I was reading this documentation page, http://developer.android.com/reference/android/util/Log.html.
The section here caught my eye:
Tip: Don't forget that when you make a call like
Log.v(TAG, "index=" + i);
that when you're building the string to pass into Log.d, the compiler
uses a StringBuilder and at least three allocations occur: the
StringBuilder itself, the buffer, and the String object.
Realistically, there is also another buffer allocation and copy, and
even more pressure on the gc. That means that if your log message is
filtered out, you might be doing significant work and incurring
significant overhead.
This implies that the Android compiler is taking string concatenations (+) and converting them into StringBuilder and append statements.
Is my assumption correct or is it still better to use StringBuilder manually instead of string concatenation?
The compiler does exactly what you suggest is implied. You can print the bytecodes of the generated .class file (using javap -c) and see the calls to construct and use a StringBuilder.
However, it's generally worth doing it manually when the string concatenations are spread over several lines of code. The compiler usually allocates a separate StringBuilder for every String-valued expression involving +.
Ted Hopp's answer is good, but it took reading it a few times to understand. Here is a rephrased answer that is hopefully more clear.
String concatenation (ie, using +, as in String myString = "Hello " + "World";) uses a StringBuilder in the background along with other allocations. Thus, for anything other than a simple one-time concatenation, it would be better to use a StringBuilder yourself.
For example,
StringBuilder myString = new StringBuilder();
myString.append("Hello ");
myString.append("to ");
myString.append("everyone ");
myString.append("in ");
myString.append("the ");
myString.append("world!");
is better than
String myString = "Hello " + "to " + "everyone " + "in " + "the " + "world!";
