kotlin学习笔记第一章

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第一章 搭建 Kotlin 开发环境

一、Kotlin简介：

Kotlin是一种基于JVM的新型编程语言，它完全兼容Java语言。
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Kotlin 优先意味着什么？

在构建新的 Android 开发工具和内容（例如 Jetpack 库、示例、文档和培训内容）时，google会在设计层面考虑到 Kotlin 用户

Java 语言 Kotlin
平台 SDK 支持 是 是
Android Studio 支持 是 是
Lint 是 是
==引导式文档支持 是 是
API 文档支持 是 是
AndroidX 支持 是 是
AndroidX Kotlin 特有 API（KTX、协程等） 无 是
在线培训 尽力而为 是
示例 尽力而为 是
多平台项目 否 是
Jetpack Compose 否 是

Kotlin能做什么?

如果对 Kotlin 的能力仅仅停留在 JVM平 台，那是片面的。如今的 Kotlin 已经从当初的更好 Java 目标完成了它华丽的大变身，他们的目标已经瞄准了星辰大海。目前 Kotlin 可以适用于移动端跨平台、原生 JVM、服务端开发、Web 开发、Android 开发、数据科学等多个领域。此外近年来 Kotlin 团队已经将重心转移到了语言层面的跨平台，多平台的支持。

Kotlin与Java的异同

打印日志

Java 

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System.out.print("hello world"); 
System.out.println("hello world");

Kotlin 

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print("hello world") println("hello world")

定义变量与常量常

Java 

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String name = "hello world";
final String name = "hello world";

Kotlin 

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var name = "hello world" 
val name = "hello world"

null声明

Java 

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String otherName; 
otherName = null;

Kotlin

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var otherName : String? 
otherName = null

空判断
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if (text != null) { 
    int length = text.length();
}

Kotlin
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text?.let { 
    val length = text.length
} // 
or 
simply val length = text?.length

字符串拼接
Java
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String firstName = "Android"; 
String lastName = "Architect"; 
String message = "My name is: " + firstName + " " + lastName;

Kotlin
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val firstName = "Android" 
val lastName = "Architect" 
val message = "My name is: $firstName $lastName"

换行
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String text = "First Line\n" + "Second Line\n" + "Third Line";

Kotlin 

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val text = """ |First Line |Second Line |Third Line """.trimMargin()

三元表达式

Java 

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String text = x > 5 ? "x > 5" : "x <= 5";

Kotlin 

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val text = if (x > 5) "x > 5" else "x <= 5"

操作符

java 

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final int andResult = a & b; 
final int orResult = a | b; 
final int xorResult = a ^ b; 
final int rightShift = a >> 2;
final int leftShift = a << 2; 
final int unsignedRightShift = a >>> 2;

Kotlin 

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val andResult = a and b 
val orResult = a or b 
val xorResult = a xor b 
val rightShift = a shr 2 
val leftShift = a shl 2 
val unsignedRightShift = a ushr 2

类型判断和转换 (声明式)

Java 

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Car car = (Car) object;

Kotlin 

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var car = object as Car

类型判断和转换 (隐式)

Java 

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if (object instanceof Car) {
    Car car = (Car) object; 
}

Kotlin

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if (object is Car) {
    var car = object // 自动识别 
}

多重条件

Java 

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if (score >= 0 && score <= 300) { }

Kotlin 

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if (score in 0..300) { }

更灵活的case语句

Java

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int score = // some score;String grade; 
switch (score) { 
    case 10: 
    case 9: 
    grade = "Excellent"; 
    break; 
    case 8: 
    case 7: 
    case 6: 
    grade = "Good"; 
    break; 
    case 5: 
    case 4: 
    grade = "OK";
    break; 
    case 3:
    case 2:
    case 1:
    grade = "Fail";
    break;
    default:
    grade = "Fail";
}

Kotlin

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var score = // some score 
var grade = when (score) { 
    9, 10 -> "Excellent"
    in 6..8 -> "Good" 
    4, 5 -> "OK" 
    in 1..3 -> "Fail"
    else -> "Fail"
}

for循环

Java

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for (int i = 1; i <= 10 ; i++) { } 
for (int i = 1; i < 10 ; i++) { } 
for (int i = 10; i >= 0 ; i--) { } 
for (int i = 1; i <= 10 ; i+=2) { } 
for (int i = 10; i >= 0 ; i-=2) { } 
for (String item : collection) { }
for (Map.Entry entry: map.entrySet()) { }

Kotlin
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for (i in 1..10) { }
for (i in 1 until 10) { }
for (i in 10 downTo 0) { }
for (i in 1..10 step 2) { }
for (i in 10 downTo 0 step 2) { }
for (item in collection) { }
for ((key, value) in map) { }

更方便的集合操作
Java
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final List listOfNumber = Arrays.asList(1, 2, 3, 4); 
final Map keyValue = new HashMap();
map.put(1, "Android"); 
map.put(2, "Ali");
map.put(3, "Mindorks"); // Java 9 
final List listOfNumber = List.of(1, 2, 3, 4);
final Map keyValue = Map.of(1, "Android", 2, "Ali", 3, "Mindorks");

Kotlin
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val listOfNumber = listOf(1, 2, 3, 4)
val keyValue = mapOf(1 to "Android", 2 to "Ali", 3 to "Mindorks")

遍历
Java
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// Java 7 and below 
for (Car car : cars) { 
    System.out.println(car.speed); 
}
// Java 8+ 
cars.forEach(car -> System.out.println(car.speed)); 
// Java 7 and below 
for (Car car : cars) {
    if (car.speed > 100) {
        System.out.println(car.speed);
    }
}
// Java 8+ 
cars.stream()
    .filter(car -> car.speed > 100)
    .forEach(car -> System.out.println(car.speed));

Kotlin
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cars.forEach { println(it.speed) } 
cars.filter { it.speed > 100 } 
    .forEach { println(it.speed)}

方法定义
Java
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void doSomething() { 
   // logic here
}
void doSomething(int... numbers) { 
   // logic here 
}

Kotlin

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fun doSomething() { 
    // logic here 
} 
fun doSomething(vararg numbers: Int) {
    // logic here 
}

带返回值的方法

Java

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int getScore() { 
   // logic here 
   return score; 
}

Kotlin
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fun getScore(): Int {
   // logic here 
   return score 
}
// as a single-expression function 
fun getScore(): Int = score

无结束符号
Java
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int getScore(int value) { 
   // logic here 
   return 2 * value; 
}

Kotlin
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fun getScore(value: Int): Int {
   // logic here 
   return 2 * value 
} 
// as a single-expression function 
fun getScore(value: Int): Int = 2 * value

constructor 构造器
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public class Utils { 
   private Utils() {
        // This utility class is not publicly instantiable 
   } 
   public static int getScore(int value) {
       return 2 * value; 
   } 
}

Kotlin
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class Utils private constructor() {
   companion object {
       fun getScore(value: Int): Int {
           return 2 * value
       }
   }
} 
// another way 
object Utils {
   fun getScore(value: Int): Int {
       return 2 * value
   }
}

Get Set 构造器
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 public class Developer {
   private String name;
   private int age;
   public Developer(String name, int age) {
       this.name = name; this.age = age;
   }
   public String getName() {
       return name;
   }
   public void setName(String name) {
       this.name = name;
   }
   public int getAge() {
       return age;
   }
   public void setAge(int age) {
       this.age = age;
   }
}

Kotlin
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data class Developer(val name: String, val age: Int)