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nullablekotlin

Kotlin mutableMap.put returns nullable

In kotlin standard lib we have MutableMap interface which has this method

public abstract fun put(key: K, value: V): V?

Why is it returning nullable value if it accepts not nullable one? Is it done for java interop?

Solution

  • Take a look at the definition 

    /**
 * Associates the specified [value] with the specified [key] in the map.
 *
 * @return the previous value associated with the key, or `null` if the key was not present in the map.
 */
public fun put(key: K, value: V): V?

    So 

    fun main(args: Array<String>) {
    var m: MutableMap<Int, String> = mutableMapOf(Pair(1, "a"))
    val prev1Value = m.put(1, "b")
    val prev2Value = m.put(2, "c")

    println(m)
    println("Previous value of 1 was: $prev1Value")
    println("Previous value of 2 was: $prev2Value")
}

    Prints:

    {1=b, 2=c}
Previous value of 1 was: a
Previous value of 2 was: null