Why is this Java variable assignment illegal?

Question

Here is the code

import java.util.*;
 class Example
 {
  public static void main(String args[])
  {
    final int x=127; int y=100;
    byte b;
    b=x; //Legal ?
    b=y; //Illegal 
  }
 }

Can you explain why b=y is illegal? I think final means no further change, is that correct?

Can you explain which lines do you expect to be legal and illegal? Otherwise it's quite hard to guess which aspect of this code it is that you are confused about. — Sweeper
– Sweeper, Commented Jan 28, 2020 at 19:32
You don't mention even if you have tried to compile it. Have you? What do you understand from the compiler output result? — Victor Polo De Gyves Montero
– Victor Polo De Gyves Montero, Commented Jan 28, 2020 at 19:33
compile error: incompatible types: possible lossy conversion. — Yohombu
– Yohombu, Commented Jan 28, 2020 at 19:36
So, you're trying to store an int value (bigger) into a byte (smaller) box. Nothing related with the 'final' word. You'll find it useful to read the Java Tutorial at docs.oracle.com/javase/tutorial/java/nutsandbolts/… — Victor Polo De Gyves Montero
– Victor Polo De Gyves Montero, Commented Jan 28, 2020 at 19:37

Jordan · Accepted Answer · 2020-01-28 19:43:23Z

9

The final keyword does mean "no further change", so you're correct there. The issue here has to do with data "width".

An int can hold more data than a byte (i.e. it is "wider" than a byte). This means that when you do b = x, you're "narrowing" the width of the int to fit into the byte. This only works if the compiler can guarantee that the int is small enough to fit into the byte, which requires the int to be <= 127, and to also be final (so that it cannot later be changed to be > 127).

In your code, both x and y are narrow enough to fit into a byte, but only x is final, so it's the only one the compiler allows to be directly assigned to a byte variable.

  final int x=127; final int y=100; // Made y final
  byte b;
  b=x; //Legal
  b=y; //Also legal

  final int x=200; final int y=100; // Made x too big
  byte b;
  b=x; //Illegal
  b=y; //Legal

edited Jan 28, 2020 at 19:43

answered Jan 28, 2020 at 19:35

Jordan

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