why in java static final variable in interface can be overridden by a local var?

Question

I know static means, there is only one instance of it in the memory. I know final means it can not be changed or subclassed,I also know any variable defined in a java interface is static final

so now here is the question , why can I over ride final static variable "a" in the interface "MyFace" in the "XFace" class?

Example:

public interface MyFace {
    static final int a = 15;

    void smile();
}

then here in the class I can easily over ride the a, with a local a,

public class XFace implements MyFace {

    @Override
    public void smile() {
        int a=3;  // over riding interface's a variable and suprsingly it works !
        System.out.println(a*2);  // will print 6
    }

why can I define int a=3 in smile() method ? isn't "a" final and static ? how could it be overridden ?

Answer 1

It's not overridden, it's shadowed, meaning that there's a closer variable with the same simple name that takes precedence. You can still use the static final by using its longer name, MyFace.a.

Answer 2

Just look at bytecode of two different usages (with local variable a and without it). As you may see that If only static a is defined compiler will directly use 30 (3: bipush 30) If local variable a is defined local variable 1 local1 is pushed then multiply operation is done.

   5:   iload_1
       6:   iconst_2
       7:   imul

if you want to use both of them you have to Usage Class name where static variable is defined.

System.out.println(a*2);  // will print 6
System.out.println(MyFace.a); //will print 15

Here is a the example code local variable a is defined

public class XFace implements MyFace {

    @Override
    public void smile() {
        int a=3; 
        System.out.println(a*2);  // will print 6

    }
}

Compiled from "XFace.java"
public class XFace extends java.lang.Object implements MyFace{
public XFace();
  Code:
   0:   aload_0
   1:   invokespecial   #10; //Method java/lang/Object."<init>":()V
   4:   return

public void smile();
  Code:
   0:   iconst_3
   1:   istore_1
   2:   getstatic       #17; //Field java/lang/System.out:Ljava/io/PrintStream;
   5:   iload_1
   6:   iconst_2
   7:   imul
   8:   invokevirtual   #23; //Method java/io/PrintStream.println:(I)V
   11:  return


}


*********************************************

Here is a the example code static variable a is defined

public class XFace implements MyFace {

    @Override
    public void smile() {
        //int a=3; 
        System.out.println(a*2);  // will print 30

    }
}

Compiled from "XFace.java"
public class XFace extends java.lang.Object implements MyFace{
public XFace();
  Code:
   0:   aload_0
   1:   invokespecial   #10; //Method java/lang/Object."<init>":()V
   4:   return

public void smile();
  Code:
   0:   getstatic       #17; //Field java/lang/System.out:Ljava/io/PrintStream;
   3:   bipush  30
   5:   invokevirtual   #23; //Method java/io/PrintStream.println:(I)V
   8:   return

}

to print byte code from compiled class file use javap tool under $JDK_HOME/bin