尝试使用其他类中的方法时出现空指针异常

Question

EDIT: [SOLVED] Thanks again -- I think I learned something new that I should have known about arrays:

When creating object[] something = new object[int]; something all point to null and need to be explicitly told what to point to.

After browsing through possible solutions I still haven't found the answer to my question:

I am attempting to call a method from another class but it seems that it isn't initialized/instantiated. I thought I did but apparently not? I keep getting a NPE

Main method (countingCards.java):

public class deck {
     card[] deckCard = new card[51];

    /** Rules for the deck of card.
      * 1. You cannot have same value of cards with same suite.
      * --- i.e) no two queens of hearts
     **/

    public void test() {
        System.out.println( deckCard.length );
        System.out.println( deckCard[1].getValue() );
    }

Card class:

public class card {
    private String value = "hello";
    private String suite = "suiteHello";

    // Method to return value
    public String getValue() {
        return value;
    }
    // Method to return suite
    public String getSuite() {
        return suite;
    }
    // Method to set value
    public void setValue(String s) {
        value = s;
    }
    // Method to set suite
    public void setSuite(String s) {
        suite = s;
    }
    // Method to test
    public void testing() {
        value = "test";
        System.out.println( value );
    }
}

deck class:

public class deck {
    card[] deckCard = new card[51];

    /** Rules for the deck of card.
      * 1. You cannot have same value of cards with same suite.
      * --- i.e) no two queens of hearts
     **/

    public void test() {
        System.out.println( deckCard.length );
        System.out.println( deckCard[1].getValue() );
    }
}

The Exception I get says

 Exception in thread "main" java.lang.NullPointerException
 at deck.test(deck.java:11)
 at countingCards.main(countingCards.java:4)

Anyone have any idea what's going on with my code?

Answer 1

You have not initialized the elements of the array. You need something like this in your deck class.

int l = deckCard.length();

for (i = 0; i<l; i++) {
    deckCard[i] = new Card();
}

PD: the name class should be Card with uppercase.

Answer 2

deckCard[1].getValue()引发NullPointerException因为在您明确初始化它们之前， deckCard数组的所有 51个元素都为null 。

Answer 3

You need to actually add values to this array, otherwise it will always return null . Try making a quick for-loop and add some random variables to it, then try it.

You can't get the value for something that was never initially created.