java - 为什么编译器说接口(interface)中的公共(public)静态字段是 "final"尽管它不是

Question

请看下面的代码——

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();
    }
}

但是，编译器提示 TestInterface.NON_CONST_B 是最终的 --

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

为什么？

Answer 1

关于:

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 --

---

但事实上，无论您是否显式声明它，它都是最终的，因为它是在接口(interface)中声明的。接口(interface)中不能有非最终变量(非常量)。它也是公共(public)和静态的，无论是否已明确声明为公共(public)静态。

根据 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.