How Java do the string concatenation using "+"?

Question

I read about the way Java works with += operator, using StringBuilder. Is it the same with a ("a" + "b") operation?

Answer 1

No. It's no the same using StringBuilder than doing "a" + "b".

In Java, String instances are immutable.

So, if you do:

String c = "a" + "b";

You are creating new Strings every time you concatenate.

On the other hand, StringBuilder is like a buffer that can grow as it needs when appending new Strings.

StringBuilder c = new StringBuilder();
c.append("a");
c.append("b"); // c is only created once and appended "a" and "b".

Rule of the thumb is (changed thanks to the comments I got):

If you are going to concatenate a lot (i.e., concatenate inside a loop, or generating a big XML formed by several string concatenated variables), do use StringBuilder. Otherwise, simple concatenation (using + operator) will be just fine.

Compiler optimizations also play a huge role when compiling this kind of code.

Here's further explanation on the topic. And more StackOVerflow questions on the issue:

One, Two, Three.

Answer 2

Yes, it's the same, but the compiler can additionally optimize concatenations of literals before issuing the code, so "a"+"b" can be just issued as "ab" directly.

Answer 3

Strings are more commonly concatenated with the + operator, as in "Hello," + " world" + "!"

Source

Answer 4

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Answer 5

If you’re only building long strings, then using StringBuilder is recommended. For shorter strings, with fixed creations (such as res = a + " " + b + " : " + c;) it should be okay. For iterations over a group of values, I’d seriously recommend StringBuilder.

Answer 6

For concatenating a fixed number of strings in one expression with +, the compiler will produce code using a single StringBuilder.

E.g. the line

String d = a + b + c;

results in the same bytecode as the line

String d = new StringBuilder().append(a).append(b).append(c).toString();

when compiled using the javac compiler. (The Eclipse compiler produces somewhat more optimized code by invoking new StringBuilder(a), thus saving one method call.)

As mentioned in other answers, the compiler will concatenate string literals like "a" + "b" into one string itself, producing bytecode that contains "ab" instead.

As mentioned everywhere on the net, you should not use + to build up one string within a loop, because you are copying the beginning of the string over and over to new strings. In this situation you should use one StringBuilder which you declare outside the loop.

Answer 7

"a" + "b" operation

Though readable, easy to format and straight forward, concatenating strings with "+" is considered to be bad in Java.

Each time you append something via '+' (String.concat()) a new String is created, the old String content is copied, the new content is appended, and the old String is discarded. The bigger the String gets the longer it takes - there is more to copy and more garbage is produced. Note: if you are just concatenating a few (say 3,4) strings and not building a string via a loop or just writing some test application, you could still stick with "+"

Using StringBuilder

When performing extensive String manipulation (or appending through a loop), replacing "+" with StringBuilder.append is likely recommended. The intermediate objects mentioned in case of "+" are not created during append() method call.

Also to be noted that optimizations in the Sun Java compiler, which automatically creates StringBuilders (StringBuffers < 5.0) when it sees String concatenations. But that is just Sun Java compiler.