Incremental Java
Expressions

Evaluate them.

What do YOU do with expressions?

Evaluate them!

What do you DO with expressions?

EVALUATE THEM!

We can write to a box to change the value in the box. We can read from the box to see what value is in the box.

But this seems boring. Surely, we need the ability to do computation.

For example, suppose we want to compute 3 + 5. This is an expression. To evaluate such an expression is to find its value. Thus, 3 + 5 evaluates to 8.

The kind of expressions you create depend on the type. For example, you can add or multiply two int, but you can't multiply two char values.

Let's define expressions a bit more rigorously.

• Literals are expressions (e.g., 7 is an expression)
• Variable names are expressions (e.g., cost is an expression)
• If expr has type int or double. then - expr is also an expression. (That's a minus sign in front of the expression). The minus sign is called a unary operator. Unary means one, since the minus sign is applied to one expression.
• If expr₁ and expr₂ are expressions of the type int or double, then expr₁ OP expr₂ is an expression where OP is one of: +, -, *, and /. (* is multiplication and / is division). These four operators are called binary operators since they operate
• Parentheses are used to determine which operator applies to which expression.

We know boxes have types, and because we can name boxes using variable names, then variable names have type too.

As we can see from the definition above, expressions have a type too. We'll talk about soon.

Here are some examples of expressions:

• 7 (7 is a literal)
• 7 + x (assuming x is an int variable)
• (7 + x) / y (assuming x and y are int variable)

A binary operator has two operands. It has a left operand (which is an expression) and a right operand (also an expression). Usually the types of the operands must match.

For example, in x + 10, the left operand is x while the right operand is 10.

There are four common binary operators: +, -, *, and /. This is addition, subtraction, multiplication, and division.

Interestingly, - is two different operators. Depending on the situation, it can be unary minus, as in - x, or it can be binary subtraction, as in x - y.

- is said to be "overloaded". This means that it has more than one meaning, depending on the situation.

Here's a quiz. Consider x - - - y. How many of the dashes are unary minus? How many are binary subtraction? And which are which?