Simple calculation returning NullPointer exception... why?

Question

(I'm new to Java so please forgive my ignorance).

I have a very simple class:

public class Bill {
    private String Total;
    private int Tip;
    private int People;
    Bill() {
        Total = "0";
        Tip=0;
        People=0;
    }
    Bill (String total, int people, int tip) {
        Total = total;
        Tip = tip;
        People = people;
    }

    //the problem is here somewhere.
    private Double Split = (Double.parseDouble(Total) * ((double)Tip / 100)) / (double)People;

    public String getTotal() {
        return Total;
    }

    public double getSplit() {
        return Split;
    }

    public double getTip() {
        return Tip;
    }

}

When I call the 'getSplit()' method, the runtime crashes with a NullPointer exception:

09-03 19:37:02.609: E/AndroidRuntime(11325): Caused by: java.lang.NullPointerException
09-03 19:37:02.609: E/AndroidRuntime(11325):    at java.lang.StringToReal.parseDouble(StringToReal.java:244)
09-03 19:37:02.609: E/AndroidRuntime(11325):    at java.lang.Double.parseDouble(Double.java:295)
09-03 19:37:02.609: E/AndroidRuntime(11325):    at williamgill.de.helloworld.Bill.<init>(Bill.java:21)
09-03 19:37:02.609: E/AndroidRuntime(11325):    at williamgill.de.helloworld.DisplayMessageActivity.onCreate(DisplayMessageActivity.java:38)

I seem to be doing something obviously wrong with the casting of types - but I can't for the life of me think what.

Sotirios Delimanolis · Accepted Answer · 2013-09-03 17:43:22Z

5

At the time this line executes

private Double Split = (Double.parseDouble(Total) * ((double)Tip / 100)) / (double)People;

The fields Total and others have not been initialized by the constructor and have a default value of null. parseDouble() must throw a NullPointerException if its argument is null (I cannot verify this, it might also come from dereferencing).

Declare that field

private Double Split;

and do the assignment in the constructor

Bill (String total, int people, int tip) {
    Total = total;
    Tip = tip;
    People = people;
    Split = (Double.parseDouble(Total) * ((double)Tip / 100)) / (double)People;
}

Advice: Java non-constant instance variables should start with a lowercase alphabetical character.

answered Sep 3, 2013 at 17:43

Sotirios Delimanolis

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8 Comments

Luiggi Mendoza Over a year ago

Another advice: constructors should be public.

Lee Meador Over a year ago

And variables, like your methods, should begin with a lower case letter according to the Java convention.

Will Gill Over a year ago

thank you - that solves my problem. Makes total sense once you say it like that... :) Thanks again!

Sotirios Delimanolis Over a year ago

@WillGill You are welcome. Please heed the advice given in the comments and follow Java conventions.

Will Gill Over a year ago

Speaking of capital letters - is there a difference between Double and double?

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