How to Join Strings in Java: Simple Methods and Examples

In Java, you can join strings using the + operator, the String.concat() method, or the String.join() method for multiple strings. The String.join() method is useful to join many strings with a delimiter easily.

Syntax

Here are common ways to join strings in Java:

• String result = str1 + str2; - uses the plus operator to join two strings.
• String result = str1.concat(str2); - calls the concat method on a string.
• String result = String.join(delimiter, str1, str2, ...); - joins multiple strings with a delimiter.

java

String result1 = "Hello" + " World";
String result2 = "Hello".concat(" World");
String result3 = String.join(", ", "Apple", "Banana", "Cherry");

Example

This example shows how to join strings using the plus operator, concat method, and String.join method.

java

public class JoinStringsExample {
    public static void main(String[] args) {
        String a = "Hello";
        String b = "World";

        // Using + operator
        String joined1 = a + " " + b;

        // Using concat method
        String joined2 = a.concat(" ").concat(b);

        // Using String.join method
        String joined3 = String.join(", ", "Apple", "Banana", "Cherry");

        System.out.println(joined1);
        System.out.println(joined2);
        System.out.println(joined3);
    }
}

Output

Hello World
Hello World
Apple, Banana, Cherry

Common Pitfalls

Common mistakes when joining strings include:

• Using + operator in loops, which creates many temporary strings and slows performance.
• Forgetting to add spaces or delimiters between strings, causing words to run together.
• Using concat() with null values, which throws NullPointerException.

Use StringBuilder for joining strings in loops for better performance.

java

public class PitfallExample {
    public static void main(String[] args) {
        String[] words = {"Java", "is", "fun"};

        // Inefficient way (creates many strings)
        String sentence = "";
        for (String word : words) {
            sentence += word + " ";
        }
        System.out.println(sentence.trim());

        // Better way using StringBuilder
        StringBuilder sb = new StringBuilder();
        for (String word : words) {
            sb.append(word).append(" ");
        }
        System.out.println(sb.toString().trim());
    }
}

Output

Java is fun
Java is fun

Quick Reference

Summary of string joining methods in Java:

Method	Usage	Notes
+ operator	str1 + str2	Simple for few strings, not efficient in loops
String.concat()	str1.concat(str2)	Joins two strings, throws NullPointerException if argument is null
String.join()	String.join(", ", str1, str2, ...)	Joins multiple strings with delimiter, introduced in Java 8
StringBuilder	Use append() in loops	Efficient for joining many strings or in loops

Key Takeaways

Use + operator or concat() for simple string joining.

Use String.join() to join multiple strings with a delimiter easily.

Avoid using + operator inside loops; prefer StringBuilder for better performance.

Remember to handle null values to avoid exceptions.

String.join() requires Java 8 or newer.