空指针异常。 为什么？

Question

I am a java newbie, so forgive me if this question may sound stupid. I am learning.

I am trying to calculate this sum but I am getting a strange error message. Can you help me find me where it is? Thank you so much

public class myfirstjavaclass {

    public static void main(String[] args) {
        Integer myfirstinteger = new Integer(1);
        Integer mysecondinteger = new Integer(2);
        Integer mythirdinteger = null;
        Integer result = myfirstinteger/mythirdinteger;
    }

}

Exception in thread "main" java.lang.NullPointerException
at myfirstjavaclass.main(myfirstjavaclass.java:8)

Answer 1

You should not be using Integer (the object type) here, as it can be null (which you don't need and trip over here).

When you dereference a null in Java, you get a NullPointerException.

In this case, it is a bit tricky, because automatic unboxing is involved (fancy name for conversion between the primitive types and their object wrappers). What happens under the hood is that

Integer result = myfirstinteger/mythirdinteger;

is really compiled as

Integer result = Integer.valueOf(
     myfirstinteger.intValue() / mythirdinteger.intValue());

The call to intValue() fails on a null pointer.

Just use int (the primitive).

public static void main(String[] args) {
    int myfirstinteger = 1;
    int mysecondinteger = 2;
    int mythirdinteger = 0;
    int result = myfirstinteger/mythirdinteger; 
       // will still fail, you cannot divide by 0
}

Answer 2

it seems to me your third Integer is assigned to null.

BTW, what do you really want to do? If you want to calculate a sum like you said in your question, see the code below

public static void main(String[] args) {
    int first = 1;
    int second = 2;
    int third = 0;
    int sum = first + second + third;
}

If you want to calculate product, make sure you are not dividing by 0

public static void main(String[] args) {
    int first = 1;
    int second = 2;
    int product = first / second; // this product is 0, because you are forcing an int
    double product2 = (double) first / second; // this will be 0.5
}

Answer 3

"Null" means "this variable doesn't reference anything", which is another way of saying "this variable has no value". It doesn't mean "the value is zero".

A NullPointerException is what Java gives you when you use a variable that doesn't reference anything, in a context that requires the variable to have a value. Dividing a number by the value of a variable is a context which requires the variable to have a value - hence the exception.

Answer 4

Because you divided by null , of course. What were you expecting to happen?