Category: Java

== or equals with Java enum

When you compare objects in Java you should prefer the equals()-method to == in general. The reason is that you get reference equality (like with ==) by default but you are able to change that behaviour. You can override equals() (DO NOT FORGET TO OVERRIDE hashCode() too because otherwise you will break the general class contract) to reflect logical equality which is often what you want, e.g when comparing some string constant with user input.

With primitive types like double and int you are more or less limited to == which is fine for those immutable value types.

But what is the right thing to to with the enum type introduced in Java 5?
Since enums look like a class with methods, fields and the like you might want to use equals() instead of ==. Now this is a special case where using reference equality is actually safer and thus better than logical equality.

Above (please mind the stupid example) we can see that comparing the EState enum with an ILamp using equals() is accepted perfectly by the compiler even though the condition never can be true in practice. Using == the compiler screams and tells us that we are comparing apples with oranges.

Using Flying Saucer PDF offline

Flying saucer is a nice tool for quick PDF generation from a (X)HTML page. Everything worked fine when we tested it at home but when we had a demo at a client’s site, no PDF could be generated. The problem was caused by a little snippet in the header of the HTML:

<!DOCTYPE html PUBLIC
"-//W3C//DTD XHTML 1.0 Transitional//EN"
"http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">

The DTD declaration! So we took a look at the flying saucer issue database and found someone who had the same problem.
But another solution is even simpler:
Xerces has a default setting which tells the parser to load external dtds.
Turning this off solved our offline problems:

  DocumentBuilderFactory builderFactory =
    DocumentBuilderFactory.newInstance();
  builderFactory.setFeature(
    "http://apache.org/xml/features/nonvalidating/load-external-dtd",
    false);