java - 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.