It means the same thing as: provided that 'frame' is not declared as a local variable within the block of code. That is, 'frame' is a predefined instance field. The 'this.' part is implicitly understood from within a non-static method when not explicitly given, It refers to the object created from the class that the method is being executed for (or on). All non-static methods (and constructor blocks) are executed on or for an instance (whether new or not) of the class. Field references are similar except that they can be overshadowed. That is, a local variable may have the same name as a field that was previously declared for an object. In that case, to reference the predefined field, one must explicitly give the object using 'this.', without which, the local variable is referenced.