Kotlin快速上手篇之Android&Kotlin常用开发对比

Posted 2022-11-28 思忆(GeorgeQin)

变量定义与常用数据类型

java：
int i = 1;
long l = 2;
boolean b = true;
float f = 0;
double d = 0;
char c = 'A';
String s = "text";
Intent intent = new Intent();

Kotlin：
var i : Int = 1
var l : Long = 2
var b : Boolean = true
var f : Float = 0F
var d : Double = 0.0
var c : Char = 'A'
var s : String = "text"
var intent = Intent() //不写也可自动推导

常量定义

java：
final String text = "";

Kotlin：
val text = ""

静态常量定义

java：
static final String TEST = "";

Kotlin：
const val TEST = ""
或
@JvmField
val TEST=""

类和继承

java：
MainActivity.class 
Kotlin：
MainActivity::class.java

java
public class MainActivity extends AppCompatActivity 

kotlin
class MainActivity : AppCompatActivity()   

类与及相关属性的声明

java
public class Order 
    private long OrderId;

    public long getOrderId() 
        return OrderId;
    

    public void setOrderId(long orderId) 
        OrderId = orderId;
    


kotlin
class Order 
    //不想生成get set方法用val
    var orderId: Long? = null
    //私有化set 如果需要的话
//    private set

抽象类和接口

Java 
public abstract class BaseActivity extends AppCompatActivity 
    abstract void test();


Kotlin 
abstract class BaseActivity : AppCompatActivity() 
    abstract fun test()


java
public interface Callback 
    void onOk();
    void onFail();


kotlin
interface Callback 
    fun onOk()
    fun onFail()
    //可加入默认声明
    fun test()
        kotlin.io.println (111)
    

构造方法

java
public class MyView extends View 
    public MyView(Context context) 
        this(context, null);
    

    public MyView(Context context, @Nullable AttributeSet attrs) 
        this(context, attrs, 0);
    

    public MyView(Context context, @Nullable AttributeSet attrs, int defStyleAttr) 
        super(context, attrs, defStyleAttr);
    


kotlin
class MyView : View 

    constructor(context : Context) : this(context, null) 
    

    constructor(context : Context, attrs : AttributeSet?) : this(context, attrs, 0) 
    

    constructor(context : Context, attrs : AttributeSet?, defStyleAttr : Int) : super(context, attrs, defStyleAttr) 
    

内部类：

Java 
public class MainActivity extends AppCompatActivity 
    public class MyTask 
    


Kotlin
class MainActivity : AppCompatActivity() 
    inner class MyTask 
    

方法与重写：

java
public void test(String message) 


kotlin  
如果是Unit也可忽略不写
fun test(message : String) : Unit 

//还可以添加默认值
fun test(message : String="默认值") 

java
public class MainActivity extends AppCompatActivity 

    @Override
    protected void onCreate(@Nullable Bundle savedInstanceState) 
        super.onCreate(savedInstanceState);
    


Kotlin 
class MainActivity : AppCompatActivity() 

    override fun onCreate(savedInstanceState: Bundle?) 
        super.onCreate(savedInstanceState)
    

可变参数

java
public int add(int... array) 
 。。。


Kotlin 
fun add(vararg array: Int) : Int 
。。。。

静态方法：

java
public class Toasts 

    public static void show() 
    


kotlin 
jvmstatic
object Toasts 

    @JvmStatic
    fun show() 
    

或 伴生对象 
class Toasts

    companion object  
        fun show()
        
    

类型检测

java
if ("" instanceof String) 

kotlin
if ("" is String) 

强制转换

java
List<Object> list = new ArrayList<>();
String data = (String) list.get(0);

kotlin
var list = ArrayList<Any>()
var data:String = list[0] as String 
或
var data:String = list[0] ？as String 

模板语法：

java
int number = 100;
System.out.println(String.format("测试样本是%d", number));

kotlin
var number = 100
//可在括号中添加表达式
println("测试样本是$number")
或
println("测试样本是$number")

比较：

kotlin	java
a==b	a.equals(b)
a===b	a==b

循环

Java 
String[] array = "1", "2", "3";
for (int i = 1; i < array.length; i++) 
    System.out.println(array[i]);

或
for (String text : array) 
    System.out.println(text);


Kotlin 
val array = arrayListOf("1", "2", "3")
for (i in 1..array.size - 1) 
    println(array[i])

或
for (i in 1 until array.size) 
    println(array[i])

或
for (text in array) 
    println(text)

判断：

Java 
int count = 1;
switch (count) 
    case 0:
        System.out.println(count);
        break;
    case 1:
    case 2:
        System.out.println(count);
        break;
    default:
        System.out.println(count);
        break;


Kotlin 
var count = 1
when (count) 
    0 -> 
        println(count)
    
    in 1..2 -> 
        println(count)
    
    else -> 
        println(count)
    

枚举

java
public enum OrderStatus 
    ACCEPT,
    FETCH,
    DISPATCH,
    FINISH


kotlin
enum class OrderStatus
    ACCEPT,
    FETCH,
    DISPATCH,
    FINISH


或密闭类(超级枚举)
sealed class OrderStatus 
    object ACCEPT : OrderStatus()
    object FETCH : OrderStatus();
    object DISPATCH : OrderStatus()
    object FINISH : OrderStatus();