They allow for fractions and decimals, which is the only time I recommend using them. As you can see, they have two ranges. One range is for the positive exponents, and one for negative. They are handy when computing anything which requires a fraction. You can use any word as long as it is not a keyword.
Variable names can also be very long.
This is the signal to the Java compiler that separates all the commands in your program. Each line above is a command to allocate a new variable. Now we will set the variables we declared above. Be aware that if you try to assign a value which does NOT fit within a variables range you will get a compiler error. Now is a good time to briefly discuss how chars are assigned.