Using + to concat string in Java

Question

I have a huge string as below :

String str = "aaaaaaa" 
        + "bbbbbbbb" 
        + "cccccccc" 
    .... 

        + "zzzzzzzz";  // about 200 lines long

Is it a big waste of memory? Will it be better to put it in one line or use StringBuilder? This is done to create a long sql.

Answer 1

No. The compiler will perform the concatenation at compile time if all your strings are constant (as they are in your question). So that ends up performing better than the StringBuilder version.

It will use StringBuilder (or StringBuffer) to perform the concatenation of any non-constant parts otherwise.

So the compiled version of:

String x = "a" + "b";

should be exactly the same as:

String x = "ab";

Note that the "non-constant" parts can mean that you can sometimes get (very minor) extra efficiencies using bracketing wisely. For example:

int value = ...;
String x = "a" + value + "b" + "c";

... is less efficient than:

int value = ...;
String x = "a" + value + ("b" + "c");

... as the first appends "b" and "c" separately. The efficient version is equivalent tO:

int value = ...;
String x = "a" + value + "bc";

I can't remember ever seeing this used as a legitimate micro-optimization, but it's at least a point of interest.

Answer 2

In current versions of Java, each + (concatenation) operation gets automatically compiled to a StringBuilder append() operation. However, if it's inside a loop, it will create a new StringBuilder for each iteration, negating its effect - so you'd be better of managing the StringBuilder explicitly.