1. Inline Classes — Compact Abstractions

Example — Inline classes allow us to create lightweight wrappers around primitive types without runtime overhead.

Inline Class Example

Explanation — In this example, we define an inline class Meter that wraps a Double value. The toCentimeter function showcases how inline classes can provide specialized functionality without introducing performance overhead.

2. Type Aliases — Enhancing Readability

Example — Type aliases allow us to create meaningful names for existing types, improving code comprehension.

Type Alias Example

Explanation — Here, we create a type alias EmployeeId for the String type. This enhances readability and clarifies the purpose of the employeeId parameter.

3. Contracts — Unlocking Smarter In-lining

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Example — Contracts in Kotlin allow us to define assumptions about functions, enabling the compiler to optimize function calls.

Kotlin Contracts Example

Explanation — Above block of code demonstrates the usage of Kotlin Contracts for improving code correctness and safety by expressing preconditions and post conditions that are enforced by the Kotlin compiler at compile-time. Contracts are annotations that help describe the expectations and guarantees of functions.

We a simple data class Request defined. It holds a single property arg of type String.

The validate function takes a nullable Request object as an argument. Within this function:

  • A contract block is defined using contract { ... }. This block specifies a contract for the function.
  • returns() specifies that the contract relates to the return value of the function.
  • implies is a clause that defines a logical implication. In this case, it means that if the function returns, then the condition (request != null) must hold true.

If the condition (request != null) is not satisfied (i.e., if the request is null), the function throws an IllegalArgumentException.

In the main function:

  • A new Request object named request is created.
  • The validate function is called with this request object as an argument.
  • Since the request is not null, the contract condition is satisfied, and the function proceeds to the println statement, printing the value of request.arg.

Additionally, there are comments in the code indicating that you can uncomment some lines to see how the code behaves when the contract is violated.

By using the contract feature, you’re providing the compiler with additional insights about the code’s behavior, which can lead to improved performance and safety in your Kotlin programs.

4. Sealed Classes — A Hierarchical Approach to Enums

Example — Sealed classes provide a hierarchical approach to defining enums, allowing us to represent complex states with their own data.

Sealed Class Example

Explanation — In this example, we define a sealed class Result that has two sub-classes — Success and Error. The handleResult function uses a when expression to handle instances of Result and provides custom behavior for each case.

5. Delegated Properties — Property Management Made Elegant

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Example — Delegated properties enable us to manage properties with reusable behavior, enhancing code readability and modularity.

Delegated Property Example

Explanation — In this example, we have a Temperature class with a property called value. Instead of directly storing the temperature value, we use a delegated property to manage it. The delegated property is an instance of the ObservableProperty class.

  1. The ObservableProperty class defines two important operators — getValue and setValue. These operators handle the behavior of getting and setting the property value.
  2. When the property is accessed (get), the getValue operator is invoked. It prints a message indicating that the property is being retrieved and returns the current value stored in the currentValue variable.
  3. When the property is assigned (set), the setValue operator is invoked. It prints a message indicating that the property is being set to a new value, and then updates the currentValue variable with the new value.

In the main function, we create an instance of the Temperature class. When we access and update the value property, the delegated property’s behavior defined in the ObservableProperty class is executed. This provides a clean way to encapsulate the property’s behavior and enables us to manage properties with reusable logic.

By using delegated properties, you can achieve elegant and modular property management, leading to improved code organization and maintainability.

6. Pattern Matching with When Expressions

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Example — Kotlin’s when expression allows for powerful pattern matching and branching.

When Expression Example

Explanation — In this example, we define a sealed class Animal with sub-classes Dog , Cat and Lion. The soundOfAnimal function uses when expressions to match the type of animal and provide corresponding sounds.

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