How do I concatenate strings

In this Q&A, we'll go over the options within Java to concatenate two or more Strings.  Please note that String class is immutable.  All options create a new character buffer and copy characters,

Option I - Use + operator

Java provides a special + operator to concatenate Strings.

Option II - Use StringBuilder

StringBuilder is a mutable character set.  One can append character sets to the buffer and eventually create a string object using toString() method.

Option III - Use StringBuffer

StringBuffer is similar to StringBuilder.  Its a synchronized, thread-safe cousin of StringBuilder

Option IV - Using String.concat

String class provides a method to concatenate two strings.  It creates a new buffer, concatenates two character buffers and returns a new String object

Option V and VI - Using String.join method or StringJoiner class

String.join() method and StringJoiner class provide functionality to concatenate Strings or Array of Strings along with a delimiter.

Most roads lead to StringBuilder

In Java 8, in addition to option II, options I, V, VI eventually use StringBuilder to concatenate character buffers.  Options I, V, VI add a small layer of functionality over StringBuilder.

Demo code

package com.javahowdoi.string;

import java.util.StringJoiner;

import static junit.framework.TestCase.assertEquals;

public class ConcatDemo {

    public static void main(String[] args) {

        String s1 = "hello";

        String s2 = "world";

        // Option I

        String s3 = s1 + s2;

        assertEquals("helloworld match", "helloworld", s3 );

        // Option II

        StringBuilder sb = new StringBuilder(32);

        s3 = sb.append(s1).append(s2).toString();

        assertEquals("helloworld match", "helloworld", s3 );

        // Option III

        StringBuffer sb1 = new StringBuffer();

        s3 = sb1.append(s1).append(s2).toString();

        assertEquals("helloworld match", "helloworld", s3 );

        // Option IV

        s3 = s1.concat(s2);

        assertEquals("helloworld match", "helloworld", s3 );

        // Option V

        s3 = String.join("", s1, s2);

        String[] l = {s1, s2};

        assertEquals("helloworld match", "helloworld", s3 );

        // Option VI

        StringJoiner j = new StringJoiner("", s1, s2);

        s3 = j.toString();

        assertEquals("helloworld match", "helloworld", s3 );

    }

}