ClassCastException in Kotlin even though the conditional statement checks that it's of right ty-CodePudding

I'm burning cycles trying to understand why I'm getting a cast error when I know for a fact that it's a string.

Anyone else seen this in Kotlin? (it fails at the second to last line).

val stringValue =
    if (attributesMap[it] !is String) { // same as !(obj is String)
        gson.toJson(attributesMap[it])
    } else if (attributesMap[it] == null) {
        ""
    } else {
        if (attributesMap[it] is String)
            logger.info("it's a string wth...${attributesMap[it]}")
            
        attributesMap[it] // <--- The stacktrace shows this line as the culprit, and the above "it's a string line" is printed out as well
    }

Here's the error:

java.lang.ClassCastException: class java.util.ArrayList cannot be cast to class java.lang.String (java.util.ArrayList and java.lang.String are in module java.base of loader 'bootstrap')

CodePudding user response：

That is why it is always good idea to put { } in if conditions. Given that you have no { }, the condition if (attributesMap[it] is String) only applies to a single line below. I think you wanted to write the following:

val stringValue =
    if (attributesMap[it] !is String) { // same as !(obj is String)
        gson.toJson(attributesMap[it])
    } else if (attributesMap[it] == null) {
        ""
    } else {
        if (attributesMap[it] is String) {
            logger.info("it's a string wth...${attributesMap[it]}")
            attributesMap[it] // <--- The stacktrace shows this line as the culprit, and the above "it's a string line" is printed out as well
        }
    }

Or maybe:

val stringValue =
    if (attributesMap[it] !is String) { // same as !(obj is String)
        gson.toJson(attributesMap[it])
    } else if (attributesMap[it] == null) {
        ""
    } else if (attributesMap[it] is String) {
        logger.info("it's a string wth...${attributesMap[it]}")
        attributesMap[it] // <--- The stacktrace shows this line as the culprit, and the above "it's a string line" is printed out as well
    } else {
        // something else
    }

CodePudding user response：

As @João Dias mentioned in his answer, the problem is the way you have written this if statement.

if (attributesMap[it] is String)
    logger.info("it's a string wth...${attributesMap[it]}")
            
attributesMap[it] // <--- The stacktrace shows this line as the culprit, and the above "it's a string line" is printed out as well

attributesMap[it] // <--- The stacktrace shows this line as the culprit, and

is falling outside of the if statement, so the right way is to put this inside a curly bracket like

if (attributesMap[it] is String) {
    logger.info("it's a string wth...${attributesMap[it]}")
            
    attributesMap[it] // <--- The stacktrace shows this line as the culprit, and the above "it's a string line" is printed out as well
} else {
   //
}

CodePudding user response：

I guess the problem is that attributesMap[it] is not a property itself. It's actually a function call attributesMap.get(it). That's why compiler is not able to smart cast it.

Also, I am not sure of your order of if statements. If attributesMap[it] !is String is false, attributesMap[it] == null will definitely be false.

Try this code, see if it works for you.

val value = attributes[it] // Store the value beforehand
val stringValue =
    if(value == null)
        ""
    else if(value is String)
        value
    else
        gson.toJson(value)

You can also replace this if-else ladder with a when statement.

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