Kotlin and Java are both influential languages in the world of software development, particularly in Android app development. Here’s an overview of their key differences:

1. Null Safety

  • Kotlin:
    • Designed with null safety in mind.
    • Variables are non-null by default.
    • Can explicitly declare a variable as nullable using ?.
    • Example: var a: String? = null
  • Java:
    • Null references can be a source of errors.
    • Requires more boilerplate code to handle nulls safely.

2. Immutable Collections

  • Kotlin:
    • Distinction between mutable and immutable collections.
    • Promotes the use of immutable collections by default.
  • Java:
    • No native support for immutable collections in earlier versions (prior to Java 9).
    • Mutable collections are standard.

3. Extension Functions

  • Kotlin:
    • Allows adding new functions to existing classes without modifying their code.
    • Example: fun String.myCustomFunction() { ... }
  • Java:
    • No native support for extension functions.
    • Similar functionality achieved through utility classes or design patterns like Decorator.

4. Top-Level Functions and Properties

  • Kotlin:
    • Supports functions and properties outside of class definitions (top-level).
    • Offers better organization and reduces unnecessary boilerplate.
  • Java:
    • Requires that functions and variables be part of a class.
    • Often leads to the creation of utility classes with static methods.

5. Data Classes

  • Kotlin:
    • data keyword to easily create classes meant for holding data.
    • Automatically provides equals(), hashCode(), toString(), and copy() methods.
  • Java:
    • Requires manual implementation of data handling methods.
    • Java 16 introduced record classes which offer similar functionality.

6. Syntax Differences

  • Kotlin:
    • More concise syntax.
    • Example: No need for semicolons at the end of each statement.
    • Type inference reduces the need to explicitly specify types.
  • Java:
    • More verbose syntax.
    • Requires explicit type declarations more often.

7. Lambda Expressions and Higher-Order Functions

  • Kotlin:
    • Native support for lambda expressions and higher-order functions.
    • Makes functional programming style more natural and concise.
  • Java:
    • Lambda expressions added in Java 8.
    • Higher-order functions achievable but less intuitive.

8. Coroutines

  • Kotlin:
    • Inbuilt support for coroutines for asynchronous programming.
    • Lightweight and efficient compared to Java threads.
  • Java:
    • Relies on threads for asynchronous programming.
    • Can use libraries like CompletableFutures, but more cumbersome than Kotlin coroutines.

9. Copy Function in Data Classes

  • Kotlin:
    • copy() function in data classes to easily create modified copies.
  • Java:
    • No native equivalent. Requires manual implementation.

10. Functional Programming

  • Kotlin:
    • Embraces functional programming features like first-class functions, immutability.
  • Java:
    • Functional programming introduced in later versions, not as idiomatic as in Kotlin.

11. Smart Casting

  • Kotlin:
    • Automatic casting of types if they have been checked in control structures.
  • Java:
    • Requires explicit casting.

12. Checked Exceptions

  • Kotlin:
    • Does not have checked exceptions.
    • Leads to cleaner and more concise error handling.
  • Java:
    • Requires catching or declaring checked exceptions.
    • Can lead to verbose error handling code.