Java StringJoiner & String.join()

Java provides convenient ways to combine multiple strings into a single string with delimiters, prefixes, or suffixes. Two common approaches are:

1. StringJoiner (introduced in Java 8)
2. String.join() (also Java 8+)

These are useful when building comma-separated lists, CSV data, or formatted output.

1. Using StringJoiner

StringJoiner allows you to join strings with a delimiter, and optionally, a prefix and suffix.

Syntax

StringJoiner joiner = new StringJoiner(delimiter);

• delimiter → String inserted between each element.
• Optional: StringJoiner joiner = new StringJoiner(delimiter, prefix, suffix);

Example – Basic Usage

import java.util.StringJoiner;

public class StringJoinerExample {
    public static void main(String[] args) {
        StringJoiner joiner = new StringJoiner(", "); // Comma delimiter
        joiner.add("Java");
        joiner.add("Python");
        joiner.add("C++");

        System.out.println(joiner.toString());
    }
}

Output:

Java, Python, C++

Example – With Prefix & Suffix

StringJoiner joiner = new StringJoiner(" | ", "[", "]");
joiner.add("Apple");
joiner.add("Banana");
joiner.add("Cherry");

System.out.println(joiner);

Output:

[Apple | Banana | Cherry]

Notes:

• add() → Adds elements.
• toString() → Returns the combined string.
• Prefix and suffix help format lists neatly.

2. Using String.join()

String.join() is a simpler alternative to join strings directly from arrays or collections.

Syntax

String joined = String.join(delimiter, elements...);

• delimiter → Separator between elements
• elements → Strings, array, or collection

Example – Joining an Array of Strings

public class StringJoinExample {
    public static void main(String[] args) {
        String[] languages = {"Java", "Python", "C++"};
        String result = String.join(", ", languages);

        System.out.println(result);
    }
}

Output:

Java, Python, C++

Example – Joining a List of Strings

import java.util.*;

public class StringJoinList {
    public static void main(String[] args) {
        List<String> fruits = Arrays.asList("Apple", "Banana", "Cherry");
        String joined = String.join(" | ", fruits);

        System.out.println(joined);
    }
}

Output:

Apple | Banana | Cherry

Notes:

• Works for arrays, lists, or any iterable collection.
• Cleaner and shorter than StringJoiner if prefix/suffix is not needed.

Points to Remember

• Both StringJoiner and String.join() require Java 8 or later.
• Use StringJoiner when you need prefix and suffix in the final string.
• Use String.join() for simple concatenation of array or list elements.
• Helps in creating CSV strings, HTML lists, or readable outputs.

🏆 Top 5 Interview Questions – StringJoiner and String.join()

1. What is the purpose of StringJoiner in Java?

Answer:

• StringJoiner is used to combine multiple strings with a delimiter, and optionally a prefix and suffix.
• Useful for creating comma-separated lists, CSV data, or formatted output.

2. How is String.join() different from StringJoiner?

Answer:

3. How do you join a list of strings using a delimiter?

Answer:

List<String> fruits = Arrays.asList("Apple", "Banana", "Cherry");
String joined = String.join(" | ", fruits);
System.out.println(joined); // Output: Apple | Banana | Cherry

4. How do you add a prefix and suffix when joining strings?

Answer:

StringJoiner joiner = new StringJoiner(", ", "[", "]");
joiner.add("Java");
joiner.add("Python");
joiner.add("C++");
System.out.println(joiner); // Output: [Java, Python, C++]

5. When should you prefer StringJoiner over String.join()?

Answer:

• Use StringJoiner when you need prefix and suffix formatting in addition to the delimiter.
• Use String.join() for simple concatenation of array or list elements without additional formatting.