如何在if条件下避免NullPointerException

Question

I have to take few actions depending upon the if condition. Say I have an enum "VoucherType"

Now I have a code, that is executed depending upon the condition:-

private boolean verifyGiveAwayAccounting(GiveAwayMoneyVerificationEvent event) {

    if(event.getVoucherType().equals(VoucherType.GIVE_AWAY_MONEY_ON_SIGNUP)){
        someAction();
    }
    return verifySystemAccountTransaction(event);
}

I have to perform someAction() if the event is of type "GIVE_AWAY_MONEY_ON_SIGNUP" . But I do not have to do anything extra is event type is anything other than "GIVE_AWAY_MONEY_ON_SIGNUP". So when I call this code I set the voucherType to "GIVE_AWAY_MONEY_ON_SIGNUP" and someAction() is executed.

But for any other type of events, I get null pointer exceptions in the if condition as I never set the voucher type as I do not want to do anything special. So in order to avoid the nullPointerException I set the Voucher code to something dummy(other voucherType values) which I never use in any conditions. I there a sophisticated way I can eliminate the nullPointerException without initializing the VoucherType in event?

Answer 1

you should always test if your object is not null before making any test on this object's attributes.

if(enum != null && enum.Football) {
  //some action
}

Answer 2

If event is never null , in this case perhaps you can invert your test :

private boolean verifyGiveAwayAccounting(GiveAwayMoneyVerificationEvent event) {
    if(VoucherType.GIVE_AWAY_MONEY_ON_SIGNUP.equals(event.getVoucherType())){
            someAction();
    }
    return verifySystemAccountTransaction(event);
}

otherwise you should test if event is not null before :

private boolean verifyGiveAwayAccounting(GiveAwayMoneyVerificationEvent event) {
    if(event != null && VoucherType.GIVE_AWAY_MONEY_ON_SIGNUP.equals(event.getVoucherType())){
            someAction();
    }
    return verifySystemAccountTransaction(event);
}

Answer 3

As defined by WikiBooks:

A NullPointerException is thrown when an application attempts to use an object reference, having the null value. These include: Calling an instance method on the object referred by a null reference.

If you do not instantiate the enum value, it will have a value of null . Thus, the program is attempting to reference an object that contains null , which throws a NullPointerException .

Therefore, no, there is no way to avoid your NullPointerException . You need to instantiate variables before attempting to reference them.

Answer 4

I would check if the enum is not null before attempting to check the value of enum.Football.

    void method(){
        if(enum!=null && enum.Football){
            SomeAction();
        }
    }

Answer 5

Apart from the already mentioned answers of checking for null, another possibility would be to actually create an extra enum value that represents null (a "dummy value") and use that as the default value:

public enum VoucherType {
    UNDEFINED, 
    GIVE_AWAY_MONEY_ON_SIGNUP,
    //....
    ;
}

Define "UNDEFINED" as default value:

public class Event {
    private VoucherType voucherType = VoucherType.UNDEFINED;

    public Event() {
    }

    public VoucherType getVoucherType() {
        return this.voucherType;
    }

    public void setVoucherType(VoucherType voucherType) {
        if(voucherType==null) {
            throw new IllegalArgumentException(); // make sure that voucher type cannot be set to null
        }
        this.voucherType=voucherType;
    }
}

That way events will never have null as voucherType and instead the enum value UNDEFINED.

Warning: A lot of people would prefer recieving a NullPointerException instead of the above solution to immediatly get feedback when they forget to set the voucherType. With the above solution making the error of forgetting to set the voucherType and not realizing it (Because code throws no error) is much easier.

Also it might force you to check if the voucherType is still UNDEFINED for some operations where the requirement is that it is set to a meaningfull value.

I would actually rather check for null myself, but since you said you want other solutions I thought I post this anyway.

Answer 6

Simply invert the operands of the equals:

if(VoucherType.GIVE_AWAY_MONEY_ON_SIGNUP.equals(event.getVoucherType()))

It will never give you NullPointerException for a null value return of your getVoucherType() method. Of course you must assure that event Object is never null.

Or, as suggested in one of the comments use the == operator, it's possible for enum:

if(VoucherType.GIVE_AWAY_MONEY_ON_SIGNUP == event.getVoucherType())