检查 null 时惯用的 Kotlin

Question

在Java，我会做这样的事情：

class Person {

    private Record record;

    public String name() {
      record().get("name");
    }

    private Record record() {
      if (record == null) {
        refetch();
      }
      return record;
    }

    private void refetch() {
      record = service.doSomething()
    }

}

在 Kotlin 中，我有这个等效代码：

class Person(private var record: Record?) {

    fun name() : String {
      record().get("name");
    }

    private fun record() : Record {
      record ?: refetch()
      return record!!;
    }

    private fun refetch() {
      record = service.doSomething()
    }

}

如您所见，我正在使用 !! 运算符，但我不太喜欢。还有另一种更惯用的方法吗？

如果我简单地遵循 Java 方式 (if (record == null))，我会得到这个错误：

Smart cast to "Record" is impossible, because "record" is a mutable property that could have been changed by this time

Answer 1

在惯用的 Kotlin 中，您将使用 lazy delegated property:

class Person(private val service: Service) {

    private val record by lazy { service.doSomething() }

    fun name() = record.get("name");
}

Answer 2

你 Java 代码没有编译，所以我模拟了一个例子。

    class Record {
        String name;
        String address;

        Record(String name, String address) {
            this.name = name;
            this.address = address;
        }

        String getName() {
            return name;
        }
    }

    class Person {

        private Record record;

        public String name() {
            return record().getName();
        }

        private Record record() {
            if (record == null) {
                refetch();
            }
            return record;
        }

        private void refetch() {
            record = new Record("Joe Smith", "101 Broadway, NYC, NY");
        }

    }

Kotlin 强制您使用“?”声明一个值是否可为空定义它时的字符。所以它需要看起来像这样：

        internal class Record(var name: String, var address: String)

    internal class Person {

        private var record: Record? = null

        fun name(): String {
            return record()!!.name
        }

        private fun record(): Record? {
            if (record == null) {
                refetch()
            }
        return record
        }

        private fun refetch() {
            record = Record("Joe Smith", "101 Broadway, NYC, NY")
        }

    }

检查 null 时惯用的 Kotlin

Idiomatic Kotlin when checking null

java

idioms

kotlin