Why we should not use protected static in java

Question

I was going through this question Is there a way to override class variables in Java? The first comment with 36 upvotes was:

If you ever see a p

Answer 1

Well, as most of the people have answered:

• protected means - 'package-private + visibility to subclasses - the property/behaviour is INHERITED'
• static means - 'the opposite of instance - it is a CLASS property/behaviour, i.e it is NOT INHERITED'

Therefore they are slightly contradictive and incompatible.

However, recently I came up to a use case where it might make sense to use these two together. Imagine that you want to create an abstract class which is a parent for immutable types and it has a bunch of properties which are common to the subtypes. To implement immutability properly and keep readability one might decide to use the Builder pattern.

package X;
public abstract class AbstractType {
    protected Object field1;
    protected Object field2;
    ...
    protected Object fieldN;

    protected static abstract class BaseBuilder> {
        private Object field1; // = some default value here
        private Object field2; // = some default value here
        ...
        private Object fieldN; // = some default value here

        public T field1(Object value) { this.field1 = value; return self();}
        public T field2(Object value) { this.field2 = value; return self();}
        ...
        public T fieldN(Object value) { this.fieldN = value; return self();}
        protected abstract T self(); // should always return this;
        public abstract AbstractType build();
    }

    private AbstractType(BaseBuilder b) {
        this.field1 = b.field1;
        this.field2 = b.field2;
        ...
        this.fieldN = b.fieldN;
    }
}

And why protected static ? Because I want a non-abstract subtype of AbstactType which implements its own non-abstract Builder and is located outside package X to be able to access and reuse the BaseBuilder.

package Y;
public MyType1 extends AbstractType {
    private Object filedN1;

    public static class Builder extends AbstractType.BaseBuilder {
        private Object fieldN1; // = some default value here

        public Builder fieldN1(Object value) { this.fieldN1 = value; return self();}
        @Override protected Builder self() { return this; }
        @Override public MyType build() { return new MyType(this); }
    }

    private MyType(Builder b) {
        super(b);
        this.fieldN1 = b.fieldN1;
    }
}

Of course we can make the BaseBuilder public but then we come to another contradictory statements:

• We have a non-instantiatable class (abstract)
• We provide a public builder for it

So in both cases with protected static and public builder of an abstract class we combine contradictory statements. It is a matter of personal preferences.

However, I still prefer the public builder of an abstract class because the protected static to me feels more unnatural in a OOD and OOP world !