Java Calling A Method without creating an instance

问题

Please excuse me if this question looks dumb for a regular java programmer but i'm stuck with this problem. I want to call the method getPoliticCards() from the class PoliticCard in the class DrawCard(Player player). At first i used a static arraylist in PoliticCard so i had no problems, but i had to change it because i'm supposed to be able to run several sessions of the game at the same time.

public enum Color {
    BLACK, PURPLE
}
public class Player {
    private int id;
    private ArrayList<Color> politicCards;
    public Player(int id){
        this.setId(id);
        ArrayList<Color> array=new ArrayList<Color>();
        setPoliticCards(array);
    }
    public int getId() {
        return id;
    }
    public void setId(int id) {
        this.id = id;
    }

    public ArrayList<Color> getPoliticCards() {
        return politicCards;
    }
    public void setPoliticCards(ArrayList<Color> politicCards) {
        this.politicCards = politicCards;
    }
}
public class PoliticCard {
    private ArrayList<Color> politicCards;
    public PoliticCard(){
        setPoliticCards(new ArrayList<Color>());
        politicCards.add(Color.BLACK);
        politicCards.add(Color.PURPLE);
    }
    public ArrayList<Color> getPoliticCards() {
        return politicCards;
    }
    public void setPoliticCards(ArrayList<Color> politicCards) {
        this.politicCards = politicCards;
    }



}
public class DrawPoliticCard {
    public DrawPoliticCard(Player player){
        PoliticCard politicCard = new PoliticCard();//I know that to 
//call a method from another class you should create an instance,
//but isn't *new PoliticCard();* creating a new arraylist in PoliticCard()?, 
//what i want is to create the arraylist only once (like it's written in the test below) 
//and then use the same updated arraylist each time i use this constructor
        player.getPoliticCards().add(politicCard.getPoliticCards().get(0));
        politicCard.getPoliticCards().remove(0);

    }

}
public class ModelPlayerTest {

    @Test
    public void testDrawCard() {
        Player player = new Player(1);
        new PoliticCard();
        new DrawPoliticCard(player);
        assertNotNull(player.getPoliticCards().get(0));
    }


}

回答1:

Here's how you can accomplish this:

// Fully construct the ArrayList here and it gets created just once per instance.
private ArrayList<Color> politicCards = new ArrayList<Color>()
public PoliticCard() {

    // Get rid of this call
    // setPoliticCards(new ArrayList<Color>());
    politicCards.add(Color.BLACK);
    politicCards.add(Color.PURPLE);
}

EDIT:

Create one PoliticCard instance in the DrawPoliticCard class and have each player draw from the same instance.

public class DrawPoliticCard
{
    final static PoliticCard politicCard = new PoliticCard();
    public static void drawCard(Player player)
    {
        player.getPoliticCards().add(politicCard.getPoliticCards().get(0));
        politicCard.getPoliticCards().remove(0);
    }
}

Usage:

public class ModelPlayerTest {

    @Test
    public void testDrawCard() {
        Player player = new Player(1);
        Player player2 = new Player(2);

        // draw a card for player one
        DrawPoliticCard.drawCard(player);

        // draw a card for player two
        DrawPoliticCard.drawCard(player2);
    }
}

回答2:

The only way to invoke a method from a class without using a real instance of it is by invoking a static method.

回答3:

you can implement singleton for PoliticCard:

private static PoliticCard instance = null;
public static PoliticCard getInstance()
{
    if (instance == null)
        instance = new PoliticCard();
    return instance;    
}

then instead of calling new PoliticCard(); use PoliticCard.getInstance(); that will make sure you only create 1 instance of PoliticCard

来源：https://stackoverflow.com/questions/37597778/java-calling-a-method-without-creating-an-instance

标签

java

arraylist

instance