java operator precedence with assignment

Question

I would be grateful if someone could please explain why the following is occuring. Thanks a lot.

boolean b = true;
// Compiles OK.
// The LHS "assignment operand" requires no ()parentheses.
if (b=true || b==true);

// Reverse the ||'s operands, and now the code doesn't compile.
if (b==true || b=true);

// Add () around the RHS "assignment operand", and the code now compiles OK.
if (b==true || (b=true));

Edit -

BTW, the compilation error for code line #2 is: "unexpected type", and occurs where the short-circuit OR operator is located:

if (b==true || b=true);
//          ^ "unexpected type" compilation error occurs here.

Edit 2 -

Please note that the code fragments found in this question are examples of "highly artificial Java coding", and consequently would not be seen in professionally written code.

Edit 3 -

I'm new to this incredibly useful website, and I've just learnt how to make and upload screenshots of Java's compilation messages. The following image replicates the information that I provided in my first "Edit" above. It shows the compilation error for example code line #2.

Code line #2 compilation error

It would be nice to add a compiler error: if (b==true || b=true); required: variable, found: value — Eel Lee
– Eel Lee, Commented Nov 15, 2013 at 10:40
It might be useful to be clear that this is a highly artificial 'Java operators test' kind of question: I can see no situation ever where you would want any construct remotely like this and, even if so, it would be incredibly bad style to mix assignments and equality expressions in an if clause. (Plus maybe include the boolean b declaration for completeness.) — Stuart Rossiter
– Stuart Rossiter, Commented Nov 15, 2013 at 11:06
@monsieurRigsby, I have edited my O.P. in response to your suggestions. Thanks. — user2911290
– user2911290, Commented Nov 15, 2013 at 11:15

Joni · Accepted Answer · 2013-11-15 10:43:03Z

6

The assignment operator = has lower precedence than the logical or operator || so that you can use the logical operator in an assignment without extra pairs of parentheses. That is, you would want to be able to write

a = b || c;

instead of being forced to write a = (b || c).

Unfortunately, if we work with operator precedence only, this rule also applies to the left hand side of the expression. a || b = c must be parsed as

(a || b) = c;

even if what you intended was a || (b = c).

answered Nov 15, 2013 at 10:43

Joni

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3 Comments

Stuart Rossiter Over a year ago

+1 for explaining the 'logic' behind the precedence ordering.

ratchet freak Over a year ago

the handyness for using operators in the lefthand side of the assignment is to allow the use of the ternary operator to decide the target (and to make parsing easier)

Joni Over a year ago

@ratchetfreak, do you mean code like p? a: b = c? Although it's valid in C++ it's not in Java.

João Mendes · Accepted Answer · 2013-11-15 10:43:27Z

5

Assignments have the lowest precedence in Java. Thus, your first two expressions are equivalent to:

if ( b = (true || b==true) );

if ( (b==true || b) = true );

The second one doesn't compile because the expression (b==true || b) is not an lValue (something that can be assigned to).

If you add parentheses, you do the assignment before the OR, and everything works.

answered Nov 15, 2013 at 10:43

João Mendes

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Comments

Nikolay Antipov · Accepted Answer · 2013-11-15 10:51:28Z

2

Using operator precedence (http://docs.oracle.com/javase/tutorial/java/nutsandbolts/operators.html) we have this (I added parentheses to indicate precedence):

if (b=(true || (b==true))), b will be assigned to expression and returns boolean, so it fits for condition;
if (((b==true) || b)=true), left side doesn't fit for assignment operator (as it is expression rather than variable);
if (((b==true) || (b=true))), boolean comprates to boolean with OR, right boolean is boolean because b is variable and = returns the assigned value.

answered Nov 15, 2013 at 10:51

Nikolay Antipov

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