.

The difference is how you can call this function. The array variant can only be called with an array

isAscOrder(new int[] {1, 2, 3});

while the varargs variant can be called with an array or with separate arguments

isAscOrder(1, 2, 3);

and the java compiler will create the array for you under the hood.

yes, Strings are always immutable (unchangable)

But now you do...

Wish I knew the replace method......

Massive W to all Java devs on StackOverflow

And is correct that way because inputs "... are always lowercase strings, and that each has at least two letters.". If that was not the case and arbitrary inputs would be allowed we'd also need checks for null and lengths in order to avoid NullPointerException and IndexOutOfBoundsException or alike.

it work only if cases are equal

I would rather have more spaces to increase the readability of the code, but it's okay

what is the difference between type[] and type...?

Actually IntStream.of() calls Arrays.stream()
From IntStream.java :

<...>
/**
 * Returns a sequential ordered stream whose elements are the specified values.
 *
 * @param values the elements of the new stream
 * @return the new stream
 */
public static IntStream of(int... values) {
    return Arrays.stream(values);
}
<...>

I googled, and may be understood the patterns problem you mentioned. But this version is same as:
replace(char oldChar, char newChar) right?

Is this solution better than solution with Arrays.stream()?

Very efficient