When you pass an array of primitives (int[] in your case) to Arrays.asList, it creates a List with a single element - the array itself. Therefore contains(3) returns false. contains(array) would return true.
If you'll use Integer[] instead of int[], it will work.
Integer[] array = {3, 2, 5, 4};
if (Arrays.asList(array).contains(3))
{
System.out.println("The array contains 3");
}
A further explanation :
The signature of asList is List asList(T...). A primitive can't replace a generic type parameter. Therefore, when you pass to this method an int[], the entire int[] array replaces T and you get a List. On the other hand, when you pass an Integer[] to that method, Integer replaces T and you get a List.
