CSE116的学习笔记-Lec2-3:JSON

JSON
An Application of Polymorphism

JSON - Reminder

• JSON is [mostly] used to communicate betweent programming languages

• Consist of 6 types

  • String
  • Number
  • Boolean
  • Array
  • Object
  • Null

• In Python

  • json.dumps to convert from Python types to JSON string
  • json.loads to convert from JSON string to Python types

• In JavaScript

  • JSON.stringify to convert from JavaScript types to JSON string
  • JSON.parse to convert from JSON string to JavaScript types

JSON

• What about Scala?
  {“timestamp”:1550774961,”message”:”success”,”iss_position”:{“latitude”:”-36.5017”,”longitude”:”-2.8015}}
• This is valid JSON
• What Scala type do we use to store this data?
  • Map[String, String]?
  • Map[String, Long]?
  • Map[String, Map[String, String]]?
  • Map[String, Any]?? <- This is the only one that can work, but it’s very restrictive since we can only use the Any methods.
• We can’t mix types in our Scala data structures
• .. at least, not without polymorphism

JSON - Library

• We’ll install a library to help us work with JSON in Scala
  • The Play JSON library
• Library defines these Scala types
  • JsString
  • JsNumber
  • JsBoolean
  • JsArray
  • JsObject
  • JsNull
• All these types extend JsValue
• Map[String, JsValue]
• The library parses JSON strings and converts all values into one of the Js_types
• Convert value to their Scala types as needed

Reading JSON

{“timestamp”:1550774961,”message”:”success”,”iss_position”:{“latitude”:”-36.5017”,”longitude”:”-2.8015}}

import play.api.libs.{JsValue, Json}
...
val parsed: JsValue = Json.parse(response)

// unused values, but this is how we would extract message and timestamp
val message: String = {parsed \ "message"}.as[String]
val timestamp: Long = {parsed \ "timestamp"}.as[Long]

val issLocation: Map[String, String] = (parsed \ "iss_position").as[Map[String, String]]

1. Use the library to extract specific values
2. Call Json.parse
3. Parse the JSON string and converts it to a JsValue
4. Extract values at specific keys
5. Use \ to get the value at a key as a JsValue
6. Use as[type] to convert the value to the type you expect, cannot use your custom types without defining how to parse your type

Writing JSON

{“timestamp”:1550774961,”message”:”success”,”iss_position”:{“latitude”:”-36.5017”,”longitude”:”-2.8015}}

def createJSON(message: String, timestamp: Long, location: Location): String = {
    val jsonTimestamp: JsValue = Json.toJson(timestamp)
    val jsonMessage: JsValue = Json.toJson(message)

    val locationMap: Map[String, String] = Map(
        "latitude" -> location.latitude.toString,
        "longitude" -> location.longitude.toString
    )

    val jsonMap: Map[String, JsValue] = Map(
        "timestamp" -> jsonTimestamp,
        "message" -> jsonMessage,
        "iss_position" -> jsonLocation
    )

    Json.stringify(Json.toJson(jsonMap))
}

• Convert Scala types to JsValue with Json.toJson
  • Cannot use your custom types without defying how to convert your type
• Call Json.stringify to convert a type to a JSON string
  • Can be any types known to the library(Most of the common Scala types)

Maven

• We’re using a new library
• Must download it before use
• Add it to our Maven file

  <project xmlns="http://maven.apache.org/POM/4.0.0">
      <gruopId>com.mycompany.app</groupId>
      <artifactId>test1</artifactId>
      <modelVersion>4.0.0</modelVersion>
      <url>http://maven.apache.org</url>
      <version>0.0.1</versiom>
  
      <dependencies>
  
          <!-- https://mvnrepository.com/artifact/org.scalatest/scalatest -->
          <dependency>
              <groupId>org.scalatest</groupId>
              <artifactId>scalatest_2.12</artifactId>
              <version>3.0.5</version>
          </dependency>
  
      </dependencies> 
  
  </project>

• This is our current Maven file that we used to download scalatest
• We can add more dependancies to this file
  • Open the Maven sidebal, refresh, then download the new libraries
• Find new libraries at https://mvnrepository.com
  • An enrmous wealth of shared libraries
  • Search for the new liraries, paste the dependency into you pom.xml file

Lecture Question

Question: In a package named “oop.json” create and complete the “Store” class which is stared below

asJSON returns a JSON string representing an object with keys “cashInRegister” and “inventory” mapping to the values from the two state variables with the same names

fromJSON takes a JSON string in the same format returned from asJSON and sets the state variables to the values from the JSON string

package opp.json

class Store(var cashInRegister: Double, var inventory: List[String]) {

    def asJSON(): String = {
        ""
    }

    def fromJSON(jsonString: String): Unit = {

    }
}

Testing:

package tests

import org.scalatest.FunSuite
import oop.json.Store

class TestSubmission extends FunSuite {

    val EPSILON: Double = 0.000001

    def equalDoubles(d1: Double, d2: Double): Boolean = {
        (d1 - d2).abs < EPSILON
    }

    test("test the store JSON") {
        val store: Store = new Store(550.21, List("eggs", "milk", "waffles"))
        val storeJSON: String = store.asJSON()

        val store2: Store = new Store(0.0, List())
        store2.fromJSON(storeJSON)

        assert(equalDoubles(store2.cashInRegister, 550.21))
        val actualList: List[String] = store2.inventory.sorted
        val expectedList: List[String] = List("eggs", "milk", "waffles").sorted

        assert(actualList == expectedList)
    }
}