This pattern is used as a security mechanism. When we want to test a specific method, but we know that this method may have calls to other methods that produce side effects, we create a specific subclass of the class to be tested (Inheritance) and override the method that generates those unwanted changes. We could even turn this subclass into a spy and record that the undesired method has been called.
Sometimes, this pattern is often used for convenience and performance rather than for security, such as to avoid expensive calculations.
Xray is class that has a method called align() that change the direction of the object. This method calls another
method called turnOn() that turns on the Xray. We want to test the align() method, but we don't want to turn on the
Xray. So we create a subclass of Xray called SafeXray and override the turnOn() method to do nothing.
It's a variation of the Test-Specific Subclass pattern, but in this case, it's the test class that overrides the undesired behavior (the one that inherits from the class being tested).
However, we need to consider the testing framework to be used since it can construct test classes at different times, so care must be taken to ensure that all spy variables are properly reset.
The same example as the Test-Specific Subclass pattern, but in this case, we override the turnOn() method in the test.
It is used to separate the code that is difficult to test from the code that is easy to test. We delegate that code, which is difficult to test, to another class that is so small that we don't need to test it. This is generally done with the boundary we interact with, code over which we have no control, such as functions/methods of a framework. It's not even necessary for us to know what it does; we simply delegate it to another class and inject it into our class through a constructor.
We have a class called Calculator that has a method called calculateSquareRoot() that calculates the square root of
a number.
This method uses the Math.sqrt() method, which is a static method of the Math class. We want to test the sqrt()
method, but we don't want to test the Math.sqrt() method. So we create a class called MathHelper that has a method
called sqrt() that calls the Math.sqrt() method. We inject the MathHelper class into the Calculator class.