list组合的问题，求源码

String [] str = {"a","b","c","d"};
求这四个元素能组成的不重复list   a，b和 b，a   算同一个，求源码

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

public class ABCTest {
    public static void main(String[] args) {
        String[] str = {"a","b","c","d"};
        Set<String> list = new HashSet<>();
        for (int i = 0; i < str.length; i++) {
            list.add(str[i]);
            for (int j = i+1; j < str.length; j++) {
                list.add(str[i]+str[j]);
                for (int k = j+1; k < str.length; k++) {
                    list.add(str[i]+str[j]+str[k]);
                    for (int l = k+1; l < str.length; l++) {
                        list.add(str[i]+str[j]+str[k]+str[l]);
                    }
                }

            }
        }
        System.out.println(list);
    }
}

输出结果：[a, ab, bc, cd, abd, ac, b, bcd, bd, abc, acd, ad, c, d, abcd]

帮忙采纳我的回答！谢谢！！！ 

/***
 * 源自 https://www.cnblogs.com/zlingh/p/4040742.html
 */
public class SetTest {

    public static void main(String[] args) {

        String str[] = { "A", "B", "C", "D", "E" };

        int nCnt = str.length;

        int nBit = (0xFFFFFFFF >>> (32 - nCnt));

        for (int i = 1; i <= nBit; i++) {
            for (int j = 0; j < nCnt; j++) {
                if ((i << (31 - j)) >> 31 == -1) {
                    System.out.print(str[j]);
                }
            }
            System.out.println("");
        }

    }
}