How does the director in the Builder class Sets a null int to 0?

Question

I'm studying a simple example of Builder Design Pattern.

So my question is, if I try to instantiate my Phone class directly, it wont accept null for the int parameter which is understandable since it is a primitive type.

However when I use the builder and I don't set the value of an Int variable(which will remain null) the director somehow changes its value to 0.
Phone p=new PhoneBuilder().setOs("Android").setRam(2).getPhone();

p.Processor == null at this point. so in getPhone() when the phone constructor is called, the value of processor is supposed to be null I tried to watch the variable with the debugger but I can't find nor understand why it changes from null to 0.

public class Phone {    

    private String os;  
    private int ram; 
    private String processor;   
    private double screenSize;   
    private int battery;    

    public Phone(String os, int ram, String processor, double screenSize, int battery) {
        super();
        this.os = os;
        this.ram = ram;
        this.processor = processor;
        this.screenSize = screenSize;
        this.battery = battery;
    }

}

and here is my builder:

    public class PhoneBuilder {

        private String os; 
        private int ram; 
        private String processor;
        private double screenSize;
        private int battery; 

        public PhoneBuilder setOs(String os) {
            this.os = os;
            return this;
        }

        public PhoneBuilder setRam(int ram) {
            this.ram = ram;
            return this;
        //...

        public Phone getPhone() {
            return new Phone(os, ram, processor, screenSize, battery);
        }
    }

Answer 1

This has nothing to do with any design pattern. Nulls are not possible for primitives, only for objects. An int, not to be confused with an Integer, cannot be null. It was never null.

So when you say this:

However when I use the builder and I don't set the value of an Int variable(which will remain null)

You've made an incorrect assumption. It wasn't null and won't remain null.

Here's a simple example:

public class NoNullForPrimitives{
    private static int myInt;

    public static void main(String[] args){
        System.out.println(myInt); //prints 0
    }
}

For more details, see the Default Values section under https://docs.oracle.com/javase/tutorial/java/nutsandbolts/datatypes.html