CODEWITHSUNDEEP
kotlin
esu
esu

Reputation: 2450

What is difference between plus() and put() methods in kotlin map?

What is difference between plus() and put() methods in kotlin map?

I have created the map and use plus() method to add my key value pair but when I print the map, I can't find the added values. And then I have moved to put(). What is the behaviour of plus() method?

Upvotes: 6

Views: 9166

Answers (2)

José Carlos
José Carlos

Reputation: 128

A regular Map (which is immutable) can only be modified using the plus function (that can also be invoked with the + operator).

The plus operator works with pairs of values, and, by nature, does not modify the original map; it returns a new map with the new entry.

// mapA and mapB are different maps
val mapA = mapOf("a" to 1, "b" to 2)
val mapB = mapA + ("c" to 3)

When using a MutableMap, in addition to plus (which will still return a new map), there are a few more options to change a map's content: put, set (which is the [] operator) and plusAssign (which is the += operator).

The difference between set and put, as function calls, is that put returns the previous value of the key (or null), whereas set does not return anything. Also, when using bracket syntax, you can't use it as a expression body for a function. It's specially useful when creating wrapper classes to access the data inside a map:

class SomeWrapper(private val backingMap: MutableMap<String, String?>) {

    var someIntValue: Int?
        get() = backingMap["someInt"]?.toIntOrNull() 
        set(value) = backingMap.set("someInt", value.toString())

        // this is invalid because assignments are not expressions
        // set(value) = backingMap["someInt"] = value

        // this is also invalid because put would return String? instead of Unit
        // set(value) = backingMap.put("someInt", value)

}

(of course, in the example above you could just change the setter to use block syntax instead of expression syntax, it's a matter of taste)

The syntax of plusAssign is similar to plus, but it will change the original map instead:

val map = mutableMapOf("a" to 1, "b" to 2)
map += ("c" to 3)

Upvotes: 6

yole
yole

Reputation: 97258

As described in the documentation, the plus method (which is normally used in its operator form, +) returns a new map containing the elements from the original map and the key/value pair passed to it as a parameter. It does not modify the map on which it was called.

The idiomatic way to add values to a Kotlin map is not to use the put method, but to use square brackets:

map[key] = value

Upvotes: 11

Related Questions