Ternary operator, syntax error when using assignment

Question

The following 3 lines of code below compile OK. (Please note that this code is an example of "artificial Java coding", and consequently wouldn't be seen in professionally written code.)

int x, y; boolean b=true;  x = b ? y=1 : 2;  // Compiles OK. 

If I now change the code in line #3 above, so that it looks like the following code line below, the compiler generates an error.

// Change the position of the "y assignment", and now the code doesn't compile. x = b ? 1 : y=2;   

Here is the syntax error message:

Can someone please explain this behaviour (to a rookie Java learner)? Thank you.

Answer 1

Short:

This is because of operator precedence. The first case is equal to this:

x = (b ? (y=1) : 2);  // Compiles OK. 

While the second is:

x = (b ? 1 : y) = 2;   

The first one compiles indeed fine, because an assignment gets evaluated to the new value. So, if b is true, it will cause to be both x and y equal to 1. The second one is like saying: x = 1 = 2. So, yes, to fix this compiler error, add paratheses to your statement:

x = (b ? 1 : (y = 2));  // Outer () are not needed, but is clearer, IMHO. 

---

Longer:

First of all, operator precedence in Java says that the assignment operators have lower priority than the conditional ternary operator. This means that your second expression is equivalent to:

x = (b ? 1 : y) = 2; 

As you can see, this looks plain wrong. Indeed, JLS §15.26 says this:

There are 12 assignment operators; all are syntactically right-associative (they group right-to-left). Thus, a=b=c means a=(b=c), which assigns the value of c to b and then assigns the value of b to a.

The result of the first operand of an assignment operator must be a variable, or a compile-time error occurs. _{(This explains the compile time error you face)}

At run time, the result of the assignment expression is the value of the variable after the assignment has occurred. The result of an assignment expression is not itself a variable.

Applying right-associativity:

x = ((b ? 1 : y) = 2); 

Finally we can see why this generates a compiler error: the result of a ternary conditional operator is not a variable (which is actually not in the JLS as far as I can find, however the compiler tells you in a simple test case like this one: https://ideone.com/kMr7Xv).

Answer 2

See "Java operator precedence". Meanwhile, use:

x = (b ? 1 : (y=2));