When is an unassigned expression a valid statement?

Question

I've read Oracle's expressions tutorial and couldn't understand this.

It is well known that the following line of code is valid Java syntax:

new Object();

However, when I try this with a primitive expression:

(3 + 2);

Eclipse is showing a compile error of "The left-hand side of an assignment must be a variable".

This is true not only for primitives, but also for String literals:

"arbitraryString";

So what is the rule for an unassigned expression to be valid as a Java line of code?

Answer 1

The rule is in the Java Language Specification:

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

ExpressionStatement:

• StatementExpression ;

StatementExpression:

• Assignment
• PreIncrementExpression
• PreDecrementExpression
• PostIncrementExpression
• PostDecrementExpression
• MethodInvocation
• ClassInstanceCreationExpression

You see that a constructor invocation is a statement. But a String literal or mathematical expression is not.

Answer 2

Creating an object or calling or method can have side effects, I think this is the main reason for this, whereas nothing will ever happen with an arithmetic expression.

Answer 3

Line containing only

new Object(); 

or to be more precise

new SomeClass(); 

is acceptable, because code of SomeClass() constructor may be all we want.

But in case of lines containing only

"foo";

or

2;//or (2+3);

compiler knows that beside creating/reusing String literal or integer literal this code doesn't do anything else, which means it is probably some kind of programmer mistake so compiler can't accept it.

Answer 4

You're looking for the difference between expressions and expression-statements. Statements like myVoid(); can be written as a statement: these are void methods, etc. (that's the part you know). Expressions, like (3 + 2); and "arbitraryString", have no side-effects. They can only be treated as a value, as no code is executed. Expression-statements, like new Object(); can have side-effects and execute code, and you sometimes just want this code to be executed and ignore the returned value. The compiler therefore allows this.