Best way to concatenate List of String objects? [duplicate]

Answer 1

Guava is a pretty neat library from Google:

Joiner joiner = Joiner.on(", ");
joiner.join(sList);

Answer 2

Assuming it's faster to just move a pointer / set a byte to null (or however Java implements StringBuilder#setLength), rather than check a condition each time through the loop to see when to append the delimiter, you could use this method:

public static String Intersperse (Collection<?> collection, String delimiter)
{
    StringBuilder sb = new StringBuilder ();
    for (Object item : collection)
    {
        if (item == null) continue;
        sb.append (item).append (delimiter);
    }
    sb.setLength (sb.length () - delimiter.length ());
    return sb.toString ();
}

Answer 3

It seems to me that the StringBuilder will be quick and efficient.

The basic form would look something like this:

public static String concatStrings(List<String> strings)
{
    StringBuilder sb = new StringBuilder();
    for(String s: strings)
    {
        sb.append(s);
    }
    return sb.toString();       
}

If that's too simplistic (and it probably is), you can use a similar approach and add a separator like this:

    public static String concatStringsWSep(List<String> strings, String separator)
{
    StringBuilder sb = new StringBuilder();
    for(int i = 0; i < strings.size(); i++)
    {
        sb.append(strings.get(i));
        if(i < strings.size() - 1)
            sb.append(separator);
    }
    return sb.toString();               
}

I agree with the others who have responded to this question when they say that you should not rely on the toString() method of Java's ArrayList.

Answer 4

A variation on codefin's answer

public static String concatStringsWSep(Iterable<String> strings, String separator) {
    StringBuilder sb = new StringBuilder();
    String sep = "";
    for(String s: strings) {
        sb.append(sep).append(s);
        sep = separator;
    }
    return sb.toString();                           
}

Answer 5

Rather than depending on ArrayList.toString() implementation, you could write a one-liner, if you are using java 8:

String result = sList.stream()
                     .reduce("", String::concat);

If you prefer using StringBuffer instead of String since String::concat has a runtime of O(n^2), you could convert every String to StringBuffer first.

StringBuffer result = sList.stream()
                           .map(StringBuffer::new)
                           .reduce(new StringBuffer(""), StringBuffer::append);

Answer 6

Use one of the the StringUtils.join methods in Apache Commons Lang.

import org.apache.commons.lang3.StringUtils;

String result = StringUtils.join(list, ", ");

If you are fortunate enough to be using Java 8, then it's even easier...just use String.join

String result = String.join(", ", list);