为何出现kotlin.Unit输出？如何解决该Kotlin编程问题

我编写了如下Kotlin代码：

package com.example.codingisfun

fun main(){
    val phone1= MobilePhone("Android","Samsung","S30")
    val phone2= MobilePhone("IOS","Apple","Iphone 14")
}

class MobilePhone(osName: String, brand: String, model: String){
    private var battery= 30
    init {
        println("The phone $model from $brand uses $osName as its Operating System")
        println(chargeBattery(30))
        println(battery)
    }
    private fun chargeBattery(charged:Int){
        println("before charge:$battery,charged: $charged and now: ${battery+charged}")
        battery += charged
    }
}

运行后输出内容为：

The phone S30 from Samsung uses Android as its Operating System
before charge:30,charged: 30 and now: 60
kotlin.Unit
60
The phone Iphone 14 from Apple uses IOS as its Operating System
before charge:30,charged: 30 and now: 60
kotlin.Unit
60

我曾尝试调整chargeBattery的调用方式，但依旧会输出kotlin.Unit，请问如何解决这个问题？

问题原因 #

你看到的kotlin.Unit是因为chargeBattery函数没有返回值（Kotlin中无返回值的函数默认返回Unit类型），而你在init块里调用println(chargeBattery(30))，相当于把这个函数的返回值（也就是Unit）打印出来了。

解决方案 #

直接调用chargeBattery(30)即可，不需要把它放在println里面：

修改后的init代码块：

init {
    println("The phone $model from $brand uses $osName as its Operating System")
    chargeBattery(30) // 直接调用，不用包裹在println里
    println(battery)
}

修改后运行输出会变成：

The phone S30 from Samsung uses Android as its Operating System
before charge:30,charged: 30 and now: 60
60
The phone Iphone 14 from Apple uses IOS as its Operating System
before charge:30,charged: 30 and now: 60
60

这样就不会出现多余的kotlin.Unit输出了。

内容的提问来源于stack exchange，提问作者raptured_kira