I am trying to convert this piece of java code to kotlin

public int compare1(Comparable c, Object o) {
    return c.compareTo(o);
  }

to kotlin code:

fun compare1(c: Comparable<*>, o: Any?): Int {
    return c.compareTo(o)
  }

But get error

Type mismatch: inferred type is Any? but Nothing was expected

Any reason why this error occurs? Thanks

CodePudding user response：

This code in Java shows a warning, because the compiler can't know if provided comparable can compare itself to provided object. Compiler still allows this, but it doesn't guarantee it won't throw an exception at runtime.

Its equivalent in Kotlin is either:

fun compare1(c: Comparable<Any?>, o: Any?): Int {
    return c.compareTo(o)
}

With this code you'll have to do unchecked casts when calling the function. Or alternatively:

fun compare1(c: Comparable<*>, o: Any?): Int {
    return (c as Comparable<Any?>).compareTo(o)
}

Note it doesn't solve the problem. You can call this function passing an integer and a string and then you will get an exception. So use this code only if the logic of your application guarantees you always pass matching objects to the function.

Even better, try to redesign your code to use generics in a type-safe manner. In that case your function would become:

fun <T> compare1(c: Comparable<T>, o: T): Int {
    return c.compareTo(o)
}

This function is type-safe, so it doesn't allow using comparables with incorrect types. It may not work as a direct replacement of your Java function though, as the original function didn't care about the type safety. You may need to redesign other parts of your code to use this function.