Android：Kotlin与Java比较

一、Kotlin优势

1.1 优雅的解决Java空指针问题

1.1.1 编译期进行空指针检查

编译器静态分析并警告或编译Error：

1.1.2 精简防空判断代码

Java代码：

if(a != null && a.b != null && a.b.c != null) { 
    String user = a.b.c.d; } 

Kotlin代码：

val user = a?.b?.c?.d 

1.2 代码精简

1.2.1 精简Set/Get方法

Java代码：

public class User { 
    private String name; public String getName() { 
    return name; } public void setName(String name) { 
    this.name = name; } } 

Kotlin代码：

class User { 
    var name : String? = null } 

user.setName("Java"); String name = user.getName(); 

Kotlin代码：

user.name = "Kotlin" String name = user.name 

1.2.2 精简属性调用

Java代码：

RequestBean requestBean = new RequestBean(); requestBean.method = AppService.METHOD_CASHIER; requestBean.cartId = mCartId; requestBean.merchantOrderInfo = ""; requestBean.merchantOrderId = ""; 

Kotlin代码：

val requestBean = RequestBean().apply { 
    method = AppService.METHOD_CASHIER cartId = mCartId merchantOrderInfo = "" merchantOrderId = "" } 

1.2.3 精简单例模式

Java示例：

public class SingletonDemo { 
    private static SingletonDemo instance = new SingletonDemo(); private SingletonDemo() { 
    } public static SingletonDemo getInstance() { 
    return instance; } public String getTAG() { 
    return "Java"; } } 

Kotlin示例：

object SingletonDemo { 
    fun getTAG(): String { 
    return "Kotlin" } } 

二、从Java到Kotlin平滑过渡

2.1 Kotlin开发环境搭建

2.1.1 若为新项目，则IDE创建时选择Kotlin

2.1.2 若已有工程为全Java开发，可新建一个Kotlin类文件，IDE会自动配置Kotlin开发环境

2.1.3 Gradle中添加kotlin支持

project build.gradle添加支持：

classpath "org.jetbrains.kotlin:kotlin-gradle-plugin:$kotlin_version" 

module build.gradle添加支持：

implementation "org.jetbrains.kotlin:kotlin-stdlib-jdk7:$kotlin_version" 

2.2 Kotlin与Java混编及互操作

2.2.1 Kotlin调用Java

public class User { 
    private String name; public String getName() { 
    return name; } public void setName(String name) { 
    this.name = name; } public int getAge() { 
    return 10; } } 

Kotlin示例：

val user = User() user.name = "Kotlin" println(user.name) println(user.age) 

public class JDPayDeviceUtil { 
    public static String getVersionName(Context context, String packageName) { 
    PackageInfo info = getPackageInfo(context, packageName); if(info != null) { 
    return info.versionName; } return ""; } } 

Kotlin示例：

val versionName = JDPayDeviceUtil.getVersionName() println("$versionName") 

2.2.2 Java调用Kotlin

Kotlin示例：

class KotlinStatic { 
    companion object { 
    var number = 0 fun staticMethod(string: String?) { 
    println(string) } } } 

Java示例：

KotlinStatic.Companion.staticMethod("Java"); 

2.3 Kotlin与主流框架、SDK

可以平滑对接网络框架Retrofit、OkHttp、异步框架RxJava、CameraX等。很多主流框架和Google官方Demo也均使用纯Kotlin实现，如：OkHttp、CameraX。

val mediaType = MediaType.parse("text/x-markdown; charset=urg-8") val requst = Request.Builder() .url("http://...") .post(RequestBody.create(mediaType, "kotlin-okhttp")) .build() val client = OkHttpClient() client.newCall(requst).enqueue(object : Callback { 
    override fun onFailure(call: Call, e: IOException) { 
    TODO("Not yet implemented") } override fun onResponse(call: Call, response: Response) { 
    TODO("Not yet implemented") } }) 

2.4 Kotlin线程/异步操作

继承Thread类：

object : Thread() { 
    override fun run() { 
    println("running thread: ${ 
     currentThread()}") } }.start() 

Kotlin封装类：

thread(start = true) { 
    println("running thread: ${ 
     Thread.currentThread()}") } 

同步线程锁：

@Synchronized fun syncMethod() { 
    println("kotlin synchronized") } fun syncMethod() { 
    println("kotlin none synchronized") synchronized(this) { 
    println("kotlin synchronized") } } 

Log.d("Kotlin","thread id: ${ 
     mainLooper.thread.id}") runBlocking { 
    repeat(8) { 
    Log.d("Kotlin","current thread: ${ 
     Thread.currentThread().id}") delay(1000) } } Log.d("Kotlin", "Blocking Coroutines END") 

异步协程：

override fun onCreate(savedInstanceState: Bundle?) { 
    super.onCreate(savedInstanceState) setContentView(R.layout.activity_main) val job = GlobalScope.launch { 
    delay(2000) Log.d("Kotlin", "Coroutines thread id:${ 
     Thread.currentThread().id}") } Log.d("kotlin", "main thread END") } 

2.5 Kotlin的IO操作

Java示例：

public static void writeText(String fileName, String txt) { 
    try { 
    FileOutputStream outputStream = new FileOutputStream(fileName); outputStream.write(txt.getBytes()); outputStream.close(); } catch (Exception e) { 
    e.printStackTrace(); } } 

Kotlin示例：

File(fileName).writeText(txt) File(fileName).appendText(txt) File(fileName).writeBytes(arrayBytes) File(fileName).appendBytes(arrayBytes) val content = File(fileName).readText() val bytes = File(fileName).readBytes() val line = File(fileName).readLines() val bitmap = BitmapFactory.decodeFile(fileName) 

2.6 Kotlin与数据库

Kotlin同样支持SQLite、Room数据库，与Java用法基本一致。

2.7 Kotlin与H5

Kotlin加载网页：

webView.loadUrl("https://www.jd.com/") 

物理返回键返回事件：

override fun onBackPressed() { 
    if(webView.canGoBack()) { 
    webView.goBack() return } super.onBackPressed() } 

Js与Kotlin互操作：

private fun initView() { 
    val webSettings = webView.settings webSettings.javaScriptEnabled = true webView.addJavascriptInterface(JsToKotlin(), "Android") webView.loadUrl("https://www.jd.com/web/KotlinToJs.html") } private fun KotlinCallJs(string: String) { 
    webView?.loadUrl("javascript:kotlinCallJs('$string')") } internal inner class JsToKotlin { 
    @JavascriptInterface fun show() { 
    println("kotlin被js调用了") } } 

2.8 Kotlin与JNI

public class SerialPort { 
    static { 
    System.loadLibrary("nativeJD"); } public native String stringFromJNI(); } 

Kotlin示例：

class MainActivity : AppCompatActivity() { 
    override fun onCreate(savedInstanceState: Bundle?) { 
    super.onCreate(savedInstanceState) setContentView(R.layout.activity_main) val string = stringFromJNI() } external fun stringFromJNI(): String companion object { 
    init { 
    System.loadLibrary("nativeJD") } } }