Nullpointerexception when calling a method in the same file as the main method

Question

So I am messing around trying to create a game. My main method calls another method that I have placed in the same file. It worked wonderfully when tested and for some reason it just stopped working and throws a NPE at me. As a preface, I am VERY green (only on chapter 5 of my Java textbook).

Here is the relevant parts of my code. I am passing info from my main method to another method which uses another method to do a calculation. This method is passed a reference to my gameboard object which contains a string. If I pass the pushCard method a constant instead of the getSlot** method, it works perfectly. Does the NPE mean that the newBoard object that I am referencing has become null? If I place a System.out.print right before windAction() is called, it prints the correct strings and not null. I am very confused.

Any help or advice will be a big help. Thanks in advance.

public static void main (String[] args)
{
    switch (playCard)
    {               
        case "wind":
        //slotselection has already been given a value
        windAction(slotSelection.toUpperCase()); 
        break;
        // There is more code here that is not shown...............
    }
}

public static void windAction(String slotSelection)
{
    switch (slotSelection.toUpperCase())
    {
        case "A1":
        {
            if (pushCard(newBoard.getSlotA2(), newBoard.getSlotA3()) == true)
                newBoard.setSlotA3(newBoard.getSlotA2());
                newBoard.setSlotA2("EMPTY");

            if (pushCard(newBoard.getSlotB1, newBoard.getSlotC1) == true)
                newBoard.setSlotC1(newBoard.getSlotB1());
                newBoard.setSlotB1("EMPTY");

        } //end case A1
        break;

        // There is more code here that is not shown...............
    }
}




public static Boolean pushCard(String S1, String S2)

{
    Boolean result = null; 

    if ((S1 == "fire") | (S1 == "water") | (S1 == "wind")){
        if ((S2 != "fire") | (S2 != "water") | (S2 != "wind"))
            result = true;
        else
            result = false;
    }

    return result;

}//end push card method

I was trying to add that info but it would not let me. The problem is solved. Thanks guys. — Sev, Aug 01 '13 at 22:56

score 2 · Accepted Answer · answered Aug 01 '13 at 22:48

2

I believe that the NullPointerException may rise from your pushCard method -> You are using the Boolean class instead of the boolean primitive, and have a case in which it may be null.

You are using a bit-wise or operation to check for a logical or and you are checking String equality using ==, this will cause the if statement to fail and thus result will not be set:

Boolean result = null; 

if ((S1 == "fire") | (S1 == "water") | (S1 == "wind")){
    ...
}

should be:

boolean result = false; 

if ("fire".equals(S1) || "water".equals(S1) || "wind".equals(S1)){
    ...
}

Similar changes must be made for the if statement inside this one.

answered Aug 01 '13 at 22:48

Sinkingpoint

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3

Wait, how do you know this is part of the OP's source? o.O – Dennis Meng Aug 01 '13 at 22:49
2

The last method declared in the source? `public static Boolean pushCard(String S1, String S2)` – Sinkingpoint Aug 01 '13 at 22:55
That did it!! Thank you for your quick and accurate response. You rock. – Sev Aug 01 '13 at 22:55
@Quirliom I meant, since the OP hadn't posted that part of the code, how on *earth* would you know that was the problem? – Dennis Meng Aug 01 '13 at 22:56
1

Educated Guess. I've seen too many of these unboxing gone wrong problems. – Sinkingpoint Aug 01 '13 at 22:56
If so, that's one heck of a shot in the dark. – Dennis Meng Aug 01 '13 at 22:58
As an extra note, the pushCard method is working great by just changing the assignment of "result" from "null" to "false". Would revising the statement to use ".equals" be superior to using "=="? – Sev Aug 01 '13 at 23:02
@Nerves82 See http://stackoverflow.com/questions/513832/how-do-i-compare-strings-in-java – Dennis Meng Aug 01 '13 at 23:07
You should always check String equality using .equals. Reversing the condition saves from NullPointers when S1 is null – Sinkingpoint Aug 01 '13 at 23:08

Nullpointerexception when calling a method in the same file as the main method

1 Answers1