Static classes initialization

February 21, 2023

There are a couple of classes ClassA and ClassB. ClassB is used in ClassA. The third class Manager is responsible for creating instances of the class, but we need static ClassA and static ClassB in Manager.

When written this way, a NullPointerException is thrown because classA.classB == null – I can’t figure out why. I redid the Manager class so that everything works correctly, but I would like to understand why this particular option does not work.

I expected that it would work like this: when the Manager class is first accessed, its fields will be initialized, after which the get methods will return not null ClassA and ClassB objects.

class Main {
    public static void main(String[] args) {
        ClassA classA = Manager.getDefaultA();
        System.out.println(classA.classB);
    }
}
    
public class ClassA {
    ClassB classB = Manager.getDefaultB();
}
    
public class ClassB {

}
    
public class Manager {
    static ClassA classA = new ClassA();
    static ClassB classB = new ClassB();
    
    public static ClassA getDefaultA() {
        return classA;
    }
    
    public static ClassB getDefaultB() {
        return classB;
    }
}

>Solution :

A NullPointerException is being thrown because of the order in which the fields of Manager class are initialized.

In your current implementation, when the JVM loads the Manager class, it will first initialize the classA and classB fields before calling the getDefaultA() and getDefaultB() methods. Since classA and classB are initialized before calling the getDefaultB() method, the classB field inside the classA instance is still null, which causes the NullPointerException when you try to access it.

To fix this issue, try initializing classB before classA in the Manager class. This way, when classA is initialized, the classB field inside it will already have a non-null value:

Note: I removed the public‘s since they were not required to illustrate the actual underlying issue.

class Main {
    public static void main(String[] args) {
        ClassA classA = Manager.getDefaultA();
        System.out.println(classA.classB);
    }
}
    
class ClassA {
    ClassB classB = Manager.getDefaultB();
}
    
class ClassB {

}
    
class Manager {
    static ClassB classB = new ClassB();
    static ClassA classA = new ClassA();

    public static ClassA getDefaultA() {
        return classA;
    }
    
    public static ClassB getDefaultB() {
        return classB;
    }
}

Output: