Concatenate Strings in Kotlin

1. Introduction

In this short tutorial, we’ll investigate different ways of concatenating strings in Kotlin.

2. Using the plus() Method

Kotlin’s String class contains a plus() method:

operator fun plus(other: Any?): String (source)

It returns a String obtained by concatenating reference String with the String passed as an argument.

For example:

@Test
fun givenTwoStrings_concatenateWithPlusMethod_thenEquals() {
    val a = "Hello"
    val b = "Baeldung"
    val c = a.plus(" ").plus(b)

    assertEquals("Hello Baeldung", c)
}

Also, it is important to realize that if the object passed in isn’t a String, the String representation of the object will be used.

3. Using the + Operator

The simplest way of concatenating Strings in Kotlin is to use the + operator. As a result, we get a new String object composed of Strings on the left and the right side of the operator:

@Test
fun givenTwoStrings_concatenateWithPlusOperator_thenEquals() {
    val a = "Hello"
    val b = "Baeldung"
    val c = a + " " + b

    assertEquals("Hello Baeldung", c)
}

Another key point is that in Kotlin, thanks to operator overload, the + operator gets resolved to the plus() method.

In general, this is a common method for concatenating small numbers of Strings.

4. Using StringBuilder

As we know, String objects are immutable. With each concatenation using the + operator or plus() method, we get a new String object. In contrast, to avoid unnecessary String object creation, we can use a StringBuilder.

Hence, StringBuilder creates a single internal buffer that contains the final string.

Therefore, StringBuilder is more efficient when concatenating a large number of strings.

Here is a String concatenation example using StringBuilder:

@Test
fun givenTwoStrings_concatenateWithStringBuilder_thenEquals() {
    val builder = StringBuilder()
    builder.append("Hello")
           .append(" ")
           .append("Baeldung")

    assertEquals("Hello Baeldung", builder.toString())
}

Finally, we can use StringBuffer for thread-safe concatenation instead of StringBuilder.

5. Using String Templates

Kotlin also has a feature called String templates. String templates contain expressions that get evaluated to build a String.

String template expressions start with a dollar sign followed by variable’s name.

Here is an example of String concatenation using templates:

@Test
fun givenTwoStrings_concatenateWithTemplates_thenEquals() {
    val a = "Hello"
    val b = "Baeldung"
    val c = "$a $b"

    assertEquals("Hello Baeldung", c)
}

The Kotlin compiler translates this code to:

new StringBuilder().append(a).append(" ").append(b).toString()

Finally, this process is String interpolation.

6. Conclusion

In this article, we’ve learned a few ways to concatenate String objects in Kotlin.

As always, all code presented in this tutorial can be found over on GitHub.