Checking if an Object is null at the same time as a value of one of it's fields

Question

Which of these would be correct?

if(dialog != null && dialog.isShowing){}

if(dialog.isShowing && dialog != null){}

if(dialog != null){
   if(dialog.isShowing){}
}

Answer 1

The first and third ones are both OK to use because they won't process past the null check. The second one can result in a NullPointerException because it's referencing dialog before you've checked if it's null.

Answer 2

The && operator in Java will stop evaluating (from left to right) as soon as it encounters a false. Therefore in

if(dialog != null && dialog.isShowing){}

dialog.isShowing() will not be called if the dialog is null and is therefore "safe" to use.

This:

if(dialog != null){
   if(dialog.isShowing){}
}

will work as well, but generally nesting if-statements like this is avoided as it decreases readability.

Answer 3

Even though everything has been answered, for the sake of completeness: The way Java evaluates conditional clauses is called Short circuit evaluation . This means that once the result of the condition is asserted, further clauses will not be evaluated.

Edit: My statement is not completely true actually. Java uses Short circuit evaluation when using || and && (which is the standard what every programmer uses, thus my statement in the first place), but you may force Java to evaluate all statements by using & and |

Answer 4

The && operator is called a short circuit operator. This means once the result is known. ie, false && x is always false it doesn't evaluate the remaining expressions.

A variation on this is using the || short circuit, OR operation, like:

if(text == null || text.isEmpty())

Answer 5

It's shortcut logic. When first expression is false, Java will not check second expression, because whole expression is false definity.

• First case is best practice.
• Second case is incorrect.

Answer 6

You may use try and catch if you want to use the second one otherwise the first and the third one are correct.