Java string concatenation efficiency

Question

Is this bad ?

(imagine it's bigger)

int count;
//done something to count
String myString = "this " + "is " + "my " + "string" + "and " + this.methodCall() + " answer " + "is : " + count;

or is it better in a StringBuilder/StringBuffer?

score 39 · Accepted Answer · answered Jan 04 '12 at 11:39

39

Java compiler will convert it into StringBuilder to increase the performance of repeated string concatenation. http://java.sun.com/docs/books/jls/third%5Fedition/html/expressions.html#15.18.1.2

Its when you are concatenating in a loop compiler can't substitute StringBuilder by itself that's when you should consider from concatenation to StringBuilder.

answered Jan 04 '12 at 11:39

user765635

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What about this [article](http://www.pellegrino.link/2015/08/22/string-concatenation-with-java-8.html)? – gaheinrichs Jul 12 '16 at 21:41
"Its when you are concatenating in a loop compiler can't substitute StringBuilder by itself that's when you should consider from concatenation to StringBuilder" – user765635 Jul 14 '16 at 03:03
1

_Its when you are concatenating in a loop **that the** compiler can't substitute StringBuilder by itself. That's when you should consider **switching** from concatenation to **using** StringBuilder_ – Trunk Nov 12 '17 at 00:37

score 7 · Answer 2 · answered Jan 04 '12 at 11:38

7

No, it's fine. If you use Sun's Java 6 compiler it will actually use StringBuilder.

Read this article

answered Jan 04 '12 at 11:38

Klas Lindbäck

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score 7 · Answer 3 · answered Jan 04 '12 at 12:05

The Javadoc for StringBuffer states from Java 5.0

The StringBuilder class should generally be used in preference to this one, as it supports all of the same operations but it is faster, as it performs no synchronization.

The compiler will combine the string literals so its the same as writing

String myString = "this is my stringand " + this.methodCall() + " answer is : " + count;

which is the same as

String myString = new StringBuilder().append("this is my stringand ").append(methodCall()).append(" answer is : ").append(count).toString();

I wouldn't worry about the performance unless you need to eliminate garbage from your system, in which case you wouldn't use Strings here. (Its highly unlikely you need to worry about it)

score 3 · Answer 4 · answered Jan 04 '12 at 11:33

It would be more readable if you use StringBuilder/StringBuffer (choose based on your app Thread model)

if you write

    String str = "abc";
    str+="def";

Internally it will create StringBuilder for the same

   7:   invokespecial   #4; //Method java/lang/StringBuilder."<init>":()V
   10:  aload_1
   11:  invokevirtual   #5; //Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
   14:  ldc #6; //String def
   16:  invokevirtual   #5; //Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
   19:  invokevirtual   #7; //Method java/lang/StringBuilder.toString:()Ljava/lang/String;
   22:  astore_1

score 3 · Answer 5 · edited May 23 '17 at 12:09

3

(imagine it's bigger)...

String concatenation using the "+" operator creates many temporary objects and increases garbage collection. Using the StringBuffer class is more efficient.

More here: http://www.ibm.com/developerworks/websphere/library/bestpractices/string_concatenation.html

StringBuffer strBuf = new StringBuffer();
strBuf.append("this");
strBuf.append("is");
strBuf.append("my");
strBuf.append("string");
strBuf.append("and");
strBuf.append(this.methodCall());
strBuf.append(" answer ");
strBuf.append("is : ");
strBuf.append(count);

Can be related: String concatenation: concat() vs "+" operator

edited May 23 '17 at 12:09

Community

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answered Jan 04 '12 at 11:34

JohnJohnGa

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yes, but is that the case only when you are concatinating in a loop rather than building the string in one go? – Mark W Jan 04 '12 at 11:36
What is the difference? Use StringBuffer if you do many concatenation – JohnJohnGa Jan 04 '12 at 11:36
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Citing [your source](http://www.ibm.com/developerworks/websphere/library/bestpractices/string_concatenation.html) is always helpful. – Jean-François Corbett Jan 04 '12 at 11:37
1

@Jean-FrançoisCorbett you are right, I added the url. Thanks – JohnJohnGa Jan 04 '12 at 11:38
3

When it's in a single expression like this, the compiler can optimise it for you so it doesn't create intermediate String objects. (as far as I'm aware!) – Mark Rhodes Jan 04 '12 at 11:39
@MarkRhodes even if it is the same, it is more readable and easy to manipulate, it is a "best practice" – JohnJohnGa Jan 04 '12 at 11:40
True but the question is about efficiency. – Mark Rhodes Jan 04 '12 at 11:41
@MarkRhodes if it is the same (in terms of efficiency), use StringBuffer for other advantages:) – JohnJohnGa Jan 04 '12 at 11:42
3

Don't use StringBuffer unless you need thread safety (which is doubtful), The Javadoc has suggested you use StringBuilder instead since 2004. – Peter Lawrey Jan 04 '12 at 12:02

score 2 · Answer 6 · edited Dec 10 '17 at 23:04

2

Relatively new article with graph, Showing that concatenation with '+' scales pretty horribly. And as mentioned in another answer, StringBuilder will probably be more readable.

edited Dec 10 '17 at 23:04

Thor Lancaster

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answered Jan 04 '12 at 11:36

Per Fagrell

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score 1 · Answer 7 · answered Jan 04 '12 at 11:35

You should prefer StringBuilder over string concatenation, since you have method call and variable being added to Strings.

However, the quantity of simple Strings like "this " and "is" has no effect on performance, since compiler will effectively process them and create interned Strings that will end up in the bytecode. Having this said, those mentioned Strings will have no overhead on the final performance.

Mark Rhodes · Answer 8 · 2012-01-04T11:48:02.313

1

I don't think it makes any difference to performance when a string is written like this - the compiler will optimise it anyway.

edited Jan 04 '12 at 11:48

answered Jan 04 '12 at 11:36

Mark Rhodes

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Java string concatenation efficiency

8 Answers8

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