[Bluej-discuss] concatenating an index value with a note ex 4.15

[Michael Kölling]

On 4 Oct 2006, at 19:51, Sheryl wrote:

> Colleen
>
> I already tried that which didn't compile ie System.out.println 
> (index + notes.get(index));
> It says that you can't use the + operator in this situation.
>
> However, when I added the string for the colon to make it display  
> like the example in the book, it compiled.  why is this?

Having any String in the expression with the plus sign changes  
everything.

Your original expression was

   index + notes.get(index)

The types if this are

   int + Object

There is no 'plus' operation defined that takes an int and an Object  
as an operand, so this does not work. BUT: Java defines a 'plus  
operator with anything and a String. So if you write

   int + String

or

   String + Object

It all will work, since Java has a rule that says: If any of the  
operands of a + operator is a String, then the other operand will be  
converted to a String, and the '+' will be interpreted as String  
concatenation.

Thus, once you write

   index + ":" + notes.get(index)

All is fine. Java will evaluate this left to right, so first it looks at

   index + ":"

which will result in something like  "1:"  (a String), and then it  
evaluates

   "1:" + notes.get(index)

which is fine again because the first operand is now a String.

Regards,

Michael