Statements versus expressions

Kotlin utilizes expressions more widely than Java, so it is important to know the difference between a statement and an expression. A program is basically a sequence of statements and expressions. An expression produces a value, which can be used as part of another expression, variable assignment, or function parameter. An expression is a sequence of one or more operands (data that is manipulated) and zero or more operators (a token that represents a specific operation) that can be evaluated to a single value:

Let's review some examples of expressions from Kotlin:

Expression (produces a value)

Assigned value

Expression of type

a = true

true

Boolean

a = "foo" + "bar"

"foobar"

String

a = min(2, 3)

2

Integer

a = computePosition().getX()

Value returned by the getX method

Integer

 

Statements, on the other hand, perform an action and cannot be assigned to a variable, because they simply don't have a value. Statements can contain language keywords that are used to define classes (class), interfaces (interface), variables (val, var), functions (fun), loop logic (break, continue) and so on. Expressions can also be treated as a statement when the value returned by the expression is ignored (do not assign a value to the variable, do not return it from a function, do not use it as part of other expressions, and so on).

Kotlin is an expression-oriented language. This means that many constructs that are statements in Java are treated as expressions in Kotlin. The first major difference is the fact that Java and Kotlin have different ways of treating control structures. In Java they are treated as statements, while in Kotlin all control structures are treated as expressions, except for loops. This means that in Kotlin we can write very concise syntax using control structures. We will see examples in upcoming sections.