Index of an element in an array using Java

Question

The following code in Java return -1. I thought that it should return 3.

int[] array = {1,2,3,4,5,6}; 
System.out.println(Arrays.asList(array).indexOf(4));

Can you help me understand how this function works.

Thanks

Answer 1

Before Java 5, Arrays.asList used to accept an Object[]. When generics and varargs were introduced into the language this was changed to

public static <T> List<T> asList(T... arr)

In your example, T cannot be int because int is a primitive type. Unfortunately, the signature matches with T equal to int[] instead, which is a reference type. The result is that you end up with a List containing an array, not a List of integers.

In Java 4, your code would not have compiled because an int[] is not an Object[]. Not compiling is preferable to producing a strange result, and in Effective Java, Josh Bloch says retrofitting asList to be a varargs method was a mistake.