DefaultInterface.java

import org.checkerframework.checker.nullness.qual.*;
import org.checkerframework.framework.qual.DefaultQualifier;

@DefaultQualifier(org.checkerframework.checker.nullness.qual.NonNull.class)
interface Foo {
    void foo(String a, String b);
}

@DefaultQualifier(org.checkerframework.checker.nullness.qual.NonNull.class)
class DefaultInterface {

    public void test() {

        @Nullable Foo foo = null;
    }
}