Java: convert List to a String

Question

JavaScript has Array.join()

js>[\"Bill\",\"Bob\",\"Steve\"].join(\" and \")
Bill and Bob and Steve

Does Java have anything

Answer 1

With Java 8 you can do this without any third party library.

If you want to join a Collection of Strings you can use the new String.join() method:

List<String> list = Arrays.asList("foo", "bar", "baz");
String joined = String.join(" and ", list); // "foo and bar and baz"

If you have a Collection with another type than String you can use the Stream API with the joining Collector:

List<Person> list = Arrays.asList(
  new Person("John", "Smith"),
  new Person("Anna", "Martinez"),
  new Person("Paul", "Watson ")
);

String joinedFirstNames = list.stream()
  .map(Person::getFirstName)
  .collect(Collectors.joining(", ")); // "John, Anna, Paul"

The StringJoiner class may also be useful.

Answer 2

No, there's no such convenience method in the standard Java API.

Not surprisingly, Apache Commons provides such a thing in their StringUtils class in case you don't want to write it yourself.

Answer 3

Code you have is right way to do it if you want to do using JDK without any external libraries. There is no simple "one-liner" that you could use in JDK.

If you can use external libs, I recommend that you look into org.apache.commons.lang.StringUtils class in Apache Commons library.

An example of usage:

List<String> list = Arrays.asList("Bill", "Bob", "Steve");
String joinedResult = StringUtils.join(list, " and ");

Answer 4

EDIT

I also notice the toString() underlying implementation issue, and about the element containing the separator but I thought I was being paranoid.

Since I've got two comments on that regard, I'm changing my answer to:

static String join( List<String> list , String replacement  ) {
    StringBuilder b = new StringBuilder();
    for( String item: list ) { 
        b.append( replacement ).append( item );
    }
    return b.toString().substring( replacement.length() );
}

Which looks pretty similar to the original question.

So if you don't feel like adding the whole jar to your project you may use this.

I think there's nothing wrong with your original code. Actually, the alternative that everyone's is suggesting looks almost the same ( although it does a number of additional validations )

Here it is, along with the Apache 2.0 license.

public static String join(Iterator iterator, String separator) {
    // handle null, zero and one elements before building a buffer
    if (iterator == null) {
        return null;
    }
    if (!iterator.hasNext()) {
        return EMPTY;
    }
    Object first = iterator.next();
    if (!iterator.hasNext()) {
        return ObjectUtils.toString(first);
    }

    // two or more elements
    StringBuffer buf = new StringBuffer(256); // Java default is 16, probably too small
    if (first != null) {
        buf.append(first);
    }

    while (iterator.hasNext()) {
        if (separator != null) {
            buf.append(separator);
        }
        Object obj = iterator.next();
        if (obj != null) {
            buf.append(obj);
        }
    }
    return buf.toString();
}

Now we know, thank you open source

Answer 5

An orthodox way to achieve it, is by defining a new function:

public static String join(String joinStr, String... strings) {
    if (strings == null || strings.length == 0) {
        return "";
    } else if (strings.length == 1) {
        return strings[0];
    } else {
        StringBuilder sb = new StringBuilder(strings.length * 1 + strings[0].length());
        sb.append(strings[0]);
        for (int i = 1; i < strings.length; i++) {
            sb.append(joinStr).append(strings[i]);
        }
        return sb.toString();
    }
}

Sample:

String[] array = new String[] { "7, 7, 7", "Bill", "Bob", "Steve",
        "[Bill]", "1,2,3", "Apple ][","~,~" };

String joined;
joined = join(" and ","7, 7, 7", "Bill", "Bob", "Steve", "[Bill]", "1,2,3", "Apple ][","~,~");
joined = join(" and ", array); // same result

System.out.println(joined);

Output:

7, 7, 7 and Bill and Bob and Steve and [Bill] and 1,2,3 and Apple ][ and ~,~

Answer 6

Java 8 does bring the

Collectors.joining(CharSequence delimiter, CharSequence prefix, CharSequence suffix)

method, that is nullsafe by using prefix + suffix for null values.

It can be used in the following manner:

String s = stringList.stream().collect(Collectors.joining(" and ", "prefix_", "_suffix"))

The Collectors.joining(CharSequence delimiter) method just calls joining(delimiter, "", "") internally.