Java equivalents of C# String.Format() and String.Join()

I know this is a bit of a newbie question, but are there equivalents to C#'s string operations in Java?

Specifically, I'm talking about String.Format and String.Join.

The Java String object has a format method (as of 1.5), but no join method.

To get a bunch of useful String utility methods not already included you could use org.apache.commons.lang.StringUtils.

String.format. As for join, you need to write your own:

 static String join(Collection<?> s, String delimiter) {
     StringBuilder builder = new StringBuilder();
     Iterator<?> iter = s.iterator();
     while (iter.hasNext()) {
         builder.append(iter.next());
         if (!iter.hasNext()) {
           break;                  
         }
         builder.append(delimiter);
     }
     return builder.toString();
 }

The above comes from http://snippets.dzone.com/posts/show/91

Guava comes with the Joiner class.

import com.google.common.base.Joiner;

Joiner.on(separator).join(data);

As of Java 8, join() is now available as two class methods on the String class. In both cases the first argument is the delimiter.

You can pass individual CharSequences as additional arguments:

String joined = String.join(", ", "Antimony", "Arsenic", "Aluminum", "Selenium");
// "Antimony, Arsenic, Alumninum, Selenium"

Or you can pass an Iterable<? extends CharSequence>:

List<String> strings = new LinkedList<String>();
strings.add("EX");
strings.add("TER");
strings.add("MIN");
strings.add("ATE");

String joined = String.join("-", strings);
// "EX-TER-MIN-ATE"

Java 8 also adds a new class, StringJoiner, which you can use like this:

StringJoiner joiner = new StringJoiner("&");
joiner.add("x=9");
joiner.add("y=5667.7");
joiner.add("z=-33.0");

String joined = joiner.toString();
// "x=9&y=5667.7&z=-33.0"

TextUtils.join is available on Android

You can also use variable arguments for strings as follows:

  String join (String delim, String ... data) {
    StringBuilder sb = new StringBuilder();
    for (int i = 0; i < data.length; i++) {
      sb.append(data[i]);
      if (i >= data.length-1) {break;}
      sb.append(delim);
    }
    return sb.toString();
  }

As for join, I believe this might look a little less complicated:

public String join (Collection<String> c) {
    StringBuilder sb=new StringBuilder();
    for(String s: c)
        sb.append(s);
    return sb.toString();
}

I don't get to use Java 5 syntax as much as I'd like (Believe it or not, I've been using 1.0.x lately) so I may be a bit rusty, but I'm sure the concept is correct.

edit addition: String appends can be slowish, but if you are working on GUI code or some short-running routine, it really doesn't matter if you take .005 seconds or .006, so if you had a collection called "joinMe" that you want to append to an existing string "target" it wouldn't be horrific to just inline this:

for(String s : joinMe)
    target += s;

It's quite inefficient (and a bad habit), but not anything you will be able to perceive unless there are either thousands of strings or this is inside a huge loop or your code is really performance critical.

More importantly, it's easy to remember, short, quick and very readable. Performance isn't always the automatic winner in design choices.

Here is a pretty simple answer. Use += since it is less code and let the optimizer convert it to a StringBuilder for you. Using this method, you don't have to do any "is last" checks in your loop (performance improvement) and you don't have to worry about stripping off any delimiters at the end.

		Iterator<String> iter = args.iterator();
		output += iter.hasNext() ? iter.next() : "";
		while (iter.hasNext()) {
			output += "," + iter.next();
		}

I didn't want to import an entire Apache library to add a simple join function, so here's my hack.

    public String join(String delim, List<String> destinations) {
	    StringBuilder sb = new StringBuilder();
        int delimLength = delim.length();
	
	    for (String s: destinations) {
		    sb.append(s);
    		sb.append(delim);
	    }

        // we have appended the delimiter to the end 
        // in the previous for-loop. Let's now remove it.
        if (sb.length() >= delimLength) {
	        return sb.substring(0, sb.length() - delimLength);
        } else {
            return sb.toString();
        }
    }

If you wish to join (concatenate) several strings into one, you should use a StringBuilder. It is far better than using

for(String s : joinMe)
    target += s;

There is also a slight performance win over StringBuffer, since StringBuilder does not use synchronization.

For a general purpose utility method like this, it will (eventually) be called many times in many situations, so you should make it efficient and not allocate many transient objects. We've profiled many, many different Java apps and almost always find that string concatenation and string/char[] allocations take up a significant amount of time/memory.

Our reusable collection -> string method first calculates the size of the required result and then creates a StringBuilder with that initial size; this avoids unecessary doubling/copying of the internal char[] used when appending strings.

I wrote own:

public static String join(Collection<String> col, String delim) {
    StringBuilder sb = new StringBuilder();
    Iterator<String> iter = col.iterator();
    if (iter.hasNext())
        sb.append(iter.next().toString());
    while (iter.hasNext()) {
        sb.append(delim);
        sb.append(iter.next().toString());
    }
    return sb.toString();
}

but Collection isn't supported by JSP, so for tag function I wrote:

public static String join(List<?> list, String delim) {
    int len = list.size();
    if (len == 0)
        return "";
    StringBuilder sb = new StringBuilder(list.get(0).toString());
    for (int i = 1; i < len; i++) {
        sb.append(delim);
        sb.append(list.get(i).toString());
    }
    return sb.toString();
}

and put to .tld file:

<?xml version="1.0" encoding="UTF-8"?>
<taglib version="2.1" xmlns="http://java.sun.com/xml/ns/javaee"
    <function>
        <name>join</name>
        <function-class>com.core.util.ReportUtil</function-class>
        <function-signature>java.lang.String join(java.util.List, java.lang.String)</function-signature>
    </function>
</taglib>

and use it in JSP files as:

<%@taglib prefix="funnyFmt" uri="tag:com.core.util,2013:funnyFmt"%>
${funnyFmt:join(books, ", ")}

StringUtils is a pretty useful class in the Apache Commons Lang library.

There is MessageFormat.format() which works like C#'s String.Format().

I see a lot of overly complex implementations of String.Join here. If you don't have Java 1.8, and you don't want to import a new library the below implementation should suffice.

public String join(Collection<String> col, String delim) {
    StringBuilder sb = new StringBuilder();
    for ( String s : col ) {
        if ( sb.length() != 0 ) sb.append(delim);
        sb.append(s);
    }
    return sb.toString();
}

ArrayList<Double> j=new ArrayList<>; 
j.add(1);
j.add(.92);
j.add(3); 
String ntop=j.toString(); //ntop= "[1, 0.92, 3]" 

So basically, the String ntop stores the value of the entire collection with comma separators and brackets.

I would just use the string concatenation operator "+" to join two strings. s1 += s2;