一、lambda与sam

SAM 转换 (Single Abstract Method Conversions): 即SAM 转换就是 kotlin 在调用 java 代码时能使用 Lambda

java中实现了只有一个接口当参数的类

参考：

Kotlin学习笔记（五十七）SAM转换

为什么 Kotlin 调用 java 时可以使用 Lambda？

示例：

/*** kotlin与java互操作（SAM 转换）* 1.SAM 是Single Abstract Method是缩写* 2.SAM转换的条件：Java的接口，只有一个接口当参数的方法，可以使用lambda当参数* 3.对于实现了kotlin实现的java接口，在java函数中定义该参数后，kotlin引用时可以用lambda表达式使用* 4.由于java与kotlin表达式并不兼容，所以在每次添加lambda表达式都是不同对象*///java式写法val button = Button(this)//匿名内部类button.setOnClickListener(object : View.OnClickListener {override fun onClick(view: View?) {val text = when (view?.id) {R.id.button1 -> "1"R.id.button2 -> "2"else -> "3"}println("==========$text=============")}})//    public void setOnClickListener(View.OnClickListener l) {}//    public final fun setOnClickListener(l: ((v:View!)->Unit)!): Unit//sam-constructorval listener = View.OnClickListener { view ->val text = when (view.id) {R.id.button1 -> "1"R.id.button2 -> "2"else -> "3"}println("==========$text=============")}button.setOnClickListener(listener)//另一种写法//方法只有一个lambda当作参数，可去掉类型button.setOnClickListener(View.OnClickListener { view ->val text = when (view.id) {R.id.button1 -> "1"R.id.button2 -> "2"else -> "3"}println("==========$text=============")})//如果 Lambda 是一个函数的唯一参数，那么调用这个函数时可以省略圆括号button.setOnClickListener({ view ->val text = when (view.id) {R.id.button1 -> "1"R.id.button2 -> "2"else -> "3"}println("==========$text=============")})//如果 Lambda 所表示的匿名函数只有一个参数，// 那么可以省略它的声明以及->符号（默认会用it来给省略的参数名命名）button.setOnClickListener { view ->val text = when (view.id) {R.id.button1 -> "1"R.id.button2 -> "2"else -> "3"}println("==========$text=============")}//最简单的kotlin lambda写法button.setOnClickListener {val text = when (it.id) {R.id.button1 -> "1"R.id.button2 -> "2"else -> "3"}println("==========$text=============")}

参考： Kotlin修炼指南(二):lambda表达式的精髓

二、lambda表达式 与 匿名内部类实现接口

测试java：

去掉实例对象，只保留参数和语句，并用->连接

()->{}

public class Test {public static void main(String[] args) {/** 单行*/new Thread(new Runnable() {@Overridepublic void run() {System.out.println("11");}}).start();//lambdanew Thread(() -> System.out.println("2")).start();/** 多行*/new Thread(new Runnable() {@Overridepublic void run() {System.out.println("12");System.out.println("13");}}).start();//lambdanew Thread(() -> {System.out.println("22");System.out.println("23");}).start();/** 多参数*/List<String> list = Arrays.asList("I", "love", "you", "too");Collections.sort(list, new Comparator<String>() {// 接口名@Overridepublic int compare(String s1, String s2) {// 方法名if (s1 == null)return -1;if (s2 == null)return 1;return s1.length() - s2.length();}});// lambdaCollections.sort(list, (s1, s2) -> {// 方法名if (s1 == null)return -1;if (s2 == null)return 1;return s1.length() - s2.length();});}
}

测试kotlin:

    @Testfun addition_isCorrect() {assertEquals(4, 2 + 2)/**lambda表达式代替匿名内部类*//** 单行*/Thread(Runnable { println("11") }).start()//lambdaThread { println("2") }.start()/** 多行*/Thread(Runnable {println("12")println("13")}).start()//lambdaThread {println("22")println("23")}.start()/** 多参数*/val list = listOf("c#", "c++", "c")Collections.sort(list, object : Comparator<String> {override fun compare(s1: String?, s2: String?): Int {if (s1 == null)return -1if (s2 == null)return 1return s1.length - s2.length}})println("1 : $list")// lambdaval list2 = listOf("c#", "c++", "c")Collections.sort(list2, Comparator { s1, s2 ->if (s1 == null)return@Comparator -1if (s2 == null)return@Comparator 1s1.length - s2.length})println("2 : $list2")// lambda2val list3 = listOf("c#", "c++", "c")Collections.sort(list3) Comparator@{ s1, s2 ->if (s1 == null)return@Comparator -1if (s2 == null)return@Comparator 1s1.length - s2.length}println("3 : $list3")}

三、lambda表达式与接口回调

常见接口回调 ：

interface ICallback {fun onSuccess(msg: String)fun onFail(msg: String)
}class NormalCallback {var mCallback: ICallback? = nullfun setCallback(callback: ICallback) {mCallback = callback}fun init() {mCallback?.onSuccess("success message")}
}

    private fun test() {val normalCallback = NormalCallback()normalCallback.setCallback(object : ICallback {override fun onSuccess(msg: String) {println("常见接口回调 onSuccess : $msg")}override fun onFail(msg: String) {println("常见接口回调  onFail : $msg")}})normalCallback.init()}//常见接口回调 onSuccess : success message

lambda表达式测试1：一个回调

class TestCallbackA {var mCallBack: ((str: String) -> Unit)? = nullfun setCallback(myCallBack: ((str: String) -> Unit)) {this.mCallBack = myCallBack}
}

    private fun testA() {val testCallbackA = TestCallbackA()testCallbackA.setCallback {println("callback ： $it")}}

lambda表达式测试2：多回调

class TestCallbackB {var mSuccessCallback: (String) -> Unit? = {}var mFailCallback: (String) -> Unit? = {}fun setCallback(successCallback: (String) -> Unit,failCallback: (String) -> Unit) {this.mSuccessCallback = successCallbackthis.mFailCallback = failCallback}fun init() {mSuccessCallback("b success message")mFailCallback("b fail message")}
}

    private fun testB() {val testCallbackB = TestCallbackB()testCallbackB.setCallback({println("b success : $it")}, {println("b failure : $it")})testCallbackB.init()}

lambda表达式测试3：多参数

//    private var mListener1: ((Int, String) -> Unit)? = null
//    private var mListener2: ((a: Int, b: String) -> Unit)? = null
//    private var mListener3: ((a: Int, b: String) -> Unit)? = { _, _ -> }
//    private var mListener4: (a: Int, b: String) -> Unit = { _, _ -> }

class TestCallbackC {var mSuccessCallback: (String, Boolean) -> Unit? = { _, _ -> }var mFailCallback: (String) -> Unit? = {}fun setCallback(successCallback: (String, Boolean) -> Unit,failCallback: (String) -> Unit) {this.mSuccessCallback = successCallbackthis.mFailCallback = failCallback}fun init() {mSuccessCallback("c success message", false)mFailCallback("c fail message")}
}

    private fun testC() {val testCallbackC = TestCallbackC()testCallbackC.setCallback({ a, b ->println("c success : $a - $b")}, {println("c failure : $it")})testCallbackC.init()}