3. "When Words Collide" Puzzle

public class PrintWords {
    public static void main(String[] args) {
        System.out.println(Words.FIRST + "" + Words.SECOND + "" + Words.THIRD);
    }
}
class Words {
    // Compile PrintWords against this version    public static final String FIRST="the";//the constant    public static final String SECOND =null;//Not a  constant!    public static final String THIRD="set";//the constant}
 class Words {// Run against this version     public static final String FIRST="physics";
     public static final String SECOND="chemistry";
     public static final String THIRD="biology";
 }

Output:

the chemistry set

What exactly is constant variable?

primitive and string constant variable whose value is compile time constant.

final string initialised to  null is not a constant.

Not the Enums are not the constant expressions.

Solution 

 public static String ident(String s)
 {
     return s;
 }
class Words {
    // Compile PrintWords against this version    public static final String FIRST=ident("the");//the constant    public static final String SECOND =ident(null);//Not a  constant!    public static final String THIRD=ident("set");//the constant}
The Moral

Constant variables are inlined

Only primitives and strings can be constant
null is not a constant

If you change the value of a constant variable without recompiling its clients, their behaviorbecomes very confusing
Use constant variables only for entities whose value will never change

-Use ident for final fields whose value may change