Why do I need to typecast when using thenComparing in Java

Question

I'm deeply confused by why the first block of code can compile, while the second block of code can't compile.

Can compile

        int[][] arr = {{2,2}, {1,2}, {3,4}};
        List<int[]> list = Arrays.asList(arr);
        
        // sort by first element
        list.sort(Comparator.comparingInt(x -> x[0]));

Cannot compile. Error is "Array type expected; found: 'java.lang.Object'"

        int[][] arr = {{2,2}, {1,2}, {3,4}};
        List<int[]> list = Arrays.asList(arr);

        // sort by first element, then by second element
        list.sort(Comparator.comparingInt(x -> x[0]).thenComparingInt( x -> x[1]));

In order to fix the error, I had to do

list.sort(Comparator.comparingInt(x -> ((int[])x)[0]).thenComparingInt( x -> ((int[])x)[1]));

My question is, why I had to typecast if calling comparingInt and then thenComparingInt, while typecast is not needed if I was only calling comparingInt?

Answer 1

With just Comparator.comparingInt, the compiler can infer that the single generic method call and the desired type should be the same - Comparator<int[]>.

The second chained call disrupts the type inference completely. The desired type is still Comparator<int[]>, and this is what thenComparingInt should return. But the compiler is not (yet?) smart enough to determine what the return type of Comparator.comparingInt should be. That's why you get a compiler error.

Your casts work, but you can make it a bit easier by telling the compiler what generic type to use for Comparator.comparingInt. Then, thenComparingInt continues from that generic type.

There are two ways I can think of:

1. list.sort(Comparator.<int[]>comparingInt(x -> x[0]).thenComparingInt(x -> x[1]))

2. list.sort(Comparator.comparingInt((int[] x) -> x[0]).thenComparingInt(x -> x[1]))

In the first case, you just tell the compiler "the generic type for the method is int[]". In the second case, you let the compiler infer it from the argument, which you tell the compiler is a Function<int[], int> by specifying the input type.