A naming convention is a rule to follow as you decide what to name your identifiers e.g. class, package, variable, constant, method etc.
But, it is not forced to follow. So, it is known as convention not rule.
Advantage of naming conventions in java
By using standard Java naming conventions, you make your code easier to read for yourself and for other programmers. Readability of Java program is very important. It indicates that less time is spent to figure out what the code does.
Understanding CamelCase in java naming conventions
Java follows camelcase syntax for naming the class, interface, method and variable.
If name is combined with two words, second word will start with uppercase letter always e.g. actionPerformed(), firstName, ActionEvent, ActionListener etc.