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This exercise is already available in your IntelliJ as a project. What you need to do is described in the comments at the top of the file in src/main/java/.

package algorithms;

public class Anagram {

    /**
     * Count the number of occurrences of each letter of the alphabet
     * (from 'A' to 'Z') in the given string. The function must return
     * an array containing 26 elements: The item with index 0 contains
     * the number of 'A' in the string, the item with index 1 contains
     * the number of 'B', and so on.
     *
     * This function must be case-insensitive, i.e. the character 'f'
     * must be considered as the same as character 'F'.
     *
     * Characters that are neither in the range 'a' to 'z', nor in the
     * range 'A' to 'Z' must be ignored.
     *
     * The time complexity of your method must be Theta(n),
     * where n is the size of the string.
     *
     * @param s The string of interest.
     * @return An array counting the number of occurrences of each
     * letter.
     **/
    public static int[] countAlphabet(String s) {
         return null;
    }


    /**
     * Check whether one string is an anagram of another string. It is
     * strongly advised to use the function "countAlphabet()" as a
     * building block for function "isAnagram()".
     *
     * The time complexity of your method must be O(n+m),
     * where n is the size of the first string, and m is the size of
     * the second string.
     *
     * @param s1 The first string.
     * @param s2 The second string.
     * @return <code>true</code> iff. the two strings are anagrams.
     **/
    public static boolean isAnagram(String s1,
                                    String s2) {
         return false;
    }
}