Java, Simplified check if int array contains int

You could simply use ArrayUtils.contains from Apache Commons Lang library.

public boolean contains(final int[] array, final int key) {     
    return ArrayUtils.contains(array, key);
}

Here is Java 8 solution

public static boolean contains(final int[] arr, final int key) {
    return Arrays.stream(arr).anyMatch(i -> i == key);
}

It's because Arrays.asList(array) returns List<int[]>. The array argument is treated as one value you want to wrap (you get a list of arrays of ints), not as vararg.

Note that it does work with object types (not primitives):

public boolean contains(final String[] array, final String key) {
    return Arrays.asList(array).contains(key);
}

or even:

public <T>  boolean contains(final T[] array, final T key) {
    return Arrays.asList(array).contains(key);
}

But you cannot have List<int> and autoboxing is not working here.

Guava offers additional methods for primitive types. Among them a contains method which takes the same arguments as yours.

public boolean contains(final int[] array, final int key) {
    return Ints.contains(array, key);
}

You might as well statically import the guava version.

See Guava Primitives Explained