‘not a statement’ in Java

Java restricts the types of expressions that are allowed in so-called “expression statements”. Only meaningful expressions that have potential side effects are allowed. It disallows semantically meaningless statements like 0; or a + b;. They’re simply excluded from the language grammar.

A function call like foo() can, and usually does, have side effects, so it is not a meaningless statement. The compiler doesn’t deeply inspect the body of foo() to check whether it actually does anything. Calling a function can have side effects, so it is syntactically valid.

This reflects a philosophical difference between C/C++ and Java. Java prohibits various constructs which result in dead or meaningless code.

return;
foo();    // unreachable statement

C and C++ are relatively laissez faire about it all. Write whatever you want; they don’t have time to babysit you.

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Quoting from the Java Language Specification, §14.8 Expression Statements:

Certain kinds of expressions may be used as statements by following
them with semicolons.

ExpressionStatement:
    StatementExpression ;

StatementExpression:
    Assignment
    PreIncrementExpression
    PreDecrementExpression
    PostIncrementExpression
    PostDecrementExpression
    MethodInvocation
    ClassInstanceCreationExpression

An expression statement is executed by evaluating the expression; if
the expression has a value, the value is discarded.

Execution of the expression statement completes normally if and only
if evaluation of the expression completes normally.

Unlike C and C++, the Java programming language allows only certain
forms of expressions to be used as expression statements. Note that
the Java programming language does not allow a “cast to void” – void
is not a type – so the traditional C trick of writing an expression
statement such as:

(void)... ;  // incorrect!

does not work. On the other hand, the Java programming language allows
all the most useful kinds of expressions in expressions statements,
and it does not require a method invocation used as an expression
statement to invoke a void method, so such a trick is almost never
needed. If a trick is needed, either an assignment statement (§15.26)
or a local variable declaration statement (§14.4) can be used instead.