String Joiner in Java

StringJoiner is a utility class in Java that makes it easy to concatenate strings with a delimiter, optionally including a prefix and/or a suffix. This can be especially useful when trying to generate sequences like comma-separated values.

Introduced in Java 8, StringJoiner is part of the java.util package.

1. Basic Usage

Creating a StringJoiner

You can create a StringJoiner by specifying a delimiter:

StringJoiner joiner = new StringJoiner(", "); 

Adding Values

joiner.add("Alice"); joiner.add("Bob"); joiner.add("Charlie"); 

Output

System.out.println(joiner.toString()); // Outputs: Alice, Bob, Charlie 

2. Using Prefix and Suffix

StringJoiner allows you to specify an optional prefix and suffix:

StringJoiner joiner = new StringJoiner(", ", "{", "}"); joiner.add("Alice"); joiner.add("Bob"); joiner.add("Charlie"); System.out.println(joiner.toString()); // Outputs: {Alice, Bob, Charlie} 

3. Merging StringJoiner Instances

You can merge two StringJoiner instances. This comes in handy when you have multiple sections of data to concatenate:

StringJoiner joiner1 = new StringJoiner(", "); joiner1.add("Alice"); joiner1.add("Bob"); StringJoiner joiner2 = new StringJoiner(", "); joiner2.add("Charlie"); joiner2.add("David"); joiner1.merge(joiner2); System.out.println(joiner1.toString()); // Outputs: Alice, Bob, Charlie, David 

Note: The prefix and suffix of the second StringJoiner (if specified) are ignored when merging.

4. Working with Streams

StringJoiner pairs well with Java Streams. You can use the Collectors.joining() method to collect a stream into a string, which internally uses StringJoiner:

List<String> names = Arrays.asList("Alice", "Bob", "Charlie"); String result = names.stream().collect(Collectors.joining(", ")); System.out.println(result); // Outputs: Alice, Bob, Charlie 

5. Setting Default Empty Value

When the StringJoiner has no values and it's printed, by default, it will just show the prefix and suffix. You can set a default value to display when it's empty:

StringJoiner joiner = new StringJoiner(", "); joiner.setEmptyValue("No names available."); System.out.println(joiner.toString()); // Outputs: No names available. 

6. Practical Application

StringJoiner can be particularly useful when constructing strings for:

• CSV Files
• Generating SQL queries
• Creating human-readable lists
• Building JSON or XML-like structured data manually

Conclusion

StringJoiner offers an elegant way to handle string concatenation with delimiters in Java. Its compatibility with streams, prefix/suffix handling, and merging capabilities make it a powerful tool in a developer's toolkit. Always consider using StringJoiner when faced with constructing complex string sequences.

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