Difference between ArrayList() and mutable

问题:

private val repositories = mutableListOf<String>()

private val repositories = ArrayList<String>()

Both are mutable list, then what is the point of two keywords mutableListOf or ArrayList?

or is there any major difference?

回答1:

The only difference between the two is communicating your intent.

When you write val a = mutableListOf(), you're saying "I want a mutable list, and I don't particularly care about the implementation". When you write, instead, val a = ArrayList(), you're saying "I specifically want an ArrayList".

In practice, in the current implementation of Kotlin compiling to the JVM, calling mutableListOf will produce an ArrayList, and there's no difference in behaviour: once the list is built, everything will behave the same.

Now, let's say that a future version of Kotlin changes mutableListOf to return a different type of list.

Likelier than not, the Kotlin team would only make that change if they figure the new implementation works better for most use cases. mutableListOf would then have you using that new list implementation transparently, and you'd get that better behaviour for free. Go with mutableListOf if that sounds like your case.

On the other hand, maybe you spent a bunch of time thinking about your problem, and figured that ArrayList really is the best fit for your problem, and you don't want to risk getting moved to something sub-optimal. Then you probably want to either use ArrayList directly, or use the arrayListOf factory function (an ArrayList-specific analogue to mutableListOf).

回答2:

The mutableListOf<T>() is an inlined function invocation that returns MutableList<T>. _{As of today the mutableListOf does return an instance of ArrayList.}

The ArrayList<String>() is a constructor invocation and cannot be inlined.

In other words given

 val a = mutableListOf<String>()
 val b = ArrayList<String>()

• a is of type MutableList<String>
• b is of type ArrayList<String>

At runtime both a and b will hold an instance of ArrayList.

Note that inlining is particularly useful when combined with type reification which justifies the existence of listOf, mutableListOf and a like.

回答3:

As you can see in sources:

public inline fun <T> mutableListOf(): MutableList<T> = ArrayList()

So, there is no difference, just a convenience method.