Master Java’s StringJoiner: Clean, Efficient String Concatenation

Basic Usage

StringJoiner is designed for concatenating strings with a delimiter. For example, joining "hello", "guys" and "欢迎使用StringJoiner" with a comma.

hello,guys,欢迎使用StringJoiner

Before Java 8, this required verbose StringBuilder code:

StringBuilder sb = new StringBuilder();
sb.append("hello");
sb.append(",");
sb.append("guys");
sb.append(",");
sb.append("欢迎使用StringJoiner");
String str = sb.toString();

Using StringJoiner makes the code cleaner:

public static void main(String[] args) {
    StringJoiner stringJoiner = new StringJoiner(",");
    stringJoiner.add("hello");
    stringJoiner.add("guys");
    stringJoiner.add("欢迎使用StringJoiner");
    System.out.println(stringJoiner.toString());
}

hello,guys,欢迎使用StringJoiner

Detailed Introduction

Class diagram of StringJoiner:

Key fields:

prefix – string placed before the result

delimiter – separator between elements

suffix – string placed after the result

value – the concatenated string

emptyValue – value returned when no elements are added

Constructors:

Two constructors are provided: one with only a delimiter, and another with delimiter, prefix, and suffix. The emptyValue defaults to prefix + suffix.

Stream‑style API

The add method returns the StringJoiner itself, enabling fluent chaining similar to StringBuilder.

public static void main(String[] args) {
    StringJoiner stringJoiner = new StringJoiner(",")
        .add("hello")
        .add("guys")
        .add("欢迎使用StringJoiner");
    System.out.println(stringJoiner.toString());
}

Prefix and Suffix Concatenation

Specify prefix and suffix in the constructor:

public static void main(String[] args) {
    StringJoiner stringJoiner = new StringJoiner(",", "[", "]");
    stringJoiner.add("hello");
    stringJoiner.add("guys");
    stringJoiner.add("欢迎使用StringJoiner");
    System.out.println(stringJoiner.toString());
}

[hello,guys,欢迎使用StringJoiner]

Empty Value Handling

When no elements are added, the result depends on the empty value configuration.

public static void main(String[] args) {
    StringJoiner stringJoiner = new StringJoiner(",");
    System.out.println(stringJoiner.toString()); // prints empty string
}

With prefix and suffix:

public static void main(String[] args) {
    StringJoiner stringJoiner = new StringJoiner(",", "[", "]");
    System.out.println(stringJoiner.toString()); // prints []
}

Custom empty value via setEmptyValue:

public static void main(String[] args) {
    StringJoiner stringJoiner = new StringJoiner(",", "[", "]");
    stringJoiner.setEmptyValue("void");
    System.out.println(stringJoiner.toString()); // prints void
}

void

String.join()

Java 8 also provides String.join(), a thin wrapper around StringJoiner for simple concatenation without prefix/suffix or empty‑value handling.

java.lang.String#join(CharSequence, CharSequence...)

java.lang.String#join(CharSequence, Iterable extends CharSequence )

Source code illustration:

Example usage:

public static void main(String[] args) {
    String str = String.join(",", "hello", "guys", "欢迎使用StringJoiner");
    System.out.println(str);
}

hello,guys,欢迎使用StringJoiner

Summary

The article compares StringJoiner, StringBuilder, and String.join(), showing when to prefer each API. Use StringJoiner for repeated delimiter concatenation with optional prefix/suffix and empty‑value handling; use String.join() for simple cases.