String Data Type in Java and Kotlin (Immutability, Null Safety, Examples)

Introduction

The String data type is used to store a sequence of characters (text). Strings are one of the most commonly used data types in both Java and Kotlin, especially for handling user input, messages, and data processing.

String in Java

In Java, String is a class, not a primitive type. Strings in Java are immutable, meaning once created, their value cannot be changed.

  • Stored as objects in the heap
  • Immutable by design
  • Supports String Pool for memory optimization

Java Example


String name = "CodeCrush";
name = name + " Corner";

System.out.println(name);

Although it looks like the string is modified, Java actually creates a new String object.

String in Kotlin

In Kotlin, String is also a class and is immutable. Kotlin adds powerful features such as string templates and null safety.

  • Immutable by default
  • Supports string templates using $
  • Nullable and non-nullable strings

Kotlin Example


val name = "CodeCrush"
val site = "$name Corner"

println(site)

Nullable String Example


var text: String? = null
println(text?.length)

String Length & Access

Java


String message = "Hello";
System.out.println(message.length());
System.out.println(message.charAt(0));

Kotlin


val message = "Hello"
println(message.length)
println(message[0])

Java vs Kotlin – String Differences

Feature Java Kotlin
Type Class Class
Immutability Yes Yes
String Templates No Yes ($variable)
Null Safety No Yes
Character Access charAt() [] operator

Interview Questions & Answers

Q1. Are strings mutable in Java and Kotlin?

Answer: No. Strings are immutable in both Java and Kotlin, meaning their value cannot be changed once created.

Q2. What is string immutability?

Answer: Once a string is created, its value cannot be changed. Any modification creates a new object.

Q3. What are string templates in Kotlin?

Answer: They allow embedding variables directly inside strings using $ or ${expression}.

Q4. How does Kotlin handle null strings safely?

Answer: Kotlin uses nullable types (String?) and safe-call operators (?.) to prevent NullPointerException.

Conclusion

Strings are essential for almost every application. While Java and Kotlin both treat strings as immutable, Kotlin improves safety and readability with null safety and string templates, making string handling simpler and safer.

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