In your own Java program, you can make up names to your heart's delight. For example, in the code
double multiplyByTwo(double myValue) {
return myValue * 2;
}
the names multiplyByTwo and myValue are your very own identifiers.
When you create a new name, you can use letters, digits, underscores (_), and dollar signs ($). But don't start the name with a digit. If you try to start a name with a digit, Java replies with a "Please don't do that" message.
