{"id":1415,"date":"2012-06-10T15:00:00","date_gmt":"2012-06-10T15:00:00","guid":{"rendered":"http:\/\/www.javacodegeeks.com\/2012\/10\/testing-abstract-classes-and-template-method-pattern.html"},"modified":"2012-10-22T05:40:18","modified_gmt":"2012-10-22T05:40:18","slug":"testing-abstract-classes-and-template","status":"publish","type":"post","link":"https:\/\/www.javacodegeeks.com\/2012\/06\/testing-abstract-classes-and-template.html","title":{"rendered":"Testing Abstract Classes and Template Method Pattern"},"content":{"rendered":"
From wikipedia “A template method defines the program skeleton of an algorithm. One or more of the algorithm steps can be overridden by subclasses to allow differing behaviors while ensuring that the overarching algorithm is still followed”. <\/p>\n

Typically this pattern is composed by two or more classes, one that is an abstract class providing template methods (non-abstract) that have calls to abstract methods implemented by one or more concrete subclasses. <\/p>\n

Often template abstract class and concrete implementations reside in the same project, but depending on the scope of the project, these concrete objects will be implemented into another project. <\/p>\n

In this post we are going to see how to test template method pattern when concrete classes are implemented on external project, or more general how to test abstract classes. <\/p>\n

Let’s see a simple example of template method pattern. Consider a class which is responsible of receiving a vector of integers and calculate the Euclidean norm. These integers could be received from multiple sources, and is left to each project to provide a way to obtain them. <\/p>\n

The template class looks like: <\/p>\n

public abstract class AbstractCalculator {\r\n\r\n public double euclideanNorm() {\r\n\r\n  int[] vector = this.read();\r\n\r\n  int total = 0;\r\n\r\n  for(int element:vector) {\r\n   total+= (element*element);  \r\n  }\r\n\r\n  return Math.sqrt(total);\r\n }\r\n\r\n public abstract int[] read();\r\n}\r\n<\/pre>\n
Now another project could extend previous class and make an implementation of  abstract calculator by providing an implementation of read() method . 
 
<\/div> <\/div><\/p>\n
public class ConsoleCalculator extends AbstractCalculator {\r\n\r\n public int[] read() {\r\n\r\n  int [] data = new int[0];\r\n\r\n  Scanner scanner = new Scanner(System.in);\r\n\r\n  \/\/data = read requried data from console\r\n\r\n  return data; \r\n\r\n }\r\n\r\n}\r\n<\/pre>\n
Developer that has written a concrete implementation will test only read() method, he can “trust” that developer of abstract class has tested non-abstract methods. <\/p>\n
But how are we going to write unit tests over calculate method if class is abstract and an implementation of read() method is required? <\/p>\n
The first approach could be creating a fake implementation:  <\/p>\n
public class FakeCalculator extends AbstractCalculator {\r\n\r\n private int[] data;\r\n\r\n public FakeCalculator(int[] data) {\r\n  this.data = data;\r\n }\r\n\r\n public int[] read() {\r\n  return this.data;\r\n }\r\n\r\n}\r\n<\/pre>\n
This is not a bad approach, but has some disadvantages: <\/p>\n