Returning random key from HashMap

Question

I have the following class Room and the enum class Direction, how would I write a method to pick a random key from the HashMap exits and return it, or return null if there are no exits in that Room?

public class Room 
{
private String description;
private HashMap<Direction, Room> exits;        // stores exits of this room.
public Set<Character> chars;  // stores the characters that are in this room.

/**
 * Create a room described "description". Initially, it has
 * no exits. "description" is something like "a kitchen" or
 * "an open court yard".
 * @param description The room's description.
 * Pre-condition: description is not null.
 */
public Room(String description) 
{
    assert description != null : "Room.Room has null description";
    this.description = description;
    exits = new HashMap<Direction, Room>();
    chars = new HashSet<Character>();
    sane();
}
/**
 * Define an exit from this room.
 * @param direction The direction of the exit.
 * @param neighbor  The room to which the exit leads.
 * Pre-condition: neither direction nor neighbor are null; 
 * there is no room in given direction yet.
 */
public void setExit(Direction direction, Room neighbor) 
{
    assert direction != null : "Room.setExit gets null direction";
    assert neighbor != null : "Room.setExit gets null neighbor";
    assert getExit(direction) == null : "Room.setExit set for direction that has neighbor";
    sane();
    exits.put(direction, neighbor);
    sane();
    assert getExit(direction) == neighbor : "Room.setExit has wrong neighbor";
}

Direction:

public enum Direction
{
NORTH("north"), WEST("west"), SOUTH("south"), EAST("east");

private String name;

/**
 * Constructor with parameter.
 * Pre-condition: name is not null.
 */
private Direction(String name)
{
    assert name != null : "Direction.Direction has null name";
    this.name = name;
    assert toString().equals(name) : "Direction.Direction produces wrong toString";
}

/**
 * Return the direction name.
 */
public String toString()
{
    return name;
}

}

From `exits.keySet()`, grab the `Set` and its size, then [pick a random element from the set](https://stackoverflow.com/questions/124671/picking-a-random-element-from-a-set). — rgettman, Mar 22 '18 at 19:16
@super95 exits.keySet() will return Set. Before performing exits.keySet() check if exits in not null. Then follow as rgettman suggested. — Karan Khanna, Mar 22 '18 at 20:00

score 0 · Answer 1 · answered Mar 23 '18 at 06:15

Note : As your Map Key is enum Direction, You should always get the value using the same Key type Direction, Otherwise it will return null

Example: map.get(Direction.NORTH);

Solution

 Step 1: Set<Direction> directions=exits.keySet(); 
 Step 2: if(directions.keySet().size()==0){return null;} //check if there is no exit 
 Step 2: Object[] dirArray = directions.toArray();
 Step 3: Random random = new Random();
         int randIndex = random.nextInt(dirArray.length-1);
 Step 4: System.out.println(dirArray [randIndex ]); // Random Direction Here

Let me know if you have any doubt, I have tested above code snippet

Nikolai Shevchenko · Answer 2 · 2018-03-22T19:22:38.467

-2

Something like that:

private Direction getRandomDirection() {
    if (exits.isEmpty()) {
        return null;
    }
    int rnd = new Random().nextInt(exits.keySet().size());
    return new ArrayList<>(exits.keySet()).get(rnd);
}

edited Mar 22 '18 at 19:22

answered Mar 22 '18 at 19:17

Nikolai Shevchenko

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Returning random key from HashMap

2 Answers2