Java null pointer exception after checking if null

Question

Can someone explain how it is possible that I get an exception from a null pointer thrown from this line of code:

if (data != null && data.isActive()) { 

The body of the isActive () method is true:

 public Boolean isActive() { return active; } 

Thanks in advance.

Answer 1

There is a thing in java called autoboxing when primitive values ​​are wrapped with object types and vice versa.

So there is a method in your code:

 public Boolean isActive() { return active; } 

note that you are returning a Boolean (object type), not a Boolean (primitive type).

and the return value will be used in your if statement.

 if (data != null && data.isActive()) { 

when java encounters data.isActive() in your if , it tries to convert the Boolean value to a primitive boolean to apply it to your boolean operation.

But your active variable inside your isActive() method is null, so java cannot remove this variable until the Boolean primitive value, and you get a Null pointer exception .