如何检查2D对象数组是否包含某个对象？ （Java）

Question

public class Maze {
    private static final int Max_Maze_Row=20;
    private static final int Max_Maze_Column=50;
    public static Entity[][] maze =new Entity[Max_Maze_Row][Max_Maze_Column];


    public Maze(){

    }

    public static void create(String filename) throws FileNotFoundException{
        Scanner fileinput=new Scanner(new FileInputStream(filename));
        fileinput.useDelimiter("");

        while(fileinput.hasNextLine()){
            for(int row=0;row<Max_Maze_Row;row++){
                String line_content=fileinput.nextLine();
                for(int col=0;col<Max_Maze_Column;col++){
                    if(line_content.charAt(col)=='*'){
                        maze[row][col]=new Wall('*',row,col);
                    }
                    if(line_content.charAt(col)==' '){
                        maze[row][col]=new Space(' ',row,col);
                    }
                }    
            }
        }
}
public abstract class Entity {
    protected char symbol;
    protected int row;
    protected int col;

    Entity(char symbol, int row, int col){
        this.symbol=symbol;
        this.row=row;
        this.col=col;
    }

    public abstract void create();

    public char Entity_put(char newEntity, int newRow, int newCol){
        char oldEntity=this.symbol;
        this.symbol=newEntity;
        this.row=newRow;
        this.col=newCol;
        System.out.println(oldEntity);
        return oldEntity;
    }

}
public class Wall extends Entity{
    public Wall(char symbol,int row,int col){
        super(symbol,row,col);
    }
    public void create(){

    }

}

I also have a class Space which inherits from the Entity superclass. Its essentially ' '. So what happens is, the program reads from a text file, then creates wall and space entities, which have a specific row, column, character value. And these entities are stored in the Entity[][] maze. However, how can I reference these objects? For example, how can I know how many wall objects there are in the maze array? Basically, what I am trying to do is something along the lines of: Randomly pick an entity that is not a wall (ie space entity). Do something.

Answer 1

You can reference the objects in the multidimensional array using their index, then you can use instanceof to identify if they are Wall or Space.....you can write something like below. Below is a code to count the number of Walls' and Spaces' in your Maze multi dimensional array.

public static void count(Maze[][] maze) {
    int wallCount = 0;

    int spaceCount = 0;

    if (maze != null)

        for (int i = 0; i < maze.length; i++) {

            if (maze[i] != null)
                for (int j = 0; j < maze[i].length; j++) {

                    if (maze[i][j] instanceof Wall)
                        wallCount++;
                    if (maze[i][j] instanceof Space)
                        spaceCount++;
                }

        }

}

Answer 2

Edit: I think you can have another abstract function such as performAction in Entity class, and override this method in each subclass, namely Wall and Space in this case. When you traverse through the array later, you can just do

Entity entity = maze[row][col];
entity.performAction();

Java will figure out which subclass's performAction to use at runtime.

=========================================================================== One option is to store two additional ArrayList as fields. For instance,

List<Entity> walls;
List<Entity> spaces;

Initialize them inside the constructor.

public Maze() {
    this.walls = new ArrayList<Entity>();
    this.spaces = new ArrayList<Entity>();
}

Inside the most inner loop in Maze.create() , you can use these to lists to track walls and spaces.

for(int col = 0; col < Max_Maze_Column; col++){
    if (line_content.charAt(col) == '*') {
        maze[row][col] = new Wall('*', row, col);
        this.walls.add(maze[row][col]);
    } 

    if (line_content.charAt(col) == ' ') {
        maze[row][col] = new Space(' ', row, col);
        this.spaces.add(maze[row][col]);
    }
}

You can get every single wall and space through index access. For size, just call walls.size() and spaces.size() . For random access, you can use

// int max = walls.size() or spaces.size()
Random seed = new Random();
// Generate a random integer from 0 to (max - 1)
int index = seed.nextInt(max);

The second option may be using the instanceof operator when you traverse through the array later. For instance,

Entity entity = maze[row][col];

if (entity instanceof Wall) {
    Wall wall = (Wall) entity;
    // do something with wall

} else if (entity instanceof Space) {
    Space space = (Space) entity;
    // do something with space
}