Avoid basic style errors

Many beginners to Java repeat the same basic style errors. Such style errors don't make your program incorrect, but they make your program less maintainable. In essence, many beginners write code that suffers from the same basic underlying defect of not being written with compassion for the reader.
There are in fact two target platforms for all code: the runtime hardware, and the human cerebral cortex. All code 'runs' in the human brain, in the sense that all code needs to be understood by a human being. Coding style is always concerned with this second target platform, not with the first.
Some common basic style errors :

• classes too long
• methods too long
• little or no javadoc
• no convention for distinguishing between local variables, arguments, and fields
• many empty catch blocks that suppress exceptions
• using exceptions to define regular program flow
• excessive use of the instanceof operator
• using floating point data to represent money
• preferring arrays over collections (especially in JDK 5+)

Some basic style errors are much more damaging to your code than others. The single most harmful bad habit is that of excessive length. Classes that are too long are hard to understand. As a guideline, if a class is more than about 300 lines long, you should likely consider splitting it up into smaller pieces. Similarly, methods that are more than a single screen are very likely too long, and would almost always benefit from being split into several methods.