json - 使用循环将scala列表转换为json

Question

我尝试了很多使用while循环将我的scala列表转换为Json；代码如下：

 var json = null
 while (list != null) {
   json = new Gson().toJson(list)
 }

该json变量必须在循环之外访问，所以我在循环之外声明它并用 初始化null，但是 Scala 编译器给了我一个类型不匹配异常......

Answer 1

Why are you using a while loop to convert a single list to JSON? Until you explain why you need a loop (or, repeated conversions to JSON, more generally speaking), I'd suggest the following trivial snippet:

val json = new Gson().toJson(list)

Note that I've also changed var json to val json.

However, if all you want to know is how to get rid of the type mismatch exception, just change:

var json = null

to

var json: String = null

or

var json: String = _

If you don't declare json to be of type String, Scala will implicitly take it to be (i.e. infer) of type Null, and it's not possible to assign values of type String to a variable of type Null.

Answer 2

此函数适用于 List 和 Map 都使用普通的 Scala 构造：

def toJson(a: Any): String = {
  a match {
    // number
    case m: Number => m.toString
    // string
    case m: String => "\"" + m + "\""
    case m: Map[AnyRef, AnyRef] => {
"{" + (m map { x => val key = x._1; toJson(key) + ": " + toJson(m(key)) } mkString (", ")) + "}"
}

    case l: Seq[AnyRef] => { "[" + (l map (toJson(_)) mkString (",")) + "]" }

    // for anything else: tuple
    case m: Product => toJson(m.productIterator.toList)
    case m: AnyRef => "\"" + m.toString + "\""
  }
}

完整示例位于此处：https ://gist.github.com/tuxdna/7926531