Detecting the first iteration through a for-each loop in Java

Question

I'm working on a server that returns character separated lists to its client. In order to build these lists I have to detect the first iteration through a for-each loop:

    StringBuilder builder = new StringBuilder() ;
    boolean firstIterationFlag = true ;

    for ( String s : list ){
        if ( firstIterationFlag) {
            firstIterationFlag = false ;
        } else {
            builder.append(separator);
        }
        builder.append(s) ;         
   }
   return builder.toString() ;

Is there a way of doing this without the flag?

instead of a `forEach` loop, you could manually do a normal `for` loop, starting with index `1` instead of 0. — Sheriff, May 08 '12 at 16:26
Have a look at this SO question: http://stackoverflow.com/questions/63150/whats-the-best-way-to-build-a-string-of-delimited-items-in-java - it may be a better solution that yours. — Aleks G, May 08 '12 at 16:27
@vilo: that is potentially a linear-time hit to each iteration, so a O(n^2) hit to the routine just to glue some text together. — andersoj, May 08 '12 at 16:29
@andersoj: you're right. I didn't think about that. I thought it's about code polishing (small string collections). `org.apache.commons.StringUtils.join()` looks good. — viliam, May 08 '12 at 16:41

score 21 · Accepted Answer · edited Nov 09 '16 at 15:09

21

One simpler way for this situation is to note that you can always append an empty string:

// For the first iteration, use a no-op separator
String currentSeparator = "";
for (String s : list) {
    builder.append(currentSeparator);
    builder.append(s);
    // From the second iteration onwards, use this
    currentSeparator = separator;
}

Alternatively (and preferrably) use Guava's Joiner class to start with :)

This "joiner" scenario is almost always the one given for this requirement - so just use Joiner. For other scenarios, either use a regular for loop or use the condition as per your code.

edited Nov 09 '16 at 15:09

Jérôme B

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answered May 08 '12 at 16:26

Jon Skeet

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1

The ``if`` eliminated, but more difficult to grasp – Israel Unterman May 08 '12 at 16:29
I use StringUtils.join from Apache commons-lang. – Ravi Wallau May 08 '12 at 18:01
@raviWallau so did I, before I knew Guava's Joiner :-) – Sean Patrick Floyd May 08 '12 at 18:05

score 9 · Answer 2 · answered May 08 '12 at 16:27

Not using foreach, but by using the iterator manually:

Iterator<String> it = list.iterator();
if(it.hasNext()){
    doSomeThingForTheFirstTime(it.next());
    while(it.hasNext()){
        doSomethingElse(it.next);
    }
}

(btw, this is pretty much what Guava's Joiner class does internally, though on a higher level)

score 3 · Answer 3 · edited Feb 27 '17 at 16:20

3

In the specific case you cite, use Guava's Joiner instead of rolling your own...

An object which joins pieces of text (specified as an array, Iterable, varargs or even a Map) with a separator. It either appends the results to an Appendable or returns them as a String. Example:

Joiner joiner = Joiner.on(separator).skipNulls();
return joiner.join(list);

edited Feb 27 '17 at 16:20

AJPerez

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answered May 08 '12 at 16:28

andersoj

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+1 (Guava's Joiner is far superior to anything in apache commons) – Sean Patrick Floyd May 08 '12 at 16:31

score 3 · Answer 4 · answered Oct 27 '16 at 13:21

3

Java 8 Solution

If you have Java 8 available, you can simply use String#join, no need for Guava:

String.join(", ", list)

answered Oct 27 '16 at 13:21

amoebe

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score 1 · Answer 5 · answered May 08 '12 at 17:47

1

Just for the fun here is a different option.

    StringBuilder builder = new StringBuilder();
    for (String s : list) {
        builder.append(separator);
        builder.append(s);
    }
    builder.deleteCharAt(0);
    return builder.toString();

answered May 08 '12 at 17:47

Krrose27

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score 1 · Answer 6 · answered Aug 10 '16 at 15:27

1

You can try this :)

int i = 0;
for (something : manythings) {
    i++;
}

answered Aug 10 '16 at 15:27

He Yifei 何一非

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score 0 · Answer 7 · answered May 08 '12 at 16:29

I would use commons.lang StringUtils.join function (see http://commons.apache.org/lang/api-2.4/org/apache/commons/lang/StringUtils.html). I strongly recommend that you get acquainted with this and other helper libraries to avoid re-inventing the wheel.

score 0 · Answer 8 · answered May 08 '12 at 17:20

You could also use the following method:

public static String toString(List<?> list, String separator) {
    StringBuilder sb = new StringBuilder();
    Iterator<?> it = list.iterator();
    while (it.hasNext()) {
        Object next = it.next();
        sb.append(next);
        if (it.hasNext()) {
            sb.append(separator);
        }
    }
    return sb.toString();
}

The enhanced for statement is equivalent to a basic for statement of the form:

for (I #i = Expression.iterator(); #i.hasNext(); ) {
    VariableModifiersopt TargetType Identifier =
        (TargetType) #i.next();
    Statement
}

#i is an automatically generated identifier

score 0 · Answer 9 · answered Jan 16 '14 at 12:27

Well, this will not detect the first iteration in the loop, but solve the problem. Just append the seperator at the end of each string and cut off the last seperator.

StringBuilder builder = new StringBuilder() ;

for ( String s : list ) {
    builder.append(s) ;         
    builder.append(separator);
}
builder.setLength(builder.length() - separator.length());
return builder.toString() ;

Detecting the first iteration through a for-each loop in Java

9 Answers9