Java variables updating without prompt?

Question

I am trying to create a game, where a player can move around the games map pulled from a text file. It has 2 stores of the map, originalmap and playermap. originalmap is used to reference the original state of the map, so that the player can move over tiles that have a special nature. playermap is what the player sees, and the changes, such as moving and spawning the player itself happens in playermap. Below is the code where both originalmap and playermap is created. Both maps are 2d arraylists.

class map {
  //properties of the map.
  private List<List<Character>> originalmap = new ArrayList<List<Character>>();
  private List<List<Character>> playermap = new ArrayList<List<Character>>();
  //read a map file. Existence is checked when entering filepath, so no need to code any catches.
  private void createmap() {
    try {
      File mapFile = new File(this.filepath);
      Scanner textfile = new Scanner(mapFile);
      //read the entire mapfile, line by line. For every line, a mapLine converts the line into a list of characters, which is appended to the map variable.
      this.mapname = textfile.nextLine().replace("name ", "");
      String line2 = textfile.nextLine();
      this.goldreq = Integer.parseInt(Character.toString(line2.charAt(line2.length()-1)));
      while (textfile.hasNextLine()) {
        String line = textfile.nextLine();
        List<Character> mapLine = new ArrayList<Character>();
        for (int i = 0; i<line.length();i++) {
          mapLine.add(line.charAt(i));  
        }
        this.originalmap.add(mapLine); 
        this.playermap.add(mapLine);
      }
      textfile.close();
    } catch (FileNotFoundException error) {
      assert true;
    }
  }

The issue is when I try to spawn a player, it appears that the player spawns on both the originalmap and the playermap. I am at a loss as to why. Below is the code where the player is spawned.

  public void spawnPlayer() {
    Random rand = new Random();
    while (true) {        
      int spawnx = rand.nextInt(playermap.get(0).size());
      int spawny = rand.nextInt(playermap.size());
      char spawnchar = playermap.get(spawny).get(spawnx);
      if (spawnchar!= '#'||spawnchar != 'G'||spawnchar != 'B') {
        this.playergold = 0;
        this.playerposy = spawny;
        this.playerposx = spawnx;
        this.playermap.get(spawny).set(spawnx,'P');
        break;
      }
    }
  }
}

N/B: these functions are called one after another (createmap, and then spawnplayer) in the default method of the class.

I tried to print the maps, expecting to see originalmap to be unchanged and playermap to be changed. Both maps were changed, without me changing originalmap after spawning the player.

Answer 1

Here are the problem statements that are related:

In createMap:

this.originalmap.add(mapLine); 
this.playermap.add(mapLine);

In spawnPlayer:

this.playermap.get(spawny).set(spawnx,'P');

When you add the same mapLine to each List, you are not adding a duplicate of the values you are adding a reference to the same List.

The next problem statement will then grab the List from playermap and change the value, which will of course affect both playermap and originalmap because they are referencing the same exact lists.

To fix this, you simply need to create two separate List, playerMapLine and originalMapLine:

private void createmap() {
    try {
      File mapFile = new File(this.filepath);
      Scanner textfile = new Scanner(mapFile);
      //read the entire mapfile, line by line. For every line, a mapLine converts the line into a list of characters, which is appended to the map variable.
      this.mapname = textfile.nextLine().replace("name ", "");
      String line2 = textfile.nextLine();
      this.goldreq = Integer.parseInt(Character.toString(line2.charAt(line2.length()-1)));
      while (textfile.hasNextLine()) {
        String line = textfile.nextLine();
        List<Character> playerMapLine = new ArrayList<>();
        List<Character> originalMapLine = new ArrayList<>();

        for (int i = 0; i<line.length();i++) {
            playerMapLine.add(line.charAt(i));  
            originalMapLine.add(line.charAt(i));  
        }
        this.playermap.add(playerMapLine);
        this.originalmap.add(originalMapLine); 
      }
      textfile.close();
    } catch (FileNotFoundException error) {
      error.printStackTrace();
    }

Also, please do not swallow an Exception and print the statement out with error.printStackTrace(), or log it properly so when an error happens, it is clear.