为什么Java字符串比较在Java15和Java11中表现不同？

这在JDK 15 发行说明 中提到。

根据 JDK-8240094 的要求对其进行了更改：

JDK-8240094：优化空子串处理

String.substring""在某些情况下返回，但可以改进以在子字符串长度为零的所有情况下都这样做。

有关的：

JDK-8240225：优化空子串处理

优化String.substring和相关操作，如stripLeading,stripTrailing避免重复创建新的空字符串。

子任务：

JDK-8251556：发行说明：优化的空子字符串处理

的实施String.substring和相关方法stripLeading和stripTrailing在此版本中已更改为避免冗余创建一个新的空字符串。这可能会影响依赖于未指定行为和空子字符串标识的代码。

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  @fluffy `""` is the default empty string constant, no need to create a constant field for it, since string literals are always interned for you.
  


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  @fluffy The string constant pool is synthesized by the JVM at runtime and merges all constant strings.
  


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  @fluffy it doesn’t matter whether you use `""` or `SomeClass.CONSTANT`. The important point is that the `substring` method needs to perform an explicit test whether the result string has length zero and use one of those constants instead of calling `new String(…)` in that case. Without that test, declaring a constant for empty strings doesn’t help.
  


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  @fluffy having public constructors in the string class is a historical design mistake on its own (including `String()`, encouraging developers to create redundant strings even when known to be empty). However, for the string class itself, that makes little difference as even `private` constructors are accessible. Consistently using factory methods that may perform this check would help, but there is a different point of view. *Is this optimization really an improvement?* We now have an additional conditional in every `substring` operation, serving the rare corner case of an empty result…
  


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  Just curious: .NET has `String.Empty` since the earliest version, Apache Commons suggest a similar final reference `StringUtils.EMPTY`. Those JDK items and issues would probably never exist if an empty string constant would exist from the very beginning. Was there any idea of introducing the "default" empty string constant to Java?
  


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  @fluffy I had such scenarios in my application, where I wished I could constrain the use of `private` members even further, ala “this member is only for method abc and xyz”. However, in case of OpenJDK’s String implementation they were not even trying to use/enforce factory methods, internally, the constructors are used all over the place.