Java で、文字列を連結する際には足し算ではなく StringBuilder を使ったほうが速い、という話はよく聞くけど、そうじゃないこともあるよ。という例。

※ コメントを頂いたとおり、初出時ひどく間違っていました。すいません。初出時のコードで条件を揃えても差が出ませんでしたので、差が出るような条件に変更しました。

実行環境

$ java --version
openjdk 12 2019-03-19
OpenJDK Runtime Environment (build 12+33)
OpenJDK 64-Bit Server VM (build 12+33, mixed mode, sharing)
$ javac --version
javac 12

サンプルコード1

UseStringBuilder

UseStringBuilder.java

public class UseStringBuilder {
  private static int receiver(String s) {
    if (s.equals("hoge")) {
      return 1;
    } else {
      return 0;
    }
  }

  private static final String foo = "uprjkdowvyvqxlncsjgqzstikgclvgorshfrruifnnuzcfqtgtojfqsakcfvaisaysaxbrlpfczukomfxoebwypmqbkqmeevgivpdxnawthcazjobhhhydlbeiltwbmvwsykslyudqludjqlmkzilhreuxyajqgpiwebbxdnccrfmictdxolowxxnaeoucmtybdnmujrwshdezptpjpmzjdnasikhjztfnuezvipxwvywhgvmzqtrfehkkbrgganygop";
  private static final String bar = "bjsqdbkweettnvympybihyjrkhqtedvpogounxelthhyrtbpzoldujwzmmspacjtxwjwciusocwimihuumzvarvyqorunyinmqlkaefmlghcoxsvqbbazlbgavsffmqnjyaqturfgvwrfsgdcrksdexcipznpovpzilbgpxewclexahkxylffwsjiuiuthtbkrkqimcfcfzkoptzsroljeagdzgacdpgxdqwnjymwnnvqlohvsyxkbnazgedrxdjukfh";
  private static final String baz = "phblfiycdsnaahcskasevmthowdijgvfxdzuiivsubtqdwqzcbjhxvlhclkxnmpikfphceulcypiukpkrwzrnmzabydgmfwulrkjbonpcfgvjfbewdavycsjaqfsgvuqunblbzysezasrlrkeawotzjmdvemcqfditpjuolntiymrwxbgmhupoznjydqxzhghgifwnbkgosarjtuvxgneqorxzjemqtepwldrmqytkygsfetvnclqhyxoprioatxxkwo";
  private static final String qux = "rysvvsftwtolxonofdvkjjepouyayiyszgehjuqmunagaaiacrlpalcyizpwmrscldwpfiggobbsynzczfjhcqoqmlkvobljithjucwxmyzdsbrhkjpnpezxmqegbllwcdtraovdupkhbnsekkszffesmrpigwaeqqdxtnklusiwyljbhkzhktarxcraqvcvdybpunxeqnurmrbhoptgdffcfwwemqxvivxhgdnyttkivxjfzzhbqgujigmwmetnuxod";
  private static final String quux = "aodwvgukhmcwrzxwpmlucjnnwtjviytbtbahyexaefkmmweozifczxnzxafwbyjgoknssrtrdoihwhxmktsykfgapffmzqujlgtovsikgjwjtzcuxkgmequueqlnjratiqrznxpyomvsyiccbiyqrwbelbhpvvpgscqzpdsnqrohphdiomjcwivxddpyqelcdnapzlixbqdsfrehamzhyaavpuxjklllvulfqyostchdrgjgdbugfoeuknbsbrntkevf";

  public static void main(String[] args) {
    int sum = 0;
    String[] bars = new String[] { "baz", "qux" };
    for (int i = 0; i < Integer.parseInt(args[0]); ++i) {
      sum += receiver(new StringBuilder()
      .append("hoge")
      .append( foo )
      .append( bar )
      .append( baz )
      .append( qux )
      .append( quux )
      .toString() );
    }
    System.out.println(sum);
  }
}

UseStringPlus

UseStringPlus.java

public class UseStringPlus {
  private static int receiver( String s ){
    if ( s.equals("hoge") ){
      return 1;
    } else {
      return 0;
    }
  }
  private static final String foo = "uprjkdowvyvqxlncsjgqzstikgclvgorshfrruifnnuzcfqtgtojfqsakcfvaisaysaxbrlpfczukomfxoebwypmqbkqmeevgivpdxnawthcazjobhhhydlbeiltwbmvwsykslyudqludjqlmkzilhreuxyajqgpiwebbxdnccrfmictdxolowxxnaeoucmtybdnmujrwshdezptpjpmzjdnasikhjztfnuezvipxwvywhgvmzqtrfehkkbrgganygop";
  private static final String bar = "bjsqdbkweettnvympybihyjrkhqtedvpogounxelthhyrtbpzoldujwzmmspacjtxwjwciusocwimihuumzvarvyqorunyinmqlkaefmlghcoxsvqbbazlbgavsffmqnjyaqturfgvwrfsgdcrksdexcipznpovpzilbgpxewclexahkxylffwsjiuiuthtbkrkqimcfcfzkoptzsroljeagdzgacdpgxdqwnjymwnnvqlohvsyxkbnazgedrxdjukfh";
  private static final String baz = "phblfiycdsnaahcskasevmthowdijgvfxdzuiivsubtqdwqzcbjhxvlhclkxnmpikfphceulcypiukpkrwzrnmzabydgmfwulrkjbonpcfgvjfbewdavycsjaqfsgvuqunblbzysezasrlrkeawotzjmdvemcqfditpjuolntiymrwxbgmhupoznjydqxzhghgifwnbkgosarjtuvxgneqorxzjemqtepwldrmqytkygsfetvnclqhyxoprioatxxkwo";
  private static final String qux = "rysvvsftwtolxonofdvkjjepouyayiyszgehjuqmunagaaiacrlpalcyizpwmrscldwpfiggobbsynzczfjhcqoqmlkvobljithjucwxmyzdsbrhkjpnpezxmqegbllwcdtraovdupkhbnsekkszffesmrpigwaeqqdxtnklusiwyljbhkzhktarxcraqvcvdybpunxeqnurmrbhoptgdffcfwwemqxvivxhgdnyttkivxjfzzhbqgujigmwmetnuxod";
  private static final String quux = "aodwvgukhmcwrzxwpmlucjnnwtjviytbtbahyexaefkmmweozifczxnzxafwbyjgoknssrtrdoihwhxmktsykfgapffmzqujlgtovsikgjwjtzcuxkgmequueqlnjratiqrznxpyomvsyiccbiyqrwbelbhpvvpgscqzpdsnqrohphdiomjcwivxddpyqelcdnapzlixbqdsfrehamzhyaavpuxjklllvulfqyostchdrgjgdbugfoeuknbsbrntkevf";
  public static void main( String[] args ){
    int sum=0;
    String[] bars = new String[]{ "baz", "qux" };
    for( int i=0 ; i<Integer.parseInt(args[0]) ; ++i ){
      sum += receiver( 
        "hoge"
        + foo 
        + bar 
        + baz 
        + qux 
        + quux 
       );
    }
    System.out.println(sum);
  }
}

走らせる人

run.rb

%w( UseStringBuilder UseStringPlus ).each do |name|
  p name
  puts( %x( javac #{name}.java && time java #{name} 10000000 ) )
end

実行結果

"UseStringBuilder"

real	0m2.594s
user	0m2.488s
sys	0m0.356s
0
"UseStringPlus"

real	0m0.227s
user	0m0.239s
sys	0m0.036s
0

サンプルコード2

コメントを受けて、コンパイル時に結合が行われないようなサンプルでの実験。

UseStringBuilder2

UseStringBuilder2.java

public class UseStringBuilder2 {
  private static int receiver(String s) {
    if (s.equals("hoge")) {
      return 1;
    } else {
      return 0;
    }
  }

  private static String makestr(int len, char c0){
    StringBuilder b = new StringBuilder();
    for( int i=0 ; i<len*1024 ; ++i ){
      b.append(c0+(char)i);
    }
    return b.toString();
  }

  public static void main(String[] args) {
    String foo = makestr(1, args[1].charAt(0));
    String bar = makestr(2, args[1].charAt(0));
    String baz = makestr(4, args[1].charAt(0));
    String qux = makestr(8, args[1].charAt(0));
    String quux = makestr(16, args[1].charAt(0));
    String corge = makestr(32, args[1].charAt(0));
    int sum = 0;
    String[] bars = new String[] { "baz", "qux" };
    for (int i = 0; i < Integer.parseInt(args[0]); ++i) {
      sum += receiver(new StringBuilder()
      .append("hoge")
      .append( foo )
      .append( bar )
      .append( baz )
      .append( qux )
      .append( quux )
      .append( corge )
      .toString() );
    }
    System.out.println(sum);
  }
}

UseStringPlus2

UseStringPlus2.java

public class UseStringPlus2 {
  private static int receiver( String s ){
    if ( s.equals("hoge") ){
      return 1;
    } else {
      return 0;
    }
  }
  private static String makestr(int len, char c0){
    StringBuilder b = new StringBuilder();
    for( int i=0 ; i<len*1024 ; ++i ){
      b.append(c0+(char)i);
    }
    return b.toString();
  }
  public static void main( String[] args ){
    String foo = makestr(1, args[1].charAt(0));
    String bar = makestr(2, args[1].charAt(0));
    String baz = makestr(4, args[1].charAt(0));
    String qux = makestr(8, args[1].charAt(0));
    String quux = makestr(16, args[1].charAt(0));
    String corge = makestr(32, args[1].charAt(0));
    int sum=0;
    for( int i=0 ; i<Integer.parseInt(args[0]) ; ++i ){
      sum += receiver( 
        "hoge"
        + foo 
        + bar 
        + baz 
        + qux 
        + quux 
        + corge
       );
    }
    System.out.println(sum);
  }
}

走らせる人

%w( UseStringBuilder2 UseStringPlus2 ).each do |name|
  p name
  puts( %x( javac #{name}.java && time java #{name} 10000 a ) )
end

実行結果

"UseStringBuilder2"

real	0m1.838s
user	0m1.608s
sys	0m0.450s
0
"UseStringPlus2"

real	0m0.917s
user	0m0.903s
sys	0m0.263s
0

というわけで

というわけで、上記のようなケースでは、足し算で連結したほうが速い。

Java のバージョン等によって逆転したりとか、いろいろあると思う。