Saw this line in a class method and my first reaction was to ridicule the developer that wrote it.. But then, I figured I should make sure I was right first.
public void dataViewActivated(DataViewEvent e) {
if (this != null)
// Do some work
}
Will that line ever evaluate to false?
No it can't. If you're using this, then you're in the instance so this isn't null.
The JLS says :
When used as a primary expression, the keyword this denotes a value that is a reference to the object for which the instance method was invoked (§15.12), or to the object being constructed.
If you invoked a method from an object, then the object exists or you would have a NullPointerException before (or it's a static method but then, you can't use this in it).
Resources :
No. To call a method of an instance of a class, the instance has to exist. The instance is implicitly passed as a parameter to the method, referenced by this. If this was null then there'd have been no instance to call a method of.
In static class methods, "this" isn't defined since "this" is associated with instances and not classes. I believe it would give a compiler error to attempt to use "this" keyword in static context.
If you compile with -target 1.3 or earlier, then an outer this may be null. Or at least it used to...
Within the context of a static method (as others have already pointed out), including within an application Main method, the "this" keyword would likely have nothing appropriate to reference, and would return null.
Otherwise, assuming that your code is within an instance method of property of a class, and thus can only run in the context of an instantiated object, this always reference that object.
I believe this would apply to Java or C#. Not sure about others...
I don't know about in Java, but in C++ (although yes this test would be silly to do at the start of a function) later on it may make sense as it is possible that an object can delete itself halfway through its own code and then continue running that code. For example, the following compiles with no errors under gcc 3.4.4 but if run will crash with a NULL access error due to 'this' becoming a NULL pointer.
void foo::bar()
{
delete this;
this->some_function();
}
When you invoke a method on null reference, the NullPointerException will be thrown from Java VM. This is by specification so if your Java VM strictly complies to the specification, this would never be null.
tl;dr, "this" can only be called from a non-static method and we all know that a non-static method is called from some sort of object which cannot be null.