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Can someone tell me why this use of the ternary operator is incorrect? Operands 2 and 3 return a boolean.

public class Something {
...
private static final double REFERENCE_FRAME_MID_X = 0;
private static final double REFERENCE_FRAME_MID_Y = 0;

private boolean findInsideOrOutsideGeneralEllipse(Point2D destCirclePos) {
    List<Boolean> returnValue = new ArrayList<>();
    Point2D referenceFrameCenter = new Point2D.Double(REFERENCE_FRAME_MID_X, REFERENCE_FRAME_MID_Y);
    Ellipse2D insideAreaEllipse2D = getEllipse2D(referenceFrameCenter.getX(), referenceFrameCenter.getY(),
                                                    destCirclePos.distance(referenceFrameCenter));

    // doesn't work
    insideAreaEllipse2D.contains(destCirclePos) ? returnValue.add(true) : returnValue.add(false);

    // works
    if (insideAreaEllipse2D.contains(destCirclePos)) {
        returnValue.add(true);
    } else {
        returnValue.add(false);
    }
}
...
}
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  • 14
    you just want, returnValue.add(insideAreaEllipse2D.contains(destCirclePos)); Commented Sep 25, 2013 at 16:30
  • 1
    @nachokk I know it can be done that way, but I'd like to understand exactly why that is a compile-time error... Commented Sep 25, 2013 at 16:32
  • 4
    conditional operator does not exist to conditionally execute the expressions, but rather to conditionally return the (non-void) result of one of the expressions. Commented Sep 25, 2013 at 16:33
  • 2
    ternary operator must be used for assignment, otherwise you will get a compiler error. Commented Sep 25, 2013 at 16:34
  • 1
    What do you mean by "doesn't work"? The result of add is not the added value, but a boolean indicating whether or not the collection changed. The value of the expression will always be true, since the empty collection has a value added to it (either true or false). Commented Sep 25, 2013 at 16:34

2 Answers 2

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10

Usage of Java ternary operation condition should looks like 

result = testCondition ? value1 : value2

it's java-language specification.

Equality, Relational, and Conditional Operators

In the following example, this operator should be read as: "If someCondition is true, assign the value of value1 to result. Otherwise, assign the value of value2 to result

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7 Comments

I can't speak as to why you got downvoted, but there's no requirement that the result of a ternary expression be used as the right hand side of an assignment. condition ? val1 : val2 is an expression, and while it can't be used as a statement, there are lots of places you can use expressions, not just assignments. E.g., you could System.out.println( condition ? val1 : val2 );.
@JoshuaTaylor your code is equivalent to String s = condition ? val1 : val2; System.out.println(s);. It's the same that I said
@JoshuaTaylor note that you're already passing the result of condition ? val1 : val2 into a temporary variable that will be used in System.out.println( ... ). Try just having condition ? val1 : val2; with nothing on the left side. If you make it work, let us know to email James Gosling about this :).
@LuiggiMendoza. Joshua is talking about the places where you can use conditional operator. And it's not only in assignment context. The usage of ?: as argument to method, is not an assignment context, but a method invocation context.
@LuiggiMendoza I'm aware that the conditional expression isn't a statement. My point is that an expression can be used in other places than an assignment. Rohit Jain's answer quotes the specification about the grammar, and makes this clearer.
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5

From JLS - Conditional Operator:

In fact, by the grammar of expression statements (§14.8), it is not permitted for a conditional expression to appear in any context where an invocation of a void method could appear.

Grammar of expression statements from JLS - 14.8:

Certain kinds of expressions may be used as statements by following them with semicolons:

ExpressionStatement:
      StatementExpression ;

StatementExpression:
       Assignment
       PreIncrementExpression
       PreDecrementExpression
       PostIncrementExpression
       PostDecrementExpression
       MethodInvocation
       ClassInstanceCreationExpression

An expression statement is executed by evaluating the expression; if the expression has a value, the value is discarded. Execution of the expression statement completes normally if and only if evaluation of the expression completes normally.

Unlike C and C++, the Java programming language allows only certain forms of expressions to be used as expression statements.

Now the way you are using the conditional operator is not a valid expression statement, as inferred from it's grammar. And hence you get the compiler error. You have to use it in any of the above mentioned context.

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6 Comments

I have explicitly pointed out that returnvalue.add() returns a boolean.
@deinocheirus The point isn't that the types of the expressions aren't compatible, but that the ternary operator expression (the conditional expression) isn't a statement, but an expression, and in your code it appears in a place where a statement is expected.
@deinocheirus Yes, but your code still isn't a StatementExpression.
@JoshuaTaylor he edited his answer to the point my comment no longer makes sense.
@Axel the same thing :)
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