Question

I had to create an output depending on an boolean state like

  String smily = null;
  StringBuffer buff = new StringBuffer();
  buff.append(", " + smily == null ? ":)" : ":("); //$NON-NLS-1$

  System.out.println(buff.toString());

The problem is the String creation statement

  ", " + smily == null ? ":)" : ":("

I tested it in 2 different eclipse environments (and may be also 2 diofferent java version, this i did not checked) and the result was different.

Result 1:

🙁

Result 2:

false:(

Of course, if i added brackets it is working

 buff.append(", " + (smily == null ? ":)" : ":(")); //$NON-NLS-1$

Expected Result:

, 🙂

Can please somebody explain to me, why java interprets the statement that way?

Thanks

Answer 1

If you check the operator precedence (see this tutorial), then you will notice that addition (+) comes before equality (==). In other words, Java will first evaluate ", " + smily => ", null" before evaluating equality, therefor ", " + smily == null evaluates to false, and so the ternary operator evaluates to ":(".

BTW: You could have avoided this by not concatenating strings before adding them to the StringBuffer (the whole point of a StringBuffer is to make concatenation cheaper):

String smily = null;
StringBuffer buff = new StringBuffer();
buff.append(", ");
buff.append(smily == null ? ":)" : ":(");