How to print multiple attrributes from a hashMap that is a property inside a toString override - kotlin

Question

I am learning Kotlin and writing code to check my understanding. I'm trying to use a toString override to print the values of a hashMap that is a property of a class. I can't get it to work. Instead I get output like "kotlin.Unit() -> kotlin.Unit". Also, I don't understand why the values of the hashMap ARE printing out before the toString output. I don't know where that output is coming from. Please help me. Thanks. Below is my code and the output I'm getting.
Code:
package ch07.ExpandoObject
import java.beans.PropertyChangeListener
import java.beans.PropertyChangeSupport
import kotlin.properties.Delegates
import kotlin.reflect.KProperty
class Person(
val name: String = "",
age: Int? = null,
var isMarried: Boolean? = null ,_attributes: kotlin.collections.HashMap<String,String>? = hashMapOf<String, String>()
)
:PropertyChangeAware()
{
var _attributes : kotlin.collections.HashMap<String,String>? = hashMapOf<String, String>()
fun setAttribute(attrName: String, value: String) {
_attributes!!.set(attrName, value)
_attributes!!.set("name", this.name)
}
override fun toString() = "Person(name=\"${name?:""}\", age=${age?:99999}, isMarried=$isMarried) " +
"${_attributes?.get("name")} " + "$name " +
this._attributes!!.forEach { (attrName, value) -> println("$attrName = $value") } +
{
for ((attrName, value) in this._attributes!!) {
println("attribute $attrName = ${this._attributes!![attrName]}")
}
}
val _age = ObservableProperty(propName = "age", propValue = age, changeSupport = changeSupport)
private val observer = {
prop: KProperty<*>, oldValue: Int, newValue: Int ->
changeSupport.firePropertyChange(prop.name, oldValue, newValue)
}
var age: Int by Delegates.observable(initialValue = age?:99999,onChange = observer)
}
class ObservableProperty(val propName: String,
var propValue: Int?, val changeSupport: PropertyChangeSupport
) {
fun getValue(): Int? = propValue
fun setValue( newValue: Int) {
val oldValue = propValue
propValue = newValue
changeSupport.firePropertyChange(propName, oldValue, newValue)
}
}
open class PropertyChangeAware {
val changeSupport = PropertyChangeSupport(this)
fun addPropertyChangeListener(listener: PropertyChangeListener) {
changeSupport.addPropertyChangeListener(listener)
}
fun removePropertyChangeListener(listener: PropertyChangeListener) {
changeSupport.removePropertyChangeListener(listener)
}
}
fun main(args: Array<String>) {
val p = Person("Bob", 89, isMarried = false)
val data = mapOf("lastname" to "Jones", "company" to "JetBrains")
for ((attrName, value) in data)
p.setAttribute(attrName, value)
println(p)
}
Here is the current output:
name = Bob
company = JetBrains
lastname = Jones
Person(name="Bob", age=89, isMarried=false) Bob Bob kotlin.Unit() -> kotlin.Unit
Thanks, again, for any help.

Answer 1

You should not use print() or println() functions inside toString() because they output their arguments to the standard output immediately instead of appending them to the string returned to the caller.
Let's examine the output kotlin.Unit() -> kotlin.Unit you're getting. It consists of two parts:
kotlin.Unit is the string representation of attributes!!.forEach { ... } expression. forEach function returns without value, and in Kotlin it's expressed by returning the Unit object value. Its string representation is appended to the string you're returning.
the second part, () -> kotlin.Unit, is also the string representation of the lambda function expression { for((attrName, value) in ...) }. This function takes no parameters, and returns without value, which means that its type is () -> Unit. Note that in Kotlin the block { ... } declares a local lambda function. If you instead want to run the code inside of that block, use the run function: run { ... }
The goal of toString function is to build the string representation of an object. And for that you can use buildString function:
override fun toString() = buildString {
append("Person(name=\"${name?:""}\", age=${age?:99999}, isMarried=$isMarried) ")
append("${_attributes?.get("name")} ").append("$name ")
this._attributes!!.forEach { (attrName, value) -> append("$attrName = $value") }
for ((attrName, value) in this._attributes!!) {
append("attribute $attrName = ${this._attributes!![attrName]}")
}
}
This function creates a StringBuilder and passes it as a receiver to its functional argument, where you call append or appendln on that receiver. Then buildString converts that string builder to a string and returns it.