[Java] About String and StringBuilder

Summary with lottery

This is a memo for yourself for studying

--About the difference between String and StringBuilder in the first place --About stringsbuilder equals

The conclusion is --StringBuilder is recommended when joining and flipping --If not, String may be more convenient & safe --Be careful about string match judgment with String and StringBuilder!


Recently, I am studying Java grammar using the AtCoder problem.

ABC146 F -Sugoroku It is a story that starts when I came across something called StringBuilder when I wanted to invert String when I came across.

Difference between String and StringBuilder

You can easily understand this by looking at the link below. https://qiita.com/shunsuke227ono/items/e8f34c67dcffa0fa28ad https://www.javadrive.jp/start/stringbuilder/

To summarize briefly

--String is immutable and StringBuilder is mutable --String can also be described as mutable in the code (S = S + "a" etc.) --StringBuilder is more memory and time efficient for adding and inserting strings

So it seems that the use of StringBuilder is recommended, but ** I didn't see any advantages on the String side **, so that? Is it a child who doesn't need String? (Probably not).

――― 2020/02/02 postscript ――― (The title has also been changed) He pointed out in the comments and introduced ↓ good links ↓. https://qiita.com/yoshi389111/items/67354ba33f9271ef2c68

There seems to be no problem with the + operator when combining string constants or when combining with one statement. If you want to invert the string like this time, it seems better to use StringBuilder or List \ <Character >.

Addendum 2 (2020/03/27)

Added because I encountered a different problem and knowledge

ABC159 B - String Palindrome Judgment whether the character string is a palindrome (+ α). I wrote the code with the idea of whether a StringBuilder and its inverted version are the same.

Palindrome judgment code (wrong)

static boolean checkKaibun(String str){
  StringBuilder t = new StringBuilder(str);
  return (t.equals(t.reverse()));

This always returns true as far as I've tried it myself. The cause is that the StringBuilder's equals method has not been overridden.

The solution is

  1. Override the equals method
  2. Convert to String with toString and then compare

If you use a lot of StringBuilder comparisons in your code, it would be 1, but it seems that you also need to override the hashCode method (see below), which is annoying.

reference https://www.atmarkit.co.jp/ait/articles/0702/20/news103.html

The implementation in 2 is as follows.

Palindrome judgment code

static boolean checkKaibun(String str){
  StringBuilder t = new StringBuilder(str);
  return (str.equals(t.reverse().toString()));

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