这段代码基本上按降序排列数组:
val arrayList = arrayListOf(1, 5, 2)
Collections.sort(arrayList, object : Comparator<Int> {
override fun compare(x : Int, y: Int) = y - x
})
How in the world does overriding the compare method with y - x works? How does Kotlin know what y - x means to put y before x if y < x?
This actually has nothing to do with Kotlin. It's related to the Java API's Comparator interface, and how Collections.sort uses it.
Compares its two arguments for order. Returns a negative integer, zero, or a positive integer as the first argument is less than, equal to, or greater than the second.
Now let's try this out for the arguments you gave:
Collections.sort doesn't know anything about what y - x means. It simply respects the defined contract of the Comparator interface that any implementor also needs to respect (if it wants to work).
It just so happens that y - x is an implementation that does respect that contract, because Math.