What is the Java ?: operator called and what does it do?

Question

I have been working with Java a couple of years, but up until recently I haven't run across this construct:

int count = isHere ? getHereCount(index) : getAwayCount(index);

This is probably a very simple question, but can someone explain it? How do I read it? I am pretty sure I know how it works.

• if isHere is true, getHereCount() is called,
• if isHere is false getAwayCount() is called.

Correct? What is this construct called?

Answer 1

http://en.wikipedia.org/wiki/Conditional_operator

Answer 2

Correct. It's called the ternary operator. Some also call it the conditional operator.

Answer 3

Yes, it's a shorthand form of

int count;
if (isHere)
    count = getHereCount(index);
else
    count = getAwayCount(index);

It's called the conditional operator. Many people (erroneously) call it the ternary operator, because it's the only ternary (three-argument) operator in Java, C, C++, and probably many other languages. But theoretically there could be another ternary operator, whereas there can only be one conditional operator.

Edit: The official name is given in the Java Language Specification:

§15.25 Conditional Operator ? :

The conditional operator ? : uses the boolean value of one expression to decide which of two other expressions should be evaluated.

Also, here is a tidbit from the spec that hasn't been mentioned:

Note that it is a compile-time error for either the second or the third operand expression to be an invocation of a void method. In fact, it is not permitted for a conditional expression to appear in any context where an invocation of a void method could appear (§14.8).

So you cannot compress this:

if (someBool)
    doSomething();
else
    doSomethingElse();

into this:

someBool ? doSomething() : doSomethingElse();

Answer 4

int count = isHere ? getHereCount(index) : getAwayCount(index);

means :

if (isHere) {
    count = getHereCount(index);
} else {
    count = getAwayCount(index);
}

Answer 5

Yes, you are correct. ?: is typically called the "ternary conditional operator", often referred to as simply "ternary operator". It is a shorthand version of the standard if/else conditional.

Ternary Conditional Operator

Answer 6

condition ? truth : false;

If the condition is true then return the first parameter. If the condition is false, return the second parameter.

It is called the Conditional Operator and it is a type of Ternary Operation.

Answer 7

According to the Sun Java Specification, it's called the Conditional Operator. See section 15.25. You're right as to what it does.

The conditional operator ? : uses the boolean value of one expression to decide which of two other expressions should be evaluated.

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

ConditionalExpression:
        ConditionalOrExpression
        ConditionalOrExpression ? Expression : ConditionalExpression

The conditional operator has three operand expressions; ? appears between the first and second expressions, and : appears between the second and third expressions.

The first expression must be of type boolean or Boolean, or a compile-time error occurs.

Answer 8

It's the conditional operator, and it's more than just a concise way of writing if statements.

Since it is an expression that returns a value it can be used as part of other expressions.

Answer 9

Others have answered this to reasonable extent, but often with the name "ternary operator".

Being the pedant that I am, I'd like to make it clear that the name of the operator is the conditional operator. It's a ternary operator (in that it has three operands) and it happens to be the only ternary operator in Java at the moment.

However, the spec is pretty clear that its name is the conditional operator. I think it's clearer to call it by that name, as it indicates the behaviour of the operator to some extent (evaluating a condition) rather than just how many operands it has.

Answer 10

The ternary operator is one of those things C programmers created to make it possible to write powerful, compact code in as few lines as possible (like ++ and -- ). It can get pretty confusing when you have it 3 times on one line... :D

Answer 11

Not exactly correct, to be precise:

1. if isHere is true, the result of getHereCount() is returned
2. otheriwse the result of getAwayCount() is returned

That "returned" is very important. It means the methods must return a value and that value must be assigned somewhere.

Also, it's not exactly syntactically equivalent to the if-else version. For example:

String str1,str2,str3,str4;
boolean check;
//...
return str1 + (check ? str2 : str3) + str4;

If coded with if-else will always result in more bytecode.

Answer 12

I happen to really like this operator, but I never use it at work.

You always have to balance code compactness with the time spent reading it, and in that it has some pretty severe flaws.

First of all, there is the posters case. He just spent an hour posting about it and reading the responses. How longer would it have taken the author to write every ?: as an if/then throughout the course of his entire life. Not an hour to be sure.

Secondly, in C-like languages, you get in the habit of simply knowing that conditionals are the first thing in the line. I noticed this when I was using Ruby and came across lines like:

callMethodWhatever(Long + Expression + with + syntax) if conditional

If I was a long time Ruby user I probably wouldn't have had a problem with this line, but coming from C, when you see "callMethodWhatever" as the first thing in the line, you expect it to be executed. The ?: is less cryptic, but still unusual enough as to throw a reader off.

The advantage, however, is a really cool feeling in your tummy when you can write a 3-line if statement in the space of 1 of the lines. Can't deny that :) But honestly, not necessarily more readable by 90% of the people out there simply because of its' rarity.

Answer 13

Ternary, conditional; tomato, tomatoh. What it's really valuable for is variable initialization. If (like me) you're fond of initializing variables where they are defined, the conditional ternary operator (for it is both) permits you to do that in cases where there is conditionality about its value. Particularly notable in final fields, but useful elsewhere, too.

e.g.:

public class Foo {
    final double value;

    public Foo(boolean positive, double value) {
     this.value = positive ? value : -value;
    }
}

Without that operator - by whatever name - you would have to make the field non-final or write a function simply to initialize it. Actually, that's not right - it can still be initialized using if/else, at least in Java. But I find this cleaner.

Answer 14

You might be interested in a proposal for some new operators that are similar to the conditional operator. The null-safe operators will enable code like this:

String s = mayBeNull?.toString() ?: "null";

It would be especially convenient where auto-unboxing takes place.

Integer ival = ...;  // may be null
int i = ival ?: -1;  // no NPE from unboxing

It has been selected for further consideration under JDK 7's "Project Coin."