why compiler says a public static field in interface is "final" although it is not

Question

Please see the below code --

public interface TestInterface {
    public static String NON_CONST_B = "" ; 
}

public class Implemented implements TestInterface {
    public static String NON_CONST_C = "" ;
}

public class AutoFinal  {

    public static String NON_CONST_A = "" ;

    public static void main(String args[]) {
        TestInterface.NON_CONST_B = "hello-b" ;
        Implemented.NON_CONST_C = "hello-c";
        AutoFinal.NON_CONST_A = "hello-a" ;
        Implemented obj = new Implemented();
    }
}

However, the compiler complains that TestInterface.NON_CONST_B is final --

AutoFinal.java:6: error: cannot assign a value to final variable NON_CONST_B
        TestInterface.NON_CONST_B = "hello-b" ;
                 ^
1 error

why ?

Answer 1

Regarding:

public interface TestInterface {
   public static String NON_CONST_B = "" ; 
}

public class AutoFinal  {    
   public static void main(String args[]) {
      TestInterface.NON_CONST_B = "hello-b" ;
      // ....
   }
}

However, the compiler complains that TestInterface.NON_CONST_B is final --

---

But it in fact is final whether you explicitly declare it to be or not since it is declared in an interface. You can't have non-final variables (non-constants) in an interface. It's also public and static whether or not it has been explicitly declared as such.

Per the JLS 9.3 Interface Field (Constant) Declarations:

Every field declaration in the body of an interface is implicitly public, static, and final. It is permitted to redundantly specify any or all of these modifiers for such fields.