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?

• Paul Bellora: This is something you could test to find out on your own.
• Scary Wombat: 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;

Answer 1

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.

• chrylis -cautiouslyoptimistic-: Note that the code is almost illegible and needs parentheses at a minimum.
• Paul Richter: For added background information regarding ternary operators in java, click here (wikipedia article) and here (java tutorial)

Answer 2

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;
 }

Answer 3

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 ;

Answer 4

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.

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