How to check if an array (unsorted) contains a certain value?

CheckExists

package com.westudio.java;

import java.util.Arrays;
import java.util.HashSet;
import java.util.Set;

/**
 * Actually, if you really need to check if a value is contained in some array/collection efficiently,
 * a sorted list or tree can do it in O(log(n)) or hashset can do it in O(1).
 */
public class CheckExists {

    private static boolean exists(String[] strings, String str) {
        return Arrays.asList(strings).contains(str);
    }

    private static boolean existsTwo(String[] strings, String str) {
        Set<String> set = new HashSet<String>(Arrays.asList(strings));
        return set.contains(str);
    }

    private static boolean existsThree(String[] strings, String str) {
        for (String s: strings) {
            if (s.equals(str)) {
                return true;
            }
        }

        return false;
    }

    /**
     * This method can only be used when the array is sorted
     * @param strings
     * @param str
     * @return
     */
    private static boolean existsFour(String[] strings, String str) {
        int a = Arrays.binarySearch(strings, str);

        if (a < 0) {
            return false;
        }

        return true;
    }

    public static void main(String[] args) {
        String[] strs = {"a", "v", "c", "d", "e"};
        System.out.println(exists(strs, "a"));
        System.out.println(existsTwo(strs, "a"));
        System.out.println(existsThree(strs, "a"));
    }

}