Java Compiler String optimization

Question

It seems reasonable to me that the compiler is going to take something like this:

log.info("A really long logger message that is kind of a pain in the tucous " + 
    "and violates formatting standards by making the line to long");

and compile the two Strings into one. I'm pretty sure this is true but I would like to have my ducks in a row if anyone brings it up.

by the way, similar will happen for int i = 40 + 2; - the compiler will compile it as int i = 42; — user85421
– user85421, Commented Dec 26, 2024 at 8:33

Jeff Foster · Accepted Answer · 2012-12-13 17:18:37Z

11

Yes, this will be handled by the constant expression part of the Java Language Specification. In particular see part 15.18.1. String Concatenation Operator +

answered Dec 13, 2012 at 17:18

Jeff Foster

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Comments

Perception · Accepted Answer · 2012-12-13 17:19:42Z

9

Strings computed by constant expressions (§15.28) are computed at compile time and then treated as if they were literals.

Show that quote from the JLS to anyone who 'challenges' you.

answered Dec 13, 2012 at 17:19

Perception

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Evgeniy Dorofeev · Accepted Answer · 2012-12-13 17:38:52Z

4

To check if what JLS says about constant expressions is true I complied this code, Test.java

public static void main(String[] args) {
    log.warning("123" + "456");
}

then decompile Test.class with Jad and got this

public static void main(String args[])
{
    log.warning("123456");
}

that is, in Test.class there is only one literal "123456"

answered Dec 13, 2012 at 17:38

Evgeniy Dorofeev

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you can also use javap to check the compiled code