The ? expression in Java

Question

I'm taking a introduction course in Java and I have a question about the ? expression (I've done some C# programming) but I'm not that familiar with it there either.

If I have this if statement, how do I use the ? on it? I have a task where I'm supposed to compare three variables in different ways and I hate to have alot of if statements.

    //Task b (If var3 is divisible with var2)
    if(var3 % var2 == 0)
    {
        System.out.println("\n" + var3 + " is divisible with " + var2);
        System.out.println(var3 + " divided with " + var2 + " is: " + var3 / var2);
    }
    else
    {
        System.out.println("\n" + var3 + " is not divisible with " + var2);
    }

This just look messy.. It's okay to turn it in like this but I personally think it look awful.

Why do you think using the ternary operator would make this look less messy? — dhke, Jun 10 '15 at 09:04
This sounds a bit like an XY Problem. Do you actually want to avoid long chains of if statements? It honestly sounds like what you're really looking for is a `switch` — Dragondraikk, Jun 10 '15 at 09:05
I think it looks ok. If you use the terny operator `?` it will be bad readable. The terny operator sould only be used in very short terms. — Jens, Jun 10 '15 at 09:06
@MarounMaroun When you have 7 ones that look almost exactly the same I think it does. Jens Alright. Thank you. Ill stick to what I have then. — Fredrik, Jun 10 '15 at 09:06
@Fredrik Then `?` won't help you here. Consider arrays and loops. — Maroun, Jun 10 '15 at 09:07
It would be useful if you would show a situation which is *actually* a problem then, instead of one that isn't. — Jon Skeet, Jun 10 '15 at 09:07

score 2 · Answer 1 · edited Jun 10 '15 at 09:55

2

You could use it like so:

bool isDivisible = var3 % var2 == 0;

System.out.format("\n%d is %sdivisible with %d",
                   var3,
                   isDivisible ? "" : "not ",
                   var2);
if (isDivisible) {
    System.out.format(" %d divided with %d is %d", var3, var2, var3 / var2);
}

I'm using it to inject the word "not" where required, so that the first two statements are now combined. Note use of format as string concatenation gets ugly quickly.

edited Jun 10 '15 at 09:55

Kayaman

72,141
5
83
121

answered Jun 10 '15 at 09:15

weston

54,145
21
145
203

Sorry, edited (and rolled back) the answer because didn't realize `printf()` actually calls `format()`. – Kayaman Jun 10 '15 at 09:56
That's OK `printf` probably more readable actually. Though OP: you should consider also `System.out.println(String.format(...))` as will add new line characters for you. – weston Jun 10 '15 at 10:30
1

@weston I prefer the simpler `%n` for a new line, e.g., `System.out.format("this %d and that %d%n", 1, 2)` – Hummeling Engineering BV Jun 14 '15 at 21:03

score 1 · Answer 2 · answered Jun 10 '15 at 09:43

? is not complete, it is a group of ? : termed as ternary operator. Basically the syntax is as follows:

<variable> = <condition> ? <true value> : <false value>;

So your code goes like this:

String msg = 0 == var3 % var2 ?
            "\n" + var3 + " is divisible with " + var2 :
            "\n" + var3 + " is not divisible with " + var2;
System.out.println(msg);

You could also use () to avoid confusion:

String msg = (0 == var3 % var2) ?
                ("\n" + var3 + " is divisible with " + var2) :
                ("\n" + var3 + " is not divisible with " + var2);
System.out.println(msg);

score 0 · Answer 3 · answered Jun 10 '15 at 09:10

It is called ternary operator. It can be used to evaluate logical expression in a statement.Its syntax is

[condition] ? [true expression] : [false expression]

In your code example,

public static void main(String[] args) {
    int var3 = 12;
    int var2= 6;
    System.out.println( var3%var2==0 ? "divisible" : "not divisible" );   }

score 0 · Answer 4 · answered Jun 10 '15 at 09:10

This looks ok as is. If you only printed one line in each case you could replace the if/else block with:

System.out.println("\n" + var3 + " is "
                        + (var3 % var2 == 0 ? "" : " not ")
                        + " divisible with " + var2);

But since you need to print 2 lines in one case and only one in the other case your if/else construct is probably better.

Hummeling Engineering BV · Answer 5 · 2015-06-14T21:08:10.950

0

I think the ternary operator can be used for simple one-liners, not for more elaborate code:

System.out.println(var3 % var2 == 0
    ? "\n" + var3 + " is divisible with " + var2 + "\n" + var3 + " divided with " + var2 + " is: " + var3 / var2
    : "\n" + var3 + " is not divisible with " + var2);

Not a lot better is it?

Most solutions offered here strike me as a case of, what I call, 'over-automating'.

edited Jun 14 '15 at 21:08

answered Jun 10 '15 at 09:10

Hummeling Engineering BV

406
2
10

score 0 · Answer 6 · answered Jun 10 '15 at 09:10

How about this?

String text1 =  "\n" + var3 + " is divisible with " + var2 + "\n" + var3 + " divided with " + var2 + " is: " + var3 / var2;
String text2 =  "\n" + var3 + " is not divisible with " + var2;   
String res = var3 % var2 == 0 ? text1 : text2;
    System.out.println(res);

The ? expression in Java

6 Answers6