java - How could I do this better to maximise performance and efficiency (for loop) -

i have these nested loops, there way make more efficient?

i want every combination of numbers. e.g. 2, 290, 29091993..etc combinations string arraylist.

int[] combo = new int[]{2, 9, 0, 9, 1, 9, 9, 3}; (int = 0; < combo.length; i++) {     combinations.add(combo[i] + "");     (int x = 0; x < combo.length; x++) {         combinations.add(combo[i] + "" + combo[x]);         (int y = 0; y < combo.length; y++) {             combinations.add(combo[i] + "" + combo[x] + "" + combo[y]);             (int z = 0; z < combo.length; z++) {                 combinations.add(combo[i] + "" + combo[x] + "" + combo[y] + "" + combo[z]);                 (int z1 = 0; z1 < combo.length; z1++) {                     combinations.add(combo[i] + "" + combo[x] + "" + combo[y] + "" + combo[z] + "" + combo[z1]);                     (int z2 = 0; z2 < combo.length; z2++) {                         combinations.add(combo[i] + "" + combo[x] + "" + combo[y] + "" + combo[z] + "" + combo[z1] + "" + combo[z2]);                         (int z3 = 0; z3 < combo.length; z3++) {                             combinations.add(combo[i] + "" + combo[x] + "" + combo[y] + "" + combo[z] + "" + combo[z1] + "" + combo[z3]);                         }                     }                 }             }         }     } } 

void addpermutation(list<string> combinations, int level, int[] combo,         stringbuilder buffer) {     int length = 0;     if (buffer != null) {         length = buffer.length();     } else {         buffer = new stringbuilder();     }     if (level < combo.length) {         (int x : combo) {             buffer.append(integer.valueof(x));             string current = buffer.tostring();             //system.out.println(current); xxx debug purpose             combinations.add(current);             addpermutation(combinations,level+1,combo,buffer);             buffer.setlength(length);         }     } }  @test public void testpermutation(){     list<string> combinations = new linkedlist<string>();     int[] combo = new int[]{2, 9, 0};     addpermutation(combinations, 0, combo, null); } 

of corse since it's time complexity it's lot high, can't miracles.