Java - Common Gotchas [closed]

Answer 1

There are two that annoy me quite a bit.

Date/Calendar

First, the Java Date and Calendar classes are seriously messed up. I know there are proposals to fix them, I just hope they succeed.

Calendar.get(Calendar.DAY_OF_MONTH) is 1-based
Calendar.get(Calendar.MONTH) is 0-based

Auto-boxing preventing thinking

The other one is Integer vs int (this goes for any primitive version of an object). This is specifically an annoyance caused by not thinking of Integer as different from int (since you can treat them the same much of the time due to auto-boxing).

int x = 5;
int y = 5;
Integer z = new Integer(5);
Integer t = new Integer(5);

System.out.println(5 == x);     // Prints true
System.out.println(x == y);     // Prints true
System.out.println(x == z);     // Prints true (auto-boxing can be so nice)
System.out.println(5 == z);     // Prints true
System.out.println(z == t);     // Prints SOMETHING

Since z and t are objects, even they though hold the same value, they are (most likely) different objects. What you really meant is:

System.out.println(z.equals(t));   // Prints true

This one can be a pain to track down. You go debugging something, everything looks fine, and you finally end up finding that your problem is that 5 != 5 when both are objects.

Being able to say

List<Integer> stuff = new ArrayList<Integer>();

stuff.add(5);

is so nice. It made Java so much more usable to not have to put all those "new Integer(5)"s and "((Integer) list.get(3)).intValue()" lines all over the place. But those benefits come with this gotcha.

Answer 2

List<Integer> list = new java.util.ArrayList<Integer>();
list.add(1);
list.remove(1); // throws...

The old APIs were not designed with boxing in mind, so overload with primitives and objects.

Answer 3

(un)Boxing and Long/long confusion. Contrary to pre-Java 5 experience, you can get a NullPointerException on the 2nd line below.

Long msec = getSleepMsec();
Thread.sleep(msec);

If getSleepTime() returns a null, unboxing throws.

Answer 4

    System.out.println(Calendar.getInstance(TimeZone.getTimeZone("Asia/Hong_Kong")).getTime());
    System.out.println(Calendar.getInstance(TimeZone.getTimeZone("America/Jamaica")).getTime());

The output is the same.

Answer 5

IMHO 1. Using vector.add(Collection) instead of vector.addall(Collection). The first adds the collection object to vector and second one adds the contents of collection. 2. Though not related to programming exactly, the use of xml parsers that come from multiple sources like xerces, jdom. Relying on different parsers and having their jars in the classpath is a nightmare.

Answer 6

SimpleDateFormat is not thread safe.