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.
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.