Henry, thank you for your attention, but I think it isn't true. Because I feel that dom is a keyword of Java. (int  sample.dom) Why do we need to use it ? What do you say? Do you think my question is clear?
This makes me think of a story that a colleague once told me. He had been a programmer long ago, in the 1970's. He told me that they always used two- or three-letter variable names, like "a1", "a2", "a3" etc., and they would have a table on paper somewhere that would explain what each variable was used for. Maybe that was necessary in the 1970's, when computers had a very limited amount of memory and were much slower than the chip in your cell phone...
It's very important to choose good and clear variable names. A name like daysPerMonth would have been much better than something like dom. Choosing good variable names makes it much easier to understand and maintain code.
Thank you Jesper, it is a very clear explanation. Here is a good tip for java variables: Choose clear variable names for you programing process. AgeOfChildren is a better variable name than AOC. T
Thank you. At this point I want to ask "Why do we use variables?" My answer is that: "Serap, you can think variables are like tools in your kitchen. You have them to cook. The names of them is not so important. But, if you label them with good names according to their function, you would make jour job fast and easy. The important point is that how you will define and use them." Did I understand variables?