CODEWITHSUNDEEP
javaconditional-statementsconditional-operator
Bibek Ghimire
Bibek Ghimire

Reputation: 562

How are ternary expressions in Java evaluated?

I have followed the tutorial provided here and in the line:

boolean t1 = false?false:true?false:true?false:true;

the final value of t1 is false. But I've evaluated it as true. The first false gives true and that true gives false, which further finally gives true, am I right? No, I am wrong. Could you please tell me how ternary expressions are evaluated in Java?

Upvotes: 4

Views: 246

Answers (3)

Ousmane D.
Ousmane D.

Reputation: 56489

The Pseudocode below shows how the expression is evaluated:

boolean t1 = false?false:true?false:true?false:true              
           = true && (true ? false : true ? false : true)
           = true && false
           = false

Another way to look at it:

boolean t1 = false?false:true?false:true?false:true;

can be simplified to:

boolean t1 = !false && (true ? false : true ? false : true);

then simplified to:

boolean t1 = true && (true ? false : true ? false : true);

then simplified to:

boolean t1 = true && false;

which eventually results in false;

Upvotes: 1

cs95
cs95

Reputation: 403198

We can regroup these operations according to associativity rules.

boolean t1 = (false ? false : (true ? false : (true ? false : true)));
                  |                      |               | 
                  |                      |               1
                  |                      2
                  3

In theory, this is what should happen

  1. Exp 1 is evaluated first and is evaluated as false.
  2. Exp 2 is now: true? false : false so the final output is false.
  3. Exp 3 now becomes false ? false : false, which is false.

Now, to get technical, this is what actually happens:

  1. Exp 3 is false ? (don't care) : (...) -> the inner expression containing Exp 2 and Exp 1 need to be evaluated.
  2. Exp 2 is true: false, (don't care) Since we've already determined the value of Exp 2, Exp 1 no longer needs to be evaluated.
  3. Exp 3 is now reevaluated to be false ? (don't care) : false and so the final answer is false.

Upvotes: 4

Eran
Eran

Reputation: 394146

When the compiler finds a ? character, it looks for a corresponding :. The expression before the ? is the first operand of the ternary conditional operator, which represents the condition.

The expression between the ? and the : is the second operand of the operator, whose value is returned if the condition is true.

The expression after the : is the third operand of the operator, whose value is returned if the condition is false.

boolean t1 = false   ? false    :    true?false:true?false:true;

             first     second        third
             operand   operand       operand

Since first operand is false, the result is the value of the third operand true?false:true?false:true, so let's evaluate it:

true    ?   false    : true?false:true;

first       second     third
operand     operand    operand

Since first operand is true, the result is the value of the second operand - false.

BTW, the value of the third operand true?false:true is also false, so x?false:true?false:true returns false regardless of the value of x.

Upvotes: 5

Related Questions