Java 在子类中设置变量

Question

I have a Java project (let's call it Project A) that consists of a class and its subclass. I packaged this project into a .jar file, which I included as a Maven dependency in another project (let's call it Project B).

My subclass in Project A contained a variable as such:

int foo = null;

And I wanted to perform the following in Project B:

Subclass.foo = 3;

However, Java kept throwing me a java.lang.ExceptionInInitializerError whenever I tried to do this, which it said was caused by a null pointer error.

The minute I moved this variable to the base class in Project A though, it worked. I was able to set the value of foo in Project B.

I have no idea why this worked, would someone who knows about Java be willing to explain this to me?

Answer 1

That's because there is no static variable foo on Subclass . You can only use static variables with the class name like this. To use non-static variables, you have to instantiate the class with the new keyword.

Try one of the following:

Option #1:

Class 1:

public static int foo = null;

Class 2:

Subclass foo = 3;

Option #2:

Class 1:

public int foo = null;

Class 2:

Subclass subclass = new Subclass();
subclass.foo = 3;

Also, note that I added the public modifier. By default, the variable is only available to others in the same package. Given you have two different projects, I don't think that's the case here.

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Edit:

Someone ( Lino ) reminded me in the comments that int can't be null . Either replace int with Integer , or null with 0 .