String concatenate(List<?> list) {
  var joiner = new StringJoiner(", ");
  list.forEach(item -> joiner.add(item.toString()));
  return joiner.toString();
}
System.out.println(concatenate(strings));
System.out.println(concatenate(friends));

Concatenation with a Stream

If you use a Stream and the collector joining, it will use a StringJoiner internally.

import java.util.stream.Collectors;
System.out.println(strings.stream().collect(Collectors.joining(", ")));
System.out.println(friends.stream().map(Friend::toString).collect(Collectors.joining(", ")));