Replacing an if statement with a disjunction

Question

Just for fun, I was trying to replace:

if (set1.add(x) == false)
{
    set2.add(x);
}

with:

set1.add(x) || set2.add(x);

However, Eclipse complains:

Syntax error on token "||", invalid AssignmentOperator
The left-hand side of an assignment must be a variable

Could anybody shine some light onto these error messages? They don't make much sense to me.

Answer 1

There are a number of answers far, but I agree with Bohemian's answer that the most straightforward simplification (although it doesn't use ||) is this:

if ( !set1.add(x) ) set2.add(x);

That doesn't explain the error message though. Mustafa Genç comes closer on this, but I think it's worthwhile to look at the language specification here. exp1 || exp2 is an expression, and the problem here is that you're trying to use it in a context where a statement is expected. According to 14.8. Expression Statements, some kinds of expressions can be used where statements are expected by attaching a semicolon:

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.

The reason that you can't do what you're trying to do, though, is that not every expression can be used as a statement. However, it does discuss some ways to work around this. From the same section of the specification (emphasis added):

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.

This approach is what the first snipped in Reik Val's answer is using:

boolean temp = set1.add(x) || set2.add(x);

Answer 2

As @qqilihq said in the comments try to do

boolean temp = set1.add(x) || set2.add(x);

or more awkward:

if(set1.add(x) || set2.add(x));

Answer 3

According to documentation java statements which can end with a semicolon are:

Assignment expressions
Any use of ++ or --
Method invocations
Object creation expressions

What you've written is not a statement it's an expression. Here you can find more about statements and expressions. So simple but worth to look.

Answer 4

I would just:

if (!set1.add(x))
    set2.add(x);

Answer 5

The statement

boolean temp = set1.add(x) || set2.add(x);

and any variation thereof is dangerous. You'll hardly ever know what happens there. Note that the right expression is NOT evaludated iff the first expression is true. That is, the attempt to add it to set2 will only be made if it was not yet contained in set1.

EDIT: Now, reading the question again, it seems that this was exactly what you intended. So I think that the anser https://stackoverflow.com/a/21755051 by Mustafa Genç is the relevant here

Usually, you should write clearly what you want to do

boolean wasNotContainedInSet1 = set1.add(x);    
boolean wasNotContainedInSet2 = set2.add(x);    
boolean wasNotContainedInAnySet = 
    wasNotContainedInSet1 | wasNotContainedInSet2;

or

boolean wasNotContainedInSet1 = set1.add(x);    
if (!wasNotContainedInSet1) {
    set2.add(x);    
}

or whatever...