Java: Not a statement

Question

I suppose this is more a question about language theory than anything else. Why is the first statement in main legal, when the second is not? Don\'t they evaluate to be the

Answer 1

In the first statement you are actually calling a function and second statement doesn't give any value. Incase you want to process the return value, you need to call a variable for return type

Eg:

public class Main {

    public static void main(String[] args) {
        int n = foo();
    
         //do whatever you want with return 
    }

    public static int foo() {
        return 0;
    }
}

Answer 2

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.