Comparing two lists in kotlin

Question

I came across with kotlin equals function to compare two list of same type. It works fine for pure Kotlin with data classes.

I'am using a Java library in Kotlin project in which a callback method returns a list of objects for a time interval of X seconds. Trying to compare the old list with new list for every call, but equals returns false even the items are same and equal.

val mOldList: MutableList<MyObject>()? = null

override fun updatedList(list: MutableList<MyObject>){
    // other code
    if (mOldList.equals(list)) // false everytime
}

Is this because of Java's equals method from library?

Alternative suggestions for list compare would be appreciative.

Answer 1

Just fyi you can call list1 == list2 without any extra work, if your custom object is based off of a data class (which automatically overrides equals for you).

Answer 2

If you don't bother about order of elements in both lists, and your goal is to just check that two lists are of exactly same elements, without any others, you can consider two mutual containsAll calls like:

var list1 = mutableListOf<String>()
var list2 = mutableListOf<String>()

if(list1.containsAll(list2) && list2.containsAll(list1)) {
    //both lists are of the same elements
}

Answer 3

Java lists implement equals method and two lists are defined to be equal if they contain the same elements in the same order. I guess, you are missing equals method in your MyObject class.

Answer 4

Using zip

zip returns a list of pairs built from the elements of this array and the other array with the same index. The returned list has length of the shortest collection.

fun listsEqual(list1: List<Any>, list2: List<Any>): Boolean {

    if (list1.size != list2.size)
        return false

    val pairList = list1.zip(list2)

    return pairList.all { (elt1, elt2) ->
        elt1 == elt2       
    }
}