Arrays.asList(int[]) not working

When you pass an array of primitives (int[] in your case) to Arrays.asList, it creates a List<int[]> 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<T> 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<int[]>. On the other hand, when you pass an Integer[] to that method, Integer replaces T and you get a List<Integer>.

In Java 8, you don't need to convert the array at all; just turn it into a stream via Arrays#stream, then use the anyMatch predicate to see if the value you want is contained in the array.

int[] array = {3, 2, 5, 4};

if (Arrays.stream(array).anyMatch(x -> x == 3)) {
    System.out.println("The array contains 3");
}