使用 Kotlin 開發(fā)幾個月后，我后悔了！團隊決定回歸 Java

fun?inc(num?:?Int)?{
????val?num?=?2
????if?(num?>?0)?{
????????val?num?=?3
????}
????println?("num:?"?+?num)
}

void?inc(int?num)?{
????int?num?=?2;?//error:?variable?'num'?is?already?defined?in?the?scope
????if?(num?>?0)?{
????????int?num?=?3;?//error:?variable?'num'?is?already?defined?in?the?scope
????}
????System.out.println?("num:?"?+?num);
}

public?class?Shadow?{
????int?val;
????public?Shadow(int?val)?{
????????this.val?=?val;
????}
}

var?a?=?"10"

var?a?:?String?=?"10"

var?a?=?"10";

val?a:?String??=?null??????//?ok
val?b:?String?=?null???????//?compilation?error

println?(a.length)??????????//?compilation?error
println?(a?.length)?????????//?fine,?prints?null
println?(a?.length??:?0)????//?fine,?prints?0

public?class?Utils?{
????static?String?format(String?text)?{
????????return?text.isEmpty()???null?:?text;
????}
}

fun?doSth(text:?String)?{
????val?f:?String?=?Utils.format(text)???????//?compiles?but?assignment?can?throw?NPE?at?runtime
????println?("f.len?:?"?+?f.length)
}

fun?doSth(text:?String)?{
????val?f:?String?=?Utils.format(text)??:?""??//?safe?with?Elvis
????println?("f.len?:?"?+?f.length)
}

fun?doSth(text:?String)?{
????val?f:?String??=?Utils.format(text)???//?safe
????println?("f.len?:?"?+?f.length)???????//?compilation?error,?fine
????println?("f.len?:?"?+?f?.length)??????//?null-safe?with???operator
}

fun?doSth(text:?String)?{
????val?f?=?Utils.format(text)????????????//?f?type?inferred?as?String!
????println?("f.len?:?"?+?f.length)???????//?compiles?but?can?throw?NPE?at?runtime
}

fun?doSth(text:?String)?{
????val?f?=?Utils.format(text)!!??????????//?throws?NPE?when?format()?returns?null
????println?("f.len?:?"?+?f.length)
}

Gson?gson?=?new?GsonBuilder().registerTypeAdapter(LocalDate.class,?new?LocalDateAdapter()).create();

def?gson?=?new?GsonBuilder().registerTypeAdapter(LocalDate,?new?LocalDateAdapter()).create()

val?kotlinClass?:?KClass?=?LocalDate::class
val?javaClass?:?Class?=?LocalDate::class.java

val?gson?=?GsonBuilder().registerTypeAdapter(LocalDate::class.java,?LocalDateAdapter()).create()

int?inc(int?i)?{
????return?i?+?1;
}

fun?inc(i:?Int):?Int?{
????return?i?+?1
}

private?fun?getMetricValue(kafkaTemplate?:?KafkaTemplate<String,?ByteArray>,?metricName?:?String)?:?Double?{
????...
}

@Bean
fun?kafkaTemplate(
????????@Value("\${interactions.kafka.bootstrap-servers-dc1}")?bootstrapServersDc1:?String,
????????@Value("\${interactions.kafka.bootstrap-servers-dc2}")?bootstrapServersDc2:?String,
????????cloudMetadata:?CloudMetadata,
????????@Value("\${interactions.kafka.batch-size}")?batchSize:?Int,
????????@Value("\${interactions.kafka.linger-ms}")?lingerMs:?Int,
????????metricRegistry?:?MetricRegistry
):?KafkaTemplate?{
????val?bootstrapServer?=?if?(cloudMetadata.datacenter?==?"dc1")?{
????????bootstrapServersDc1
????}
????...
}

MongoExperimentsRepository?repository

repository?:?MongoExperimentsRepository

class?MyClass?{
????companion?object?{
????????val?logger?=?LoggerFactory.getLogger(MyClass::class.java)
????}
}

class?AppRunner?{
????companion?object?{
????????@JvmStatic?fun?main(args:?Array<String>)?{
????????????SpringApplication.run(AppRunner::class.java,?*args)
????????}
????}
}

import?java.util.Arrays;
...
List<String>?strings?=?Arrays.asList("Saab",?"Volvo");

import?com.google.common.collect.ImmutableMap;
...
Map<String,?String>?string?=?ImmutableMap.of("firstName",?"John",?"lastName",?"Doe");

const?list?=?['Saab',?'Volvo']
const?map?=?{'firstName':?'John',?'lastName'?:?'Doe'}

list?=?['Saab',?'Volvo']
map?=?{'firstName':?'John',?'lastName':?'Doe'}

def?list?=?['Saab',?'Volvo']
def?map?=?['firstName':?'John',?'lastName':?'Doe']

val?list?=?listOf("Saab",?"Volvo")
val?map?=?mapOf("firstName"?to?"John",?"lastName"?to?"Doe")

public?int?parseAndInc(String?number)?{
????return?Optional.ofNullable(number)
???????????????????.map(Integer::parseInt)
???????????????????.map(it?->?it?+?1)
???????????????????.orElse(0);
}

fun?parseAndInc(number:?String?):?Int?{
????return?number.let?{?Integer.parseInt(it)?}
?????????????????.let?{?it?->?it?+?1?}??:?0
}

fun?parseAndInc(number:?String?):?Int?{
????return?number?.let?{?Integer.parseInt(it)?}
??????????????????.let?{?it?->?it?+?1?}??:?0
}

data?class?User(val?name:?String,?val?age:?Int)

open?class?Base
class?Derived?:?Base()

buildscript?{
????dependencies?{
????????classpath?group:?'org.jetbrains.kotlin',?name:?'kotlin-allopen',?version:?"$versions.kotlin"
????}
}

class?AppRunner?{
????companion?object?{
????????@JvmStatic?fun?main(args:?Array<String>)?{
????????????SpringApplication.run(AppRunner::class.java,?*args)
????????}
????}
}

?fun?main(args：Array ){?SpringApplication.run(AppRunner?::?class.java，*?args)}?

?fun?main(args：Array ){?runApplication?(*?args)}