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Join In Java

04.09.2005

| 145556 views |

        Join a collection of objects into a String using the specified delimiter.  One thing to note is that the collection can contain any object.  The object's toString() method will be used to convert said object to its String representation.

    public static String join(Collection s, String delimiter) {
        StringBuffer buffer = new StringBuffer();
        Iterator iter = s.iterator();
        while (iter.hasNext()) {
            buffer.append(iter.next());
            if (iter.hasNext()) {
                buffer.append(delimiter);
            }
        }
        return buffer.toString();
    }

Tags:

java
String

Comments

Ahsan Gill replied on Fri, 2013/05/10 - 6:35am

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Snippets Manager replied on Tue, 2010/03/23 - 9:33am

You can also have a look at import org.apache.commons.lang.StringUtils; StringUtils.join(...) http://www.java2s.com/Code/JavaAPI/org.apache.commons.lang/StringUtilsjoinObjectarrayStringseparator.htm

Snippets Manager replied on Fri, 2010/04/16 - 10:39am

I have found that preallocating the buffer with the correct capacity is 2-5% faster on average.


    private static String join(List<? extends CharSequence> s, String delimiter) {
	int capacity = 0;
	int delimLength = delimiter.length();
	Iterator<? extends CharSequence> iter = s.iterator();
	if (iter.hasNext()) {
	    capacity += iter.next().length() + delimLength;
	}

	StringBuilder buffer = new StringBuilder(capacity);
	iter = s.iterator();
	if (iter.hasNext()) {
	    buffer.append(iter.next());
	    while (iter.hasNext()) {
		buffer.append(delimiter);
		buffer.append(iter.next());
	    }
	}
	return buffer.toString();
    }

Snippets Manager replied on Wed, 2009/11/18 - 2:22pm

The last Iterable lacks the null check so combining them together with the "hasNext()" And of course we should use the StringBuilder instead of the Threadsafe but slower StringBuffer.


    public static String join( Iterable< ? extends Object > pColl, String separator )
    {
        Iterator< ? extends Object > oIter;
        if ( pColl == null || ( !( oIter = pColl.iterator() ).hasNext() ) )
            return "";
        StringBuilder oBuilder = new StringBuilder( String.valueOf( oIter.next() ) );
        while ( oIter.hasNext() )
            oBuilder.append( separator ).append( oIter.next() );
        return oBuilder.toString();
    }

Snippets Manager replied on Tue, 2007/06/05 - 11:43am

This will also work correctly, if the Collection has nulls, or does not implements the protocol correctly.

public static 
String join(final Collection objs, final String delimiter) {
    if (objs == null || objs.isEmpty())
        return "";
    Iterator iter = objs.iterator();
    // remove the following two lines, if you expect the Collection will behave well
    if (!iter.hasNext())
        return "";
    StringBuffer buffer = new StringBuffer(String.valueOf(iter.next()));
    while (iter.hasNext())
        buffer.append(delimiter).append(String.valueOf(iter.next()));
    return buffer.toString();
}

Also it could be implemented on Iterables, not just Collections

public static 
String join(final Iterable objs, final String delimiter) {
    Iterator iter = objs.iterator();
    if (!iter.hasNext())
        return "";
    StringBuffer buffer = new StringBuffer(String.valueOf(iter.next()));
    while (iter.hasNext())
        buffer.append(delimiter).append(String.valueOf(iter.next()));
    return buffer.toString();
}

Snippets Manager replied on Fri, 2008/10/10 - 2:53pm


public static 
String join(final Collection objs, final String delimiter) {
  if (objs == null || objs.isEmpty())
    return "";
  Iterator iter = objs.iterator();
  StringBuffer buffer = new StringBuffer(iter.next().toString());
  while (iter.hasNext())
    buffer.append(delimiter).append(iter.next().toString());
  return buffer.toString();
}

Snippets Manager replied on Fri, 2008/10/10 - 2:53pm

public static String join(final Collection objs, final String delimiter) { if (objs == null || objs.isEmpty()) return ""; Iterator iter = objs.iterator(); StringBuffer buffer = new StringBuffer(iter.next().toString()); while (iter.hasNext()) buffer.append(delimiter).append(iter.next().toString()); return buffer.toString(); }

Snippets Manager replied on Tue, 2008/07/29 - 4:46pm

And we can leverage the fact that append is a telescoped operator and compose them:


public static String join(AbstractCollection s, String delimiter) {
    if (s.isEmpty()) return "";
    Iterator iter = s.iterator();
    StringBuffer buffer = new StringBuffer(iter.next());
    while (iter.hasNext()) buffer.append(delimiter).append(iter.next());
    return buffer.toString();
}

Snippets Manager replied on Tue, 2008/07/29 - 4:46pm

Let's not create the StringBuffer/Iterator unless we need to:


public static String join(AbstractCollection s, String delimiter) {
    if (s.isEmpty()) return "";
    Iterator iter = s.iterator();
    StringBuffer buffer = new StringBuffer(iter.next());
    while (iter.hasNext()) {
        buffer.append(delimiter);
        buffer.append(iter.next());
    }
    return buffer.toString();
}

Snippets Manager replied on Tue, 2008/01/15 - 3:51am

We can make it faster by taking the 'if' out of the loop:


    public static String join(AbstractCollection s, String delimiter) {
        StringBuffer buffer = new StringBuffer();
        Iterator iter = s.iterator();
        if (iter.hasNext()) {
            buffer.append(iter.next());
            while (iter.hasNext()) {
                buffer.append(delimiter);
                buffer.append(iter.next());
            }
        }
        return buffer.toString();
    }