Java string join to string

Question

Assume i have List Collection of Strings and i want to loop by this List and add each element of List to some variable of type String.

List<String> words;
//assume words have 5 elements
String summary;//variable where i want to keep all elements

for (String word : words){
    //here i want to add new word to the variable summary
}

As i know java always creates new object of String. Even if i try to change value - new object will be created anyway, am i right?

So here is a question how to join all elements of List in one variable?

docs.oracle.com/javase/7/docs/api/java/lang/StringBuilder.html — JB Nizet
– JB Nizet, Commented Aug 10, 2014 at 17:28

Jeanne Boyarsky · Accepted Answer · 2014-08-10 17:28:51Z

3

On any version of Java: Apache Commons has a class StringUtils that has a join method:

String result = StringUtils.join(words, ",")

On Java 8, you can do this natively. See this article.

answered Aug 10, 2014 at 17:28

Jeanne Boyarsky

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Comments

Eran · Accepted Answer · 2014-08-10 17:28:15Z

1

use a StringBuilder to join all the words.

List<String> words;
//assume words have 5 elements
StringBuilder summary = new StringBuilder();

for (String word : words){
    summary.append(word);
}

Finally, get the joined String with summary.toString().

Note : If you have an idea of the number of characters that would be appended to the StringBuilder, it will be more efficient to use the constructor that gets an initial size :

summary = new StringBuilder(size);.

answered Aug 10, 2014 at 17:28

Eran

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Comments

pascalh · Accepted Answer · 2014-08-10 17:28:41Z

1

I think the easiest solution would be to use a StringBuilder:

String summary;
List<String> words = ...;
StringBuilder builder = new StringBuilder();
for (String word : words) {
    builder.append(word);
}

summary = builder.toString();

answered Aug 10, 2014 at 17:28

pascalh

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Comments

Óscar López · Accepted Answer · 2014-08-10 17:30:16Z

1

The simplest way would be to use an existing functionality, for example Apache Common's StringUtils.join(). If that's not possible, this will work:

    StringBuilder sb = new StringBuilder();
    for (String word : words) {
        sb.append(word);
    }
    String summary = sb.toString();

answered Aug 10, 2014 at 17:30

Óscar López

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Aditya · Accepted Answer · 2014-08-10 17:36:38Z

1

You can try the following code,

List<String> words;
String summary = null;

for (String word : words)
{
    summary = summary + word + " , ";
}
System.out.println("List items : " + summary);

answered Aug 10, 2014 at 17:36

Aditya

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1 Comment

laune Over a year ago

Inefficient due to repeated creations of a String object.

nickool · Accepted Answer · 2016-11-28 21:21:06Z

1

An alternative to Apache commons (StringUtils) is Guava's Joiner.

For example:

 List<String> words = new ArrayList<>();
 words.add("word");
 words.add("anotherWord");

 String joinedWords = Joiner.on(",").join(words);

This may also be useful if you're not able to use Java 8.

See wiki: https://github.com/google/guava/wiki/StringsExplained

answered Nov 28, 2016 at 21:21

nickool

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Comments

Lars Kakavandi-Nielsen · Accepted Answer · 2014-08-10 17:29:39Z

0

As Eran suggest or just, use simple concatination

List<String> words;
//assume words have 5 elements
String summary = "";//variable where i want to keep all elements

for (String word : words){
    summary = summary + word;
}

answered Aug 10, 2014 at 17:29

Lars Kakavandi-Nielsen

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1 Comment

Óscar López Over a year ago

This is terribly inefficient, it'll create intermediate strings and will end up being O(n^2) (see: Schlemiel the Painter's algorithm). Use a StringBuilder instead, as shown in my answer.

tale852150 · Accepted Answer · 2014-08-10 17:31:51Z

0

Have you looked at

StringUtils.join()

as a possible solution?

answered Aug 10, 2014 at 17:31

tale852150

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laune · Accepted Answer · 2014-08-10 17:45:13Z

0

String summary = words.stream().collect( Collectors.joining() );

answered Aug 10, 2014 at 17:45

laune

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KriszDev · Accepted Answer · 2014-08-10 17:32:12Z

-1

You can simply add.

summary += word;

answered Aug 10, 2014 at 17:32

KriszDev

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1 Comment

laune Over a year ago

summary is initially null: you can't do that.

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