Java.Java8.What is StringJoiner?

✅ What is StringJoiner?

StringJoiner is a class in java.util that allows you to construct a sequence of characters separated by a delimiter, optionally with a prefix and suffix.

Think of it like a flexible alternative to StringBuilder when you’re building things like CSV rows, human-readable lists, or HTML attributes.

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📦 Package:

import java.util.StringJoiner;

🧠 Basic Syntax:

StringJoiner joiner = new StringJoiner(", ");
joiner.add("Java");
joiner.add("Python");
joiner.add("C++");

System.out.println(joiner.toString()); // Output: Java, Python, C++

✅ Constructor Options

1. Delimiter only

new StringJoiner(", ");

2. Delimiter, prefix, and suffix

new StringJoiner(", ", "[", "]");

StringJoiner j = new StringJoiner(", ", "[", "]");
j.add("red").add("green").add("blue");
System.out.println(j); // Output: [red, green, blue]

💡 Methods

Method	Description
.add(CharSequence element)	Adds a new element to the joiner
.toString()	Returns the final joined string
.merge(StringJoiner other)	Appends all elements from another joiner (ignores prefix/suffix of the other)
.setEmptyValue(String emptyValue)	Sets the string to return when no elements have been added yet

🧠 Example with .setEmptyValue()

StringJoiner sj = new StringJoiner(", ");
sj.setEmptyValue("Nothing to join");
System.out.println(sj); // Output: Nothing to join

🔁 Example with .merge()

StringJoiner j1 = new StringJoiner(", ");
j1.add("apple").add("banana");

StringJoiner j2 = new StringJoiner(", ");
j2.add("cherry").add("date");

j1.merge(j2);
System.out.println(j1); // Output: apple, banana, cherry, date

✅ When to Use StringJoiner

• When building CSV strings
• When formatting output with custom prefixes/suffixes
• When combining strings in a loop
• Under the hood, it’s used by Collectors.joining() in the Stream API

List<String> names = List.of("Alice", "Bob", "Carol");
String result = names.stream().collect(Collectors.joining(", ", "[", "]"));
System.out.println(result); // Output: [Alice, Bob, Carol]

🔚 Summary

Feature	StringJoiner
📌 Introduced in	Java 8
🔧 Used for	Building delimited strings with optional prefix/suffix
🧠 Advantages	Clean, flexible, efficient for joining strings
📚 Alternatives	StringBuilder, Collectors.joining()

📦 Internally:

Under the hood, StringJoiner uses a StringBuilder internally to store and concatenate characters.

public final class StringJoiner {
    private final String prefix;
    private final String delimiter;
    private final String suffix;
    private String emptyValue;
    private StringBuilder value; // <--- THIS IS KEY
}

So it inherits the same memory efficiency benefits as StringBuilder.