An expression produces a value. For example, expression 2+3 produces value 5.
Expressions can be as complex as you like. For example, you can write 4 * 3 + 8 * (9 + 1), which evaluates to 92.
Here are some rules to remember about expressions.
In ordinary mathematical notation, when you write xy, you mean to mulitly x times y. But in Java, you need to put a * to indicate multiplication. So you write x*y.
To divide, use the / operator. It works like ordinary division when you are dividing real numbers (of type double in Java).
But be careful. In ordinary mathematical notation, expression 3/2 has value 1.5. But in Java, any time you divide two integers, you get an integer result. Anything the comes after the decimal point is discarded. So, for example, Java expression 4/3 has value 1. (In ordinary mathematics it would be 1.33333..., but if you throw away what is to the right of the decimal point, you get 1.)
You can get the remainder when you divide one integer by another using the % operator. For example, if you divide 42 by 10, you get a quotient of 4 and a remainder of 2. So expression 42 % 10 has value 2 (the remainder).
The operators obey rules of precedence similar to those of standard mathematics. + and - are done from left to right. Operators *, / and % are also done from left to right, and have higher precedence than + or -.
Get in the habit of reading carefully. We have just said that * and / operators are done from left to right. So 5/2*3 is done from left to right. First, you compute 5/2, which, according to our rules, is 2. Then you multiply that by 3, yielding 6. If you want 5/(2*3), then you need to write the parentheses.
Questions.