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

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?

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

condition ? truth : false;

If the condition is true then evaluate the first expression. If the condition is false, evaluate the second expression.

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

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.

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.

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.

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.

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

means :

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

This construct is called Ternary Operator in Computer Science and Programing techniques.
And Wikipedia suggest the following explanation:

In computer science, a ternary operator (sometimes incorrectly called a tertiary operator) is an operator that takes three arguments. The arguments and result can be of different types. Many programming languages that use C-like syntax feature a ternary operator, ?: , which defines a conditional expression.

Not only in Java, this syntax is available within PHP, Objective-C too.

In the following link it gives the following explanation, which is quiet good to understand it:

A ternary operator is some operation operating on 3 inputs. It's a shortcut for an if-else statement, and is also known as a conditional operator.

In Perl/PHP it works as:
boolean_condition ? true_value : false_value

In C/C++ it works as:
logical expression ? action for true : action for false

This might be readable for some logical conditions which are not too complex otherwise it is better to use If-Else block with intended combination of conditional logic.

We can simplify the If-Else blocks with this Ternary operator for one code statement line.
For Example:

if ( car.isStarted() ) {
     car.goForward();
} else {
     car.startTheEngine();
}

Might be equal to the following:

( car.isStarted() ) ? car.goForward() : car.startTheEngine();

So if we refer to your statement:

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

It is actually the 100% equivalent of the following If-Else block:

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

That's it!
Hope this was helpful to somebody!
Cheers!

I happen to really like this operator, but the reader should be taken into consideration.

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 Original Asker's 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.

When it is truly an assignment based on a Boolean and values I don't have a problem with it, but it can easily be abused.

Actually it can take more than 3 arguments. For instance if we want to check wether a number is positive, negative or zero we can do this:

String m= num > 0 ? "is a POSITIVE NUMBER.": num < 0 ?"is a NEGATIVE NUMBER." :"IT's ZERO.";

which is better than using if, else if, else.

Its Ternary Operator(?:)

The ternary operator is an operator that takes three arguments. The first 
argument is a comparison argument, the second is the result upon a true 
comparison, and the third is the result upon a false comparison.

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 or "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 or "conditional operator ?:" to be absolutely unambiguous. 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.

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."

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

Conditional expressions are in a completely different style, with no explicit if in the statement.

The syntax is: boolean-expression ? expression1 : expression2;

The result of this conditional expression is

expression1 if boolean-expression is true;

otherwise the result is expression2.

Suppose you want to assign the larger number of variable num1 and num2 to max. You can simply write a statement using the conditional expression: max = (num1 > num2) ? num1 : num2;

Note: The symbols ? and : appear together in a conditional expression. They form a conditional operator and also called a ternary operator because it uses three operands. It is the only ternary operator in Java.

cited from: Intro to Java Programming 10th edition by Y. Daniel Liang page 126 - 127