Idioms to start learning with Kotlin

some idioms

1.1. Create DTOs (POJOs/POCOs)

DTO: Data transfer object, its interpretation is Data Transfer Object;
POJO: Simple javaobject , which is actually ordinary JavaBean. Its explanation is Plain Old Java Object;
POCO: POJOThe meaning of the same is the same, the only difference is the language used. Therefore, its interpretation is Plain Old C# Object.

data class Customer(val name: String, val email: String)

A Customer class is provided with the following functions:

• getters for all properties (and setters if var properties)

• equals()
• hashCode()
• toString()
• copy()
• component1(), component2(),... of all properties (explained in the Data classes section)

1.2, the default value of function parameters

fun foo(a: Int = 0, b: String = "") { ... }

1.3, filter list list

val positives = list.filter { x -> x > 0 }

or is relatively simpler to write:

val positives = list.filter { it > 0 }

Note: itwhat is it? Additional explanation will be given later. It is probably known here that it is an object representing a collection instance.

1.4. String interpolation

println("Name $name")  // 字符串模板

1.5. Instance Check

// 类型检查，自动转换类型
when (x) {
    is Foo -> ...
    is Bar -> ...
    else   -> ...
}

1.6. Traverse the map/list (in the form of key-value pairs)

for ((k, v) in map) {
    println("$k -> $v")
}

k,v can be called anything.

1.7. Use interval

for (i in 1..100) { ... }  // closed range: includes 100
for (i in 1 until 100) { ... } // half-open range: does not include 100
for (x in 2..10 step 2) { ... }
for (x in 10 downTo 1) { ... }
if (x in 1..10) { ... }

1.8, read-only list

val list = listOf("a", "b", "c")

1.9, read-only mapping

val map = mapOf("a" to 1, "b" to 2, "c" to 3)

1.10, access mapping

println(map["key"])
map["key"] = value

1.11, Lazy property

val p: String by lazy {
    // compute the string
}

1.12, extension function

fun String.spaceToCamelCase() { ... }

"Convert this to camelcase".spaceToCamelCase()

1.13. Create a singleton

object Resource {
    val name = "Name"
}

1.14, if non-space write

val files = File("Test").listFiles()

println(files?.size)

1.15, if non-empty and else shorthand

al files = File("Test").listFiles()

println(files?.size ?: "empty")

1.16, if empty execution statement

val values = ...
val email = values["email"] ?: throw IllegalStateException("Email is missing!")

1.17. Get the first data of a possibly empty collection

val emails = ... // might be empty
val mainEmail = emails.firstOrNull() ?: ""

1.18, if non-empty execution

val value = ...

value?.let {
    ... // execute this block if not null
}

1.19, if non-null, map null value

val value = ...

val mapped = value?.let { transformValue(it) } ?: defaultValueIfValueIsNull

1.20, when statement return value

fun transform(color: String): Int {
    return when (color) {
        "Red" -> 0
        "Green" -> 1
        "Blue" -> 2
        else -> throw IllegalArgumentException("Invalid color param value")
    }
}

1.21, try/catch expressions

fun test() {
    val result = try {
        count()
    } catch (e: ArithmeticException) {
        throw IllegalStateException(e)
    }

    // Working with result
}

1.22, if expression

fun foo(param: Int) {
    val result = if (param == 1) {
        "one"
    } else if (param == 2) {
        "two"
    } else {
        "three"
    }
}

1.23. Method builder style usage that returns Unit type

fun arrayOfMinusOnes(size: Int): IntArray {
    return IntArray(size).apply { fill(-1) }
}

1.24, single expression function

fun theAnswer() = 42

Equivalent to

fun theAnswer(): Int {
    return 42
}

This can also be combined effectively with other idioms to simplify code. Such as: with when expression:

fun transform(color: String): Int = when (color) {
    "Red" -> 0
    "Green" -> 1
    "Blue" -> 2
    else -> throw IllegalArgumentException("Invalid color param value")
}

1.25. Calling multiple methods in an object instance ('with')

class Turtle {
    fun penDown()
    fun penUp()
    fun turn(degrees: Double)
    fun forward(pixels: Double)
}

val myTurtle = Turtle()
with(myTurtle) { //draw a 100 pix square
    penDown()
    for(i in 1..4) {
        forward(100.0)
        turn(90.0)
    }
    penUp()
}

1.26, Java7 resources

val stream = Files.newInputStream(Paths.get("/some/file.txt"))
stream.buffered().reader().use { reader ->
    println(reader.readText())
}

1.27. Convenience forms for generic functions that require generic type information

//  public final class Gson {
//     ...
//     public <T> T fromJson(JsonElement json, Class<T> classOfT) throws JsonSyntaxException {
//     ...

inline fun <reified T: Any> Gson.fromJson(json: JsonElement): T = this.fromJson(json, T::class.java)

1.27. Consume nullable Boolean

val b: Boolean? = ...
if (b == true) {
    ...
} else {
    // `b` is false or null
}