[PATCH] enhancement proposal for String concatenation

Сергей Цыпанов sergei.tsypanov at yandex.ru
Fri Mar 8 19:22:10 UTC 2019


I have an enhancement proposal for some cases of String concatenation in Java.

Currently we concat Strings mostly using java.lang.StringBuilder. The main disadvantage of StringBuilder is underlying char array or rather a need to resize it when the capacity is about to exceed array length and subsequent copying of array content into newly allocated array.

One alternative solution existing is StringJoiner. Before JDK 9 it was a kind of decorator over StringBuilder, but later it was reworked in order to store appended Strings into String[] and overall capacity accumulated into int field. This makes it possible to allocate char[] only once and of exact size in toString() method reducing allocation cost.

My proposal is to copy-paste the code of StringJoinder into newly created class java.util.StringChain, drop the code responsible for delimiter, prefix and suffix and use it instead of StringBuilder in common StringBuilder::append concatenation pattern.

Possible use-cases for proposed code are:
- plain String concatenation
- String::chain (new methods)
- Stream.collect(Collectors.joining())
- StringConcatFactory

We can create new methods String.chain(Iterable<CharSequence>) and String.chain(CharSequence...) which allow to encapsulate boilerplate code like

  StringBuilder sb = new StringBuilder();
  for (CharSequence cs : charSequences) {
  String result = sb.toString():

into one line:

  String result = String.chain(charSequences);

As of performance I've done some measurements using JMH on my work machine (Intel i7-7700) for both Latin and non-Latin Strings of different size and count.
Here are the results:


There is a few corner cases (e.g. 1000 Strings of length 1 appended) when StringBuilder takes over StringChain constructed with default capacity of 8, but StringChain constructed with exact added Strings count almost always wins, especially when dealing with non-Latin chars (Russian in my case).

I've also created a separate repo on GitHub with benchmarks:


Key feature here is ability to allocate String array of exact size is cases we know added elements count.
Thus I think that if the change will be accepted we can add also an overloaded method String.chain(Collection<CharSequence>) as Collection::size allows to contruct StringChain of exact size.

Patch is attached.

Kind regards,
Sergei Tsypanov
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