Kotlin Onboarding Guide for Java Developers

Introduction

What is Kotlin?

Kotlin is a modern, statically-typed programming language developed by JetBrains. It runs on the JVM (Java Virtual Machine) and is fully interoperable with Java. Kotlin enhances developer productivity by reducing boilerplate code, improving null safety, and providing powerful features like coroutines and extension functions. It is widely used for backend and Android development.

Why Use Kotlin?

• Concise syntax
• Null safety to prevent NullPointerException
• Coroutines for efficient async programming
• Seamless Java interoperability
• Functional programming support
• Official support by Google for Android

Java vs. Kotlin: Key Differences

Kotlin with Spring Boot

Spring Boot provides first-class support for Kotlin. The framework offers the same capabilities as in Java but with a more concise syntax.

Spring Boot + Java Example

<span>@RestController</span>
<span>@RequestMapping</span><span>(</span><span>"/hello"</span><span>)</span>
<span>public</span> <span>class</span> <span>HelloController</span> <span>{</span>
    <span>@GetMapping</span>
    <span>public</span> <span>String</span> <span>sayHello</span><span>()</span> <span>{</span>
        <span>return</span> <span>"Hello, Java + Spring Boot!"</span><span>;</span>
    <span>}</span>
<span>}</span>
<span>@RestController</span>
<span>@RequestMapping</span><span>(</span><span>"/hello"</span><span>)</span>
<span>public</span> <span>class</span> <span>HelloController</span> <span>{</span>
    <span>@GetMapping</span>
    <span>public</span> <span>String</span> <span>sayHello</span><span>()</span> <span>{</span>
        <span>return</span> <span>"Hello, Java + Spring Boot!"</span><span>;</span>
    <span>}</span>
<span>}</span>
@RestController
@RequestMapping("/hello")
public class HelloController {
    @GetMapping
    public String sayHello() {
        return "Hello, Java + Spring Boot!";
    }
}

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Spring Boot + Kotlin Example

<span>@RestController</span>
<span>@RequestMapping</span><span>(</span><span>"/hello"</span><span>)</span>
<span>class</span> <span>HelloController</span> <span>{</span>
    <span>@GetMapping</span>
    <span>fun</span> <span>sayHello</span><span>():</span> <span>String</span> <span>=</span> <span>"Hello, Kotlin + Spring Boot!"</span>
<span>}</span>
<span>@RestController</span>
<span>@RequestMapping</span><span>(</span><span>"/hello"</span><span>)</span>
<span>class</span> <span>HelloController</span> <span>{</span>
    <span>@GetMapping</span>
    <span>fun</span> <span>sayHello</span><span>():</span> <span>String</span> <span>=</span> <span>"Hello, Kotlin + Spring Boot!"</span>
<span>}</span>
@RestController
@RequestMapping("/hello")
class HelloController {
    @GetMapping
    fun sayHello(): String = "Hello, Kotlin + Spring Boot!"
}

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Key Benefits

• Less boilerplate code
• Improved null safety
• Coroutines for async programming
• Data classes for DTOs

Migrating from Java to Kotlin

Migration Strategy

1. Convert Java to Kotlin: Use IntelliJ IDEA (Code → Convert Java File to Kotlin).
2. Refactor Converted Code: Optimize syntax, remove redundant elements.
3. Leverage Kotlin Features: Utilize coroutines, extension functions, and data classes.
4. Gradual Migration: Mix Java and Kotlin before full migration.

Kotlin Key Features

Data Classes

<span>data class</span> <span>Person</span><span>(</span><span>val</span> <span>name</span><span>:</span> <span>String</span><span>,</span> <span>val</span> <span>age</span><span>:</span> <span>Int</span><span>)</span>
<span>data class</span> <span>Person</span><span>(</span><span>val</span> <span>name</span><span>:</span> <span>String</span><span>,</span> <span>val</span> <span>age</span><span>:</span> <span>Int</span><span>)</span>
data class Person(val name: String, val age: Int)

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Null Safety

<span>var</span> <span>name</span><span>:</span> <span>String</span><span>?</span> <span>=</span> <span>null</span> <span>// Nullable type</span>
<span>println</span><span>(</span><span>name</span><span>?.</span><span>length</span><span>)</span>  <span>// Safe call operator</span>
<span>var</span> <span>name</span><span>:</span> <span>String</span><span>?</span> <span>=</span> <span>null</span> <span>// Nullable type</span>
<span>println</span><span>(</span><span>name</span><span>?.</span><span>length</span><span>)</span>  <span>// Safe call operator</span>
var name: String? = null // Nullable type
println(name?.length)  // Safe call operator

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Coroutines for Async Tasks

<span>suspend</span> <span>fun</span> <span>fetchData</span><span>()</span> <span>{</span>
    <span>withContext</span><span>(</span><span>Dispatchers</span><span>.</span><span>IO</span><span>)</span> <span>{</span>
        <span>// Fetch data from network or DB</span>
    <span>}</span>
<span>}</span>
<span>suspend</span> <span>fun</span> <span>fetchData</span><span>()</span> <span>{</span>
    <span>withContext</span><span>(</span><span>Dispatchers</span><span>.</span><span>IO</span><span>)</span> <span>{</span>
        <span>// Fetch data from network or DB</span>
    <span>}</span>
<span>}</span>
suspend fun fetchData() {
    withContext(Dispatchers.IO) {
        // Fetch data from network or DB
    }
}

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Extension Functions

<span>fun</span> <span>String</span><span>.</span><span>reverse</span><span>():</span> <span>String</span> <span>=</span> <span>this</span><span>.</span><span>reversed</span><span>()</span>
<span>println</span><span>(</span><span>"Kotlin"</span><span>.</span><span>reverse</span><span>())</span> <span>// Output: niltoK</span>
<span>fun</span> <span>String</span><span>.</span><span>reverse</span><span>():</span> <span>String</span> <span>=</span> <span>this</span><span>.</span><span>reversed</span><span>()</span>
<span>println</span><span>(</span><span>"Kotlin"</span><span>.</span><span>reverse</span><span>())</span> <span>// Output: niltoK</span>
fun String.reverse(): String = this.reversed()
println("Kotlin".reverse()) // Output: niltoK

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Default & Named Arguments

<span>fun</span> <span>greet</span><span>(</span><span>name</span><span>:</span> <span>String</span> <span>=</span> <span>"User"</span><span>)</span> <span>{</span>
    <span>println</span><span>(</span><span>"Hello, $name"</span><span>)</span>
<span>}</span>
<span>greet</span><span>()</span> <span>// Output: Hello, User</span>
<span>greet</span><span>(</span><span>"Lara"</span><span>)</span> <span>// Output: Hello, Lara</span>
<span>fun</span> <span>greet</span><span>(</span><span>name</span><span>:</span> <span>String</span> <span>=</span> <span>"User"</span><span>)</span> <span>{</span>
    <span>println</span><span>(</span><span>"Hello, $name"</span><span>)</span>
<span>}</span>
<span>greet</span><span>()</span> <span>// Output: Hello, User</span>
<span>greet</span><span>(</span><span>"Lara"</span><span>)</span> <span>// Output: Hello, Lara</span>
fun greet(name: String = "User") {
    println("Hello, $name")
}
greet() // Output: Hello, User
greet("Lara") // Output: Hello, Lara

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Setting Up a Kotlin Spring Boot Project

Using Spring Initializr

1. Go to Spring Initializr
2. Select Language: Kotlin
3. Add dependencies: Spring Web, Spring Data JPA, etc.
4. Generate the project and start coding!

Manually Creating a Kotlin Spring Boot App

spring init <span>--name</span><span>=</span>my-kotlin-app <span>--language</span><span>=</span>kotlin
spring init <span>--name</span><span>=</span>my-kotlin-app <span>--language</span><span>=</span>kotlin
spring init --name=my-kotlin-app --language=kotlin

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Best Practices

• Use val instead of var whenever possible
• Utilize data classes for DTOs
• Prefer extension functions for reusable logic
• Use coroutines for async programming
• Follow Kotlin idiomatic coding conventions

Conclusion

Kotlin is a powerful and modern alternative to Java that enhances productivity, reduces boilerplate, and improves safety. For Java developers working with Spring Boot, Kotlin is an easy transition offering better syntax, null safety, and improved async handling.

Resources

• Kotlin Docs: https://kotlinlang.org/docs/home.html
• Kotlin Playground: https://play.kotlinlang.org/
• Kotlin for Java Developers Course: https://play.kotlinlang.org/koans/overview
• JetBrains YouTube: https://www.youtube.com/c/JetBrainsTV

原文链接：Kotlin Onboarding Guide for Java Developers