俩个数组去重合并成一个数组

import java.util.*;
public class Merge {
    public static void main(String[] args) {
        int [] a = {1, 2, 7, 4, 4, 2};
        int [] b = {1, 5, 5, 6};
        int [] c = concat(a, b);
        setDistinct(c);
        listDistinct(c);
        mapDistinct(c);
    }
    //合并数组
    public static int[] concat(int [] first, int[] second) {
        int [] result = Arrays.copyOf(first, first.length + second.length);
        System.arraycopy(second, 0, result, first.length, second.length);
        return result;
    }
    //数组去重
　　//第一种：使用set集合无序不重复
    public static void setDistinct(int[] arr) {
        HashSet<Integer> set = new HashSet<Integer>();
        for (Integer i : arr) {
            set.add(i);
        }
        Integer[] newArr = set.toArray(new Integer[1]);
        System.out.println(Arrays.toString(newArr));
    }
    //第二种：使用list集合，判断元素不存在再插入
    public static void listDistinct(int [] arr) {
        List<Integer> list = new ArrayList<Integer>();
        for (int i = 0; i < arr.length; i++) {
            if (!list.contains(arr[i])) {
                list.add(arr[i]);
            }
        }
        Integer[] newArrStr = list.toArray(new Integer[1]);
        System.out.println(Arrays.toString(newArrStr));
    }
    //第三种：使用Map集合，键唯一
    public static void mapDistinct(int [] arr) {
        Map<Integer, Object> map = new HashMap<Integer, Object>();
        for (Integer i : arr) {
            map.put(i, i);
        }
        Integer[] newArrStr = map.keySet().toArray(new Integer[1]);
        System.out.println(Arrays.toString(newArrStr));
    }
}