Explain Java syntax

Question

I'm looking at a piece of Java code that contains:

User rv = tmp != null && tmp.size() > 0 ? tmp.get(0) : null;

I'm not very strong with Java syntax. My interpretation is that rv = tmp as long as tmp is null tmp's size is > 0, or else it equals null. Am I correct?

Probably if I wrote this I would use brackets to make it easier for a junior programmer to understand User rv = (tmp != null && tmp.size() > 0) ? tmp.get(0) : null; — Scary Wombat
– Scary Wombat, Commented Oct 23, 2013 at 2:16

Sergey Kalinichenko · Accepted Answer · 2013-10-23 02:14:09Z

7

Here is the "anatomy" of this expression:

rv =                                // assignment of a conditional expression
    (tmp != null && tmp.size() > 0) // condition
?   tmp.get(0)                      // on true
:   null;                           // on false

This is a common way of ensuring that there would be no exception accessing element zero of the list: the condition ensures that tmp is not null, and also that the size is one or more.

answered Oct 23, 2013 at 2:14

Sergey Kalinichenko

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2 Comments

chrylis -cautiouslyoptimistic- Over a year ago

Note that the code is almost illegible and needs parentheses at a minimum.

Paul Richter Over a year ago

For added background information regarding ternary operators in java, click here (wikipedia article) and here (java tutorial)

Corbin · Accepted Answer · 2013-10-23 02:15:01Z

3

It's a ternary conditional expression:

expr ? value-if-true : value-if-false

If expr is true, it evaluates to value-if-true, and otherwise it evaluates to value-if-valuse.

So, in this case, it's equivalent to:

 if (tmp != null && tmp.size() > 0) {
     rv = tmp.get(0);
 } else {
      rv = null;
 }

answered Oct 23, 2013 at 2:15

Corbin

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Comments

Anurag Rana · Accepted Answer · 2013-10-23 02:16:40Z

2

rv = ((tmp != null && tmp.size() > 0) ? tmp.get(0) : null);

if expression inside the inner brackets return true then rv will hold the value tmp.get(0) else it will hold the value null.

variable = expression ? a : b ;

is similar to

if expression == true 
  variable = a;
else 
   variable =b ;

answered Oct 23, 2013 at 2:16

Anurag Rana

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Comments

Rohan · Accepted Answer · 2013-10-23 02:18:55Z

0

This is called a ternary statement.

Basically,

User rv = tmp != null && tmp.size() > 0 ? tmp.get(0) : null;

is a shorter way of writing:

        User rv;

        if (tmp != null && tmp.size() > 0)
            rv = tmp.get(0);
        else
            rv = null;

The ternary statement is not faster. Although, in some cases, using it makes your code more readable.

answered Oct 23, 2013 at 2:18

Rohan

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1 Comment

Scary Wombat Over a year ago

in some cases, using it makes your code more readable. - Not in this case though (if it had parentheses it would be better)

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