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2.3.1 Static Methods

In Figure 2.19, we implemented three static methods: two factory methods, fromBraneCollision() and FromBigCrunch(), and one getter method to retrieve the total number of big bangs, getTotalCount(). These methods are very similar to instance methods we are already used to, with a few important differences:

  1. Static methods are declared with the static keyword.
  2. In the body, this is bound to the class itself, rather than the instance.
  3. Static methods cannot call any non-static methods of the same class without first instantiating it.

The final rule above is not true when the situation is reversed. Non-static methods can call static methods through use of the __self object, which is a reference to the class itself. That is, this in a static method is the same object as this.__self in a non-static method. This is demonstrated by getTotalCount()

    this.$('_count')

and __construct().

    this.__self.$('_count')

To help remember __self, consider what the name states. A class is a definition used to create an object. The body of a method is a definition, which is defined on the class. Therefore, even though the body of a method may be called in the context of an instance, it is still part of the class. As such, __self refers to the class.