Question

In my Java book, it says that “an expression is a statement that can convey a return value.” This is different than my traditional understanding. I thought an expression DOES return a value. Not CAN return a value.

this is from Sams Teach Yourself Java in 21 Days.

Answer 1

A mathematical expression always returns something, but a Java expression doesn’t have to. The Java Specification defines what exactly is meant by the term expression in the Java language. Another difference is that expressions can, and often do, have side effects in Java. A side effect is pretty much anything that happens other than returning a value.

Quoting the Java Language Specification:

Much of the work in a program is done by evaluating expressions, either for their side effects, such as assignments to variables, or for their values, which can be used as arguments or operands in larger expressions, or to affect the execution sequence in statements, or both.

For example system.out.println("Hello World"); doesn’t return a value, but it does print Hello World to the output stream. This process of outputting data is a side effect of calling println. Functional languages, in contrast, attempt to minimize dependence on side effects and stick more closely to the mathematical definition of an expression.

Quoting from the JLS again, here is the BNF grammar for an expression:

Primary:
    PrimaryNoNewArray
    ArrayCreationExpression

PrimaryNoNewArray:
    Literal
    Type . class 
    void . class 
    this
    ClassName.this
    ( Expression )
    ClassInstanceCreationExpression
    FieldAccess
    MethodInvocation
    ArrayAccess

You can see that a MethodInvocation is an expansion of PrimaryNoNewArray, which is an expansion of Primary (expression).