When I had a loop looking like this:
String[][] map = new String[][] //This is just to show what type map is
String code = "";
for(int y = 0; y < map[0].length; y++){
  for(int x = 0; x < map.length; x++){
 code += //Some signs
 code += map[x][y];
 code += //Some other signs
 
  }
}

one small change I would add:

When I had a loop looking like this:
String[][] map = new String[][] //This is just to show what type map is
String code = "";

int map0length=map[0].length;
int maplength=map.length;

for(int y = 0; y < map0length; y++){
  for(int x = 0; x < maplength; x++){
 code += //Some signs
 code += map[x][y];
 code += //Some other signs
 
  }
}