It is common knowledge that Java optimizes the substring operation for the case where you generate a lot of substrings of the same source string. It does this by using the (value, offset, count) way of storing the information. See an example below:

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In the above diagram you see the strings ‘Hello’ and ‘World!’ derived from ‘Hello World!’ and the way they are represented in the heap: there is one character array containing ‘Hello World!’ and two references to it. This method of storage is advantageous in some cases, for example for a compiler which tokenizes source files. In other instances it may lead you to an OutOfMemorError (if you are routinely reading long strings and only keeping a small part of it – but the above mechanism prevents the GC from collecting the original String buffer). Some even call it a bug. I wouldn’t go so far, but it’s certainly a leaky abstraction because you were forced to do the following to ensure that a copy was made: new String(str.substring(5, 6)).

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This all changed in May of 2012 or Java 7u6. The pendulum is swung back and now full copies are made by default. What does this mean for you?

• For most probably it is just a nice piece of Java trivia
• If you are writing parsers and such, you can not rely any more on the implicit caching provided by String. You will need to implement a similar mechanism based on buffering and a custom implementation of CharSequence
• If you were doing new String(str.substring) to force a copy of the character buffer, you can stop as soon as you update to the latest Java 7 (and you need to do that quite soon since Java 6 is being EOLd as we speak).

Thankfully the development of Java is an open process and such information is at the fingertips of everyone!

A couple of more references (since we don’t say pointers in Java) related to Strings:

• If you are storing the same string over and over again (maybe you’re parsing messages from a socket for example), you should read up on alternatives to String.intern() (and also consider reading chapter 50 from the second edition of Effective Java: Avoid strings where other types are more appropriate)
• Look into (and do benchmarks before using them!) options like UseCompressedStrings (which seems to have been removed), UseStringCache and StringCache

Hope I didn’t strung you along too much and you found this useful! Until next time
– Attila Balazs

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Reference: Changes to String.substring in Java 7 from our JCG partner Attila-Mihaly Balazs at the Java Advent Calendar blog.