Originally posted by Nathaniel Stoddard:
Interfaces are abstract, so while adding the "abstract" keyword to the definition is redundant it's still correct. As you mentioned, constructors can't be overridden, so using the "abstract" keyword on a constructor definition isn't just redundant, it's just plain incorrect, ergo the compile-time error.
Not at all what they asked.
While it would be redundant to declare an interface abstract or a private method final I would venture a guess that the author's simply didn't bother to make them explicitly compile-time errors.
From the Java Language Specification:
8.8.3 Constructor Modifiers
Unlike methods, a constructor cannot be abstract, static, final, native, strictfp, or synchronized.
188.8.131.52 abstract Interfaces
Every interface is implicitly abstract. This modifier is obsolete and should not be used in new programs.
184.108.40.206 final Methods
A private method [is] implicitly final, because it is impossible to override [it]. It is permitted but not required for the declarations of such methods to redundantly include the final keyword.