Priority of equality and conditional operators in java

Question

I run the following code snippet

System.out.println(T1() ? F1() : T2() == T3() ? F2() : T4());
public static boolean T1() { System.out.println("true1"); return true; }
public static boolean T2() { System.out.println("true2"); return true; }
public static boolean T3() { System.out.println("true3"); return true; }
public static boolean T4() { System.out.println("true4"); return true; }
public static boolean F1() { System.out.println("false1"); return false; }
public static boolean F2() { System.out.println("false2"); return false; }

I get the output

true1
false1
false

The first ternary operator been evaluated before evaluating the equality operator, but according to oracle documentation , equality operator has greater priority than the ternary one, therefore the equality operator must be evaluated before the ternary one.

Whats the reason of such code behavior?

Answer 1

JLS 15.25

The conditional operator is syntactically right-associative (it groups right-to-left). Thus, a?b:c?d:e?f:g means the same as a?b:(c?d:(e?f:g)).

And in your case :

T1() ? F1() : (T2() == T3() ? F2() : T4())
  a     b            c         d       e

T1() is evaluated true, returning true, so only F1() is evaluated next.

Answer 2

You seem to misunderstand what priority means. The fact that == has higher priority than the ternary operator does not mean that in an expression all == subexpressions are evaluated first. Priority is solely a means to omit parentheses in certain cases.

For example, the expression

a == b ? c : d

could be equivalent to either (a==b) ? c : d (a==b) ? c : d or a == (b ? c : d) . The fact that == has higher priority means that it is actually equivalent to the first.

In the expression

a ? b : c == d

there is no ambiguity, it is always equivalent to a ? b : (c == d) a ? b : (c == d) . Since the ternary operator is lazily evaluated, if a holds, c == d is never evaluated.

Answer 3

The T2() == T3() is never evaluated. Your code is equal to:

if(T1()) {
  System.out.println(F1());
}
else {
  if(T2() == T3()) {
    System.out.println(F2());
  }
  else {
    System.out.println(T4());
  }
}

Here you clearly see why it is never evaluated - because T1() is true and the else block is never entered.