Java-创建卡片组类

Question

I am having trouble with my deck class, i have code that creates a finite deck and will return a value from the array and then make it null. it also checks to see if it is null to begin with because getCard() is called multiple times. this ensures there are no duplicate cards. The problem is that when i run my code it prints empty lines which must mean it is returning null which by my understanding shoudnt be happening.

public class Deck {

  public int randomNumber;
  private String card;
  private String [] deck;
  public Deck()
  {
    deck = new String [] {"Ace Of Hearts" , "2 Of Hearts" , "3 Of Hearts" , "4 Of Hearts" , "5 Of Hearts" , "6 Of Hearts" , "7 Of Hearts" , 
  "8 Of Hearts" , "9 Of Hearts" , "10 Of Hearts" , "Jack Of Hearts" , "Queen Of Hearts" , "King Of Hearts" ,
  "Ace Of Diamonds" , "2 Of Diamonds" , "3 Of Diamonds" , "4 Of Diamonds" , "5 Of Diamonds" , "6 Of Diamonds" ,
  "7 Of Diamonds" , "8 Of Diamonds" , "9 Of Diamonds" , "10 Of Diamonds" , "Jack Of Diamonds" , "Queen Of Diamonds" ,
  "King Of Diamonds" , "Ace Of Clovers" , "2 Of Clovers" , "3 Of Clovers" , "4 Of Clovers" , "5 Of Clovers" ,
  "6 Of Clovers" , "7 Of Clovers" , "8 Of Clovers" , "9 Of Clovers" , "10 Of Clovers" , "Jack Of Clovers" ,
  "Queen Of Clovers" , "King Of Clovers" , "Ace Of Spades" , "2 Of Spades" , "3 Of Spades" , "4 Of Spades" , "5 Of Spades" ,
  "6 Of Spades" , "7 Of Spades" , "8 Of Spades" , "9 Of Spades" , "10 Of Spades" , "Jack Of Spades" ,
  "Queen Of Spades" , "King Of Spades"};
  }

  public String getCard ()
  {
    randomNumber = (int) (Math.random() * 51) + 1;
    String tempCard;
    if (deck[randomNumber] != null)
    {
      tempCard = deck[randomNumber];
      deck[randomNumber] = ("");
      return (tempCard); 
    }
else
{
  while (deck[randomNumber] == null)
  {
    randomNumber = (int) (Math.random() * 51) + 1;
  }
  tempCard = deck[randomNumber];
  deck [randomNumber] = ("");
  return (tempCard);
  }
  }
}

here is a test program to run my deck class:

import hsa.Console;

public class DeckTest {
static Console c;

  public static void main(String[] args) {

//The Console 
c = new Console();

Deck Card = new Deck();

c.println(Card.getCard());
c.println(Card.getCard());
c.println(Card.getCard());
c.println(Card.getCard());
c.println(Card.getCard());
c.println(Card.getCard());
c.println(Card.getCard());
c.println(Card.getCard());
c.println(Card.getCard());
c.println(Card.getCard());
c.println(Card.getCard());
c.println(Card.getCard());
c.println(Card.getCard());
c.println(Card.getCard());
c.println(Card.getCard());
c.println(Card.getCard());
c.println(Card.getCard());
c.println(Card.getCard());
c.println(Card.getCard());
c.println(Card.getCard());
c.println(Card.getCard());
  }
 }

Answer 1

This test is futile:

deck[randomNumber] != null

because all of your deck elements aren't null and you are replacing drawn cards with "", which is the empty string, but not null.

Use

 deck [randomNumber] = null;

Also the loop should be improved.

private Random rand = new Random();
public String getCard () {
    String tempCard = null;
    do {
        int r = rand.nextInt( deck.length );
        tempCard = deck[r];
        deck[r] = null;
    } while( tempCard == null );
    return tempCard; 
}

A test should be added so as not to try to draw more than there are in the deck.

Answer 2

There are several things that can be improved with your code.

To solve your problem: "" is not the same as null . The former is a non-null empty string. The latter is a pointer that points to nothing. You will need to use the latter to have your null-check str == null work as you intend.

Arrays are 0-indexed. For some reason, in your random number you are adding 1, making the Ace of Hearts be a card that can never be selected. What you want instead is to make sure you are multiplying by the size of the number of elements. That will create a uniformly distributed number from [0, size) and using a cast to integer will give you the correct number.

I would also suggest looking at a do-while loop instead of an if statement with a while loop. That would significantly clean up your code.