原文在 https://github.com/Zhuinden/guide-to-kotlin
kotlin的空指针安全相关
?. ?: !! 操作符
class MyAdapter: RecyclerView.Adapter<MyAdapter.ViewHolder>() {
private var items: List<Item>? = null
fun updateItems(items: List<Item>?) {
this.items = items
notifyDataSetChanged()
}
override fun onCreateViewHolder(parent: ViewGroup?, viewType: Int): ViewHolder {
return ViewHolder(LayoutInflater.from(parent!!.getContext()).inflate(R.layout.my_item, parent, false))
}
override fun onBindViewHolder(holder: ViewHolder, position: Int) {
holder.bind(items!!.get(position))
}
override fun getItemCount(): Int {
return items?.size ?: 0
}
}
强制类型转换
延迟初始化
private lateinit var realm: Realm
string
val hello: String
get() = "Hello $name, your overlord ${overlord.name} has been expecting you."
val jsonString = """
{
"hello": "world",
"another": {
"field": "field",
"boom": "boom"
}
}
""".trimIndent()
data class
data class Dog(
val name: String,
val owner: Person
)
@Parcelize
data class LoginKey(val placeholder: String = ""): Parcelable
when关键字
val value = when(number) {
0,1,2,3,4 -> 1.0
in 5..9 -> 0.75
in 10..14 -> 0.5
in 15..19 -> 0.25
20 -> 0.2
else -> 0
}
enum class Colors {
RED,
GREEN,
BLUE
}
val color = Colors.RED
when(color) {
Colors.RED -> {
...
}
else ->
...
}
}
控制语句作为表达式（when， return）
val name = tryGetName() ?: return
有名称的参数，默认参数
@JvmOverloads
fun printStrings(first: String = "Hello", second: String = "World") {
println("$first $second")
}
printStrings()
printStrings("Goodbye", "my dear")
printStrings(first = "Goodbye", second = "my dear")
printStrings(first = "Goodbye")
printStrings(second = "my dear")
如果我们使用了 @JvmOverload 那么在java中也起作用
可变参数和*操作符
java中的可变参数函数是这样的 void dosomething(String... values)
在kotlin中有一种语法实现如下:
fun doSomething(vararg values: String)
如果我们想把这个数组一个一个的传入其他函数，那么就要使用*操作符
fun animateTogether(vararg animators: Animator) = AnimatorSet().apply {
playTogether(*animators)
}
接口 和默认实现
泛型
fun <T: View> findViewById(view: View, @IdRes idRes: Int) = view.findViewById(idRes) as T
val activityType: Class<out Activity> // similar to `? extends T`
get() = when(this) {
CAR_HEADER -> CarHeader::class.java
CAR_VIEW -> CarView::class.java
DATA_VIEW -> DataView::class.java
}
}
val clazz: Class<*> = SomeClass::class.java
