A very common mistake that might seem obvious (or so I hope), yet that I've seen in many places is the one where we need to read characters from any kind of input streams and we cast an int read from that input stream to a char a little bit too early.

The usual code goes like this:

InputStream in = ...;
char c = 0;
while ((c = (char) in.read()) > -1) {
   //do something with c
}

The while loop will never break and depending on what you do within the loop probably end up in an OutOfMemoryError being raised, for instance if you keep appending the read character to a StringBuffer or something similar.

The reason for this is that the type char is the only unsigned primitive type in Java, and thus, the range of allowed values for a variable having the type char starts at 0. It's obvious then that the local variable c will never get assigned the value -1 but in case i is -1 (i.e., end of stream reached), c will get the value Character.MAX_VALUE (or 0xFFFF). Thus the solution consists of applying the type cast within the while loop only after the value check is successful, as shown below:

InputStream in = ...;
int i = 0;
while ((i = in.read()) > -1) {
   char c = (char) i;
   //do something with c
}
//EOF reached

That way the loop will always break at some point and you'll be safe.