How to Concatenate Strings in Java: Simple Guide

In Java, you can concatenate strings using the + operator or the concat() method. For better performance when joining many strings, use StringBuilder with its append() method.

Syntax

There are three 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.
• StringBuilder sb = new StringBuilder(); sb.append(str1).append(str2); String result = sb.toString(); uses StringBuilder for efficient concatenation.

java

String result1 = "Hello" + " World";
String result2 = "Hello".concat(" World");
StringBuilder sb = new StringBuilder();
sb.append("Hello").append(" World");
String result3 = sb.toString();

Example

This example shows how to concatenate strings using the plus operator, concat() method, and StringBuilder. It prints the results to the console.

java

public class StringConcatExample {
    public static void main(String[] args) {
        String str1 = "Hello";
        String str2 = "World";

        // Using + operator
        String result1 = str1 + " " + str2;

        // Using concat() method
        String result2 = str1.concat(" ").concat(str2);

        // Using StringBuilder
        StringBuilder sb = new StringBuilder();
        sb.append(str1).append(" ").append(str2);
        String result3 = sb.toString();

        System.out.println(result1);
        System.out.println(result2);
        System.out.println(result3);
    }
}

Output

Hello World
Hello World
Hello World

Common Pitfalls

One common mistake is using the + operator repeatedly in loops, which creates many temporary strings and slows down the program. Instead, use StringBuilder for better performance.

Also, concat() only accepts one string argument, so chaining multiple concatenations requires multiple calls.

java

public class WrongConcat {
    public static void main(String[] args) {
        String result = "";
        for (int i = 0; i < 3; i++) {
            // Inefficient: creates new string each loop
            result = result + i;
        }
        System.out.println(result);

        // Better way:
        StringBuilder sb = new StringBuilder();
        for (int i = 0; i < 3; i++) {
            sb.append(i);
        }
        System.out.println(sb.toString());
    }
}

Output

012
012

Quick Reference

Use this quick guide to choose the right concatenation method:

Method	Usage	Best for
+ operator	Simple joining of few strings	Quick and readable code
concat()	Joining two strings	When chaining few strings explicitly
StringBuilder	Joining many strings or in loops	Efficient and fast concatenation

Key Takeaways

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

Prefer StringBuilder when concatenating strings inside loops for better performance.

concat() only accepts one string argument at a time.

Repeated + operations in loops create many temporary strings and slow down your program.

StringBuilder's append() method is efficient and recommended for multiple concatenations.