android-studio - Gson().fromJson 出错 - “无法在没有参数的情况下调用公共 com.keikakupet.PetStatus()”

Question

我正在尝试使用 Gson 库将我的 Kotlin 类的实例存储到 JSON 文件中。但是，当我运行 Gson().fromJson 时，我收到以下错误：

java.lang.RuntimeException：无法调用没有参数的公共 com.keikakupet.PetStatus()

我对错误的理解是 Gson 要求我的类有一个不带参数的主构造函数，因此它可以构造所需的对象（在本例中为 PetStatus 对象）。但是，我有这样一个构造函数。我不确定部分问题是否在于我正在从 init 运行该方法。有谁知道我可以如何解决这个错误？

我的代码：

package com.keikakupet

import android.content.Context
import android.util.Log
import com.google.gson.Gson
import java.io.File
import java.util.*
import java.io.BufferedReader

class PetStatus constructor(){

    var maxHealth: Int = 10
    var currentHealth: Int = 10
    var healthIncrementer: Int = 2 // amount by which health increments when a task is completed
    var healthDecrementer: Int = 0 // amount by which health decrements when user approaches / misses deadline
    var isHungry: Boolean = false
    var isTired: Boolean = false
    var isSick: Boolean = false
    var isAlive: Boolean = true

    init{
        //if a json exists, use it to update PetStatus
        val context = getContext()
        var file = File(context.getFilesDir(), "PetStatus.json")
        if(file.exists()){
            Log.d("FILE_EXISTS", "File exists!")

            val bufferedReader: BufferedReader = file.bufferedReader()
            val json = bufferedReader.readText()

            val retrievedStatus = Gson().fromJson(json, PetStatus::class.java)

            Log.d("JSON_RETRIEVED", json)
        }
        else
            Log.d("FILE_DNE", "File does not exist!")
            updateJson()
    }

    // method to update pet's health and ailment upon completing a task
    fun processCompletedTask(){
        incrementHealth()
        removeAilment()
        Log.d("TASK_COMPLETE", "completed task processed")
    }

    fun getHealthPercent(): Int {
        return currentHealth / maxHealth
    }

    // method to update pet's health and ailment upon missing a task
    fun processMissedTask(){
        decrementHealth()
        addAilment()
        Log.d("TASK_MISSED", "missed task processed")
    }

    /*
    Potentially creating another method to update pet's
    health and status because of an approaching deadline.
     */

    // method to decrement the pet's health
    private fun decrementHealth(){
        currentHealth-=healthDecrementer
        if(currentHealth <= 0)
            isAlive = false
        updateJson()
    }

    // method to increment the pet's health
    private fun incrementHealth(){
        val sum = currentHealth + healthIncrementer
        if(sum > maxHealth)
            currentHealth = maxHealth
        else
            currentHealth = sum
        updateJson()
    }

    // method to add an ailment to the pet
    private fun addAilment(){
        // if no ailment, randomly assign hungry or tired
        if(!isHungry && !isTired && !isSick){
            val rand = Random()
            val randBool = rand.nextBoolean()
            if(randBool)
                isHungry = true
            else
                isTired = true
            healthDecrementer = 1
        }

        // otherwise, if hungry XOR tired, assign the other
        else if(isHungry && !isTired){
            isTired = true
            healthDecrementer = 2
        }
        else if(isTired && !isHungry){
            isHungry = true
            healthDecrementer = 2
        }

        // otherwise, if both hungry AND tired, assign sick
        else if(isHungry && isTired){
            isSick = true
            healthDecrementer = 3
        }

        updateJson()
    }

    // method to remove an ailment from the pet
    private fun removeAilment(){
        // if sick, remove sick
        if(isSick){
            isSick = false
            healthDecrementer = 2
        }

        // otherwise, if hungry and tired, remove one of the two randomly
        else if(isHungry && isTired){
            val rand = Random()
            val randBool = rand.nextBoolean()
            if(randBool)
                isHungry = false
            else
                isTired = false
            healthDecrementer = 1
        }

        // otherwise, if hungry XOR tired, remove relevant ailment
        else if(isHungry && !isTired){
            isHungry = false
            healthDecrementer = 0
        }
        else if(isTired){
            isTired = false
            healthDecrementer = 0
        }

        updateJson()
    }

    private fun updateJson(){
        val gson = Gson()
        var json: String = gson.toJson(this)
        Log.d("JSON_UPDATE", json)

        val context = getContext()
        var file = File(context.getFilesDir(), "PetStatus.json")
        file.writeText(json)

        val bufferedReader: BufferedReader = file.bufferedReader()
        json = bufferedReader.readText()
        Log.d("JSON_FROM_FILE", json)
    }

    companion object {

        private lateinit var context: Context

        fun setContext(con: Context) {
            context=con
        }

        fun getContext() : Context {
            return context
        }
    }
}

日志信息：

Caused by: java.lang.RuntimeException: Failed to invoke public com.keikakupet.PetStatus() with no args
        at com.google.gson.internal.ConstructorConstructor$3.construct(ConstructorConstructor.java:118)
        at com.google.gson.internal.bind.ReflectiveTypeAdapterFactory$Adapter.read(ReflectiveTypeAdapterFactory.java:212)
        at com.google.gson.Gson.fromJson(Gson.java:927)
        at com.google.gson.Gson.fromJson(Gson.java:892)
        at com.google.gson.Gson.fromJson(Gson.java:841)
        at com.google.gson.Gson.fromJson(Gson.java:813)
        at com.keikakupet.PetStatus.<init>(PetStatus.kt:32)
        at java.lang.reflect.Constructor.newInstance0(Native Method)
        at java.lang.reflect.Constructor.newInstance(Constructor.java:343)
        at com.google.gson.internal.ConstructorConstructor$3.construct(ConstructorConstructor.java:110)
        	... 4981 more

标签： android-studiokotlingson

Answer 1

由于您的构造函数没有参数，因此 Gson 无法在 json 的帮助下实例化您的类。

像这样重组你的类：

class PetStatus private constructor(
    var maxHealth: Int = 10,
    var currentHealth: Int = 10,
    var healthIncrementer: Int = 2, // amount by which health increments when a task is completed
    var healthDecrementer: Int = 0, // amount by which health decrements when user approaches / misses deadline
    var isHungry: Boolean = false,
    var isTired: Boolean = false,
    var isSick: Boolean = false,
    var isAlive: Boolean = true) {

    /**
     * This is optional fix, since it is a design guideline
     * recommendation, you can retain your original function as well
     * fun getHealthPercent(): Int {
     *     return currentHealth / maxHealth
     * }
     */
    val healthPercent: Int
        get() = currentHealth / maxHealth

    ...

    companion object {

        lateinit var context: Context // getters and setters for java are automatically generated

        operator fun invoke(): PetStatus {
            //if a json exists, use it to update PetStatus
            val context = context
            var file = File(context.getFilesDir(), "PetStatus.json")
            if(file.exists()){
                Log.d("FILE_EXISTS", "File exists!")

                val bufferedReader: BufferedReader = file.bufferedReader()
                val json = bufferedReader.readText()

                val retrievedStatus = Gson().fromJson(json, PetStatus::class.java)

                Log.d("JSON_RETRIEVED", json)

                return retrievedStatus
            } else {
                Log.d("FILE_DNE", "File does not exist!")
                return PetStatus()
            }
            updateJson()
        }

        operator fun invoke(maxHealth: Int, currentHealth: Int, healthIncrementer: Int, healthDecrementer: Int, isHungry: Boolean, isTired: Boolean, isSick: Boolean, isAlive: Boolean): PetStatus
            = PetStatus(maxHealth, currentHealth, healthIncrementer, healthDecrementer, isHungry, isTired, isSick, isAlive)
    }
}

现在你可以调用这个类

PetStatus()

operator fun invoke()是一种 hack (ish) 方法，我建议您实际上应该取出代码并从外部实例化该类。