Parallel Inheritance Hierarchies
The Code Smells are similar in concept to Development-level Anti-patterns. Sometimes in our code, we introduce code smell unintentionally those makes our design fragile.
Definition of Code smell
Code smell, also known as a bad smell, in computer programming code, refers to any symptom in the source code of a program that possibly indicates a deeper problem.
Martin fowler says:
"a code smell is a surface indication that usually corresponds to a deeper problem in the system"
Code smell creates lot of problems while introducing new feature or maintains the codebase,
Often developer has to write repeatable code, breaking encapsulation, breaking abstraction etc.
If code smells are not corrected.
So always refactor code smell while developing.
In this article, we discuss on “Parallel Inheritance Hierarchies” code smell.
Parallel Inheritance Hierarchies code smells occurs when an inheritance tree depends on another inheritance tree by composition and they maintain a special relation one subclass of dependent inheritance must dependent one a particular subclass of another Inheritance
Think about Engineers and their work and target, Computer Engineers work on Computers
And the target is to deliver projects whereas Civil engineer work on Structure and target is to implement structure. So Design perspective there is two parallel hierarchies a. Engineers b. Milestone
The different engineer has a different milestone and each engineer has a specified milestone special relation.
The problem is Every time you add a new Engineer in Engineer inheritance you have to introduce a new Milestone in Milestone hierarchy.
Cause of Parallel Inheritance Hierarchies smell:
1. Fail to understand the responsibility. Often due to misunderstanding (Single responsibility principle)
2. Overenthusiasm to break each functionality as a separate interface.
3. Fail to introduce proper design pattern.
Consequences of Parallel Inheritance Hierarchies:
1. Lots of duplicates code.
2. The wrong relationship sets by the client
3. Unmaintainable code base.
Refactor Strategy
. We can do it by “Move Method”, “Move Field” techniques.
Now, take an Example Where Parallel Inheritance Hierarchies code smells present. We will implement the Engineer and set Milestone for him.
Java Code
Definition of Code smell
Code smell, also known as a bad smell, in computer programming code, refers to any symptom in the source code of a program that possibly indicates a deeper problem.
Martin fowler says:
"a code smell is a surface indication that usually corresponds to a deeper problem in the system"
Code smell creates lot of problems while introducing new feature or maintains the codebase,
Often developer has to write repeatable code, breaking encapsulation, breaking abstraction etc.
If code smells are not corrected.
So always refactor code smell while developing.
In this article, we discuss on “Parallel Inheritance Hierarchies” code smell.
Parallel Inheritance Hierarchies code smells occurs when an inheritance tree depends on another inheritance tree by composition and they maintain a special relation one subclass of dependent inheritance must dependent one a particular subclass of another Inheritance
Think about Engineers and their work and target, Computer Engineers work on Computers
And the target is to deliver projects whereas Civil engineer work on Structure and target is to implement structure. So Design perspective there is two parallel hierarchies a. Engineers b. Milestone
The different engineer has a different milestone and each engineer has a specified milestone special relation.
The problem is Every time you add a new Engineer in Engineer inheritance you have to introduce a new Milestone in Milestone hierarchy.
Cause of Parallel Inheritance Hierarchies smell:
1. Fail to understand the responsibility. Often due to misunderstanding (Single responsibility principle)
2. Overenthusiasm to break each functionality as a separate interface.
3. Fail to introduce proper design pattern.
Consequences of Parallel Inheritance Hierarchies:
1. Lots of duplicates code.
2. The wrong relationship sets by the client
3. Unmaintainable code base.
Refactor Strategy
. We can do it by “Move Method”, “Move Field” techniques.
Now, take an Example Where Parallel Inheritance Hierarchies code smells present. We will implement the Engineer and set Milestone for him.
Java Code
package com.example.codesmell.parallelinheritence;
public interface Engineer {
String getType();
void setType(String type);
int getSalary();
void setSalary(int salary);
MileStone getMileStone();
void setMileStone(MileStone mileStone);
}
package com.example.codesmell.parallelinheritence;
public interface MileStone {
public String work();
public String target();
}
package com.example.codesmell.parallelinheritence;
public class ComputerEngineer implements Engineer{
private String type;
private int salary;
private MileStone mileStone;
public void setType(String type) {
this.type = type;
}
public void setSalary(int salary) {
this.salary = salary;
}
public void setMileStone(MileStone mileStone) {
this.mileStone = mileStone;
}
@Override
public String getType() {
// TODO Auto-generated method stub
return type;
}
@Override
public int getSalary() {
// TODO Auto-generated method stub
return salary;
}
@Override
public MileStone getMileStone() {
// TODO Auto-generated method stub
return mileStone;
}
@Override
public String toString() {
return "ComputerEngineer [type=" + type + ", salary=" + salary
+ ", mileStone=" + mileStone + "]";
}
}
package com.example.codesmell.parallelinheritence;
public class ComputerMileStone implements MileStone{
@Override
public String work() {
return"Build a Billing MicroService";
}
@Override
public String target() {
return"Has to be finshed in 14 PD";
}
@Override
public String toString() {
return "ComputerMileStone [work()=" + work() + ", target()=" + target()
+ "]";
}
}
package com.example.codesmell.parallelinheritence;
public class CivilEngineer implements Engineer{
private String type;
private int salary;
private MileStone mileStone;
public void setType(String type) {
this.type = type;
}
public void setSalary(int salary) {
this.salary = salary;
}
public void setMileStone(MileStone mileStone) {
this.mileStone = mileStone;
}
@Override
public String getType() {
// TODO Auto-generated method stub
return type;
}
@Override
public int getSalary() {
// TODO Auto-generated method stub
return salary;
}
@Override
public MileStone getMileStone() {
// TODO Auto-generated method stub
return mileStone;
}
@Override
public String toString() {
return "CivilEngineer [type=" + type + ", salary=" + salary
+ ", mileStone=" + mileStone + "]";
}
}
package com.example.codesmell.parallelinheritence;
public class CivilMileStone implements MileStone{
@Override
public String work() {
// TODO Auto-generated method stub
return "Create Twin Towers";
}
@Override
public String target() {
// TODO Auto-generated method stub
return "Has to be completed in 2 years";
}
@Override
public String toString() {
return "CivilMileStone [work()=" + work() + ", target()=" + target()
+ "]";
}
}
package com.example.codesmell.parallelinheritence;
public class Manager {
public static void main(String[] args) {
Engineer comp = new ComputerEngineer();
comp.setType("Computer Engineer");
comp.setSalary(50000);
comp.setMileStone(new ComputerMileStone());
Engineer civil = new ComputerEngineer();
civil.setType("Civil Engineer");
civil.setSalary(60000);
civil.setMileStone(new CivilMileStone());
System.out.println(comp);
System.out.println("********************");
System.out.println(civil);
}
}
Output :
ComputerEngineer [type=Computer Engineer, salary=50000, mileStone=ComputerMileStone [work()=Build a Billing MicroService, target()=Has to be finshed in 14 PD]]
********************
CivilEngineer [type=Civil Engineer, salary=60000, mileStone=CivilMileStone [work()=Create Twin Towers, target()=Has to be completed in 2 years]]
We create two interfaces Engineer and Milestone and create Subclass for them but the thing to notice is Every engineer has his own special milestone so as we expose the setMileStone method to client it may possible client set a wrong Milestone for an Engineer.
Another thing If we want to add a new Engineer also we need to add a new Milestone for him/her.
It is very difficult to fix this if we try to fix it may break SRP (Single Responsibility) principal.
So, In my opinion, there is no fix solution for dealing with parallel Hierarchies.
There is Three possible way we can deal with it.
Solution 1:
1. Keep the parallel hierarchies open and get used to it.
Pros:
Better way to maintain SRP.
The code will be flexible.
Cons:
To Add a new feature, we have to create two classes every time.
May change in one need to change in Other hierarchies as they are coupled.
Harder to maintain.
Solution 2:
Make it Partial Hierarchies so we can open provision for parallel hierarchies.
Pros:
Only maintain One hierarchy
When you are not sure about responsibility try to adopt it.
Provide flexibility
Cons:
May break Single Responsibility principle
Technique:
Make a concrete class and implements both interfaces. The client got an instance of this class by the static factory method.
Let see the solution:
Another thing If we want to add a new Engineer also we need to add a new Milestone for him/her.
It is very difficult to fix this if we try to fix it may break SRP (Single Responsibility) principal.
So, In my opinion, there is no fix solution for dealing with parallel Hierarchies.
There is Three possible way we can deal with it.
Solution 1:
1. Keep the parallel hierarchies open and get used to it.
Pros:
Better way to maintain SRP.
The code will be flexible.
Cons:
To Add a new feature, we have to create two classes every time.
May change in one need to change in Other hierarchies as they are coupled.
Harder to maintain.
Solution 2:
Make it Partial Hierarchies so we can open provision for parallel hierarchies.
Pros:
Only maintain One hierarchy
When you are not sure about responsibility try to adopt it.
Provide flexibility
Cons:
May break Single Responsibility principle
Technique:
Make a concrete class and implements both interfaces. The client got an instance of this class by the static factory method.
Let see the solution:
package com.example.codesmell.parallelinheritence;
public class PartialComputerEngineer implements Engineer,MileStone{
private String type;
private int salary;
@Override
public String work() {
return"Build a Billing MicroService";
}
@Override
public String target() {
return"Has to be finshed in 14 PD";
}
@Override
public String getType() {
// TODO Auto-generated method stub
return type;
}
@Override
public void setType(String type) {
this.type=type;
}
@Override
public int getSalary() {
// TODO Auto-generated method stub
return salary;
}
@Override
public void setSalary(int salary) {
this.salary=salary;
}
@Override
public MileStone getMileStone() {
// TODO Auto-generated method stub
return this;
}
@Override
public void setMileStone(MileStone mileStone) {
throw new UnsupportedOperationException("Not Supported");
}
@Override
public String toString() {
return "PartialComputerEngineer [type=" + type + ", salary=" + salary
+ ", work()=" + work() + ", target()=" + target()
+ ", getType()=" + getType() + ", getSalary()=" + getSalary()
+ "]";
}
}
package com.example.codesmell.parallelinheritence;
public class PartialCivilEngineer implements Engineer,MileStone{
private String type;
private int salary;
@Override
public String work() {
return "Create Twin Towers";
}
@Override
public String target() {
return "Has to be completed in 2 years";
}
@Override
public String getType() {
return type;
}
@Override
public void setType(String type) {
this.type=type;
}
@Override
public int getSalary() {
// TODO Auto-generated method stub
return salary;
}
@Override
public void setSalary(int salary) {
this.salary=salary;
}
@Override
public MileStone getMileStone() {
// TODO Auto-generated method stub
return this;
}
@Override
public void setMileStone(MileStone mileStone) {
throw new UnsupportedOperationException("Not Supported");
}
@Override
public String toString() {
return "PartialCivilEngineer [type=" + type + ", salary=" + salary
+ ", work()=" + work() + ", target()=" + target()
+ ", getType()=" + getType() + ", getSalary()=" + getSalary()
+ "]";
}
}
package com.example.codesmell.parallelinheritence;
public class EngineerFactory {
public static Engineer getEngineer(Class clazz) throws InstantiationException, IllegalAccessException
{
return (Engineer) clazz.newInstance();
}
}
package com.example.codesmell.parallelinheritence;
public class Manager {
public static void main(String[] args) throws InstantiationException, IllegalAccessException {
Engineer comp = EngineerFactory.getEngineer(PartialComputerEngineer.class);
comp.setType("Computer Engineer");
comp.setSalary(50000);
Engineer civil = EngineerFactory.getEngineer(PartialCivilEngineer.class);
civil.setType("Computer Engineer");
civil.setSalary(50000);
System.out.println(comp);
System.out.println("********************");
System.out.println(civil);
}
}
Output :
PartialComputerEngineer [type=Computer Engineer, salary=50000, work()=Build a Billing MicroService, target()=Has to be finshed in 14 PD, getType()=Computer Engineer, getSalary()=50000]
********************
PartialCivilEngineer [type=Computer Engineer, salary=50000, work()=Create Twin Towers, target()=Has to be completed in 2 years, getType()=Computer Engineer, getSalary()=50000]
Solution 3:
Collapse a hierarchy.
Pros:
Only maintain One hierarchy
Easy to maintain
Cons:
Break Single Responsibility principle often
Technique:
Make a common interface and move methods from another interface.
Collapse a hierarchy.
Pros:
Only maintain One hierarchy
Easy to maintain
Cons:
Break Single Responsibility principle often
Technique:
Make a common interface and move methods from another interface.
Let see the solution:
package com.example.codesmell.parallelinheritence;
public interface EngineerMileStone {
String getType();
void setType(String type);
int getSalary();
void setSalary(int salary);
public String work();
public String target();
}
package com.example.codesmell.parallelinheritence;
public class RefactorComputerEngineer implements EngineerMileStone{
private String type;
private int salary;
@Override
public String getType() {
return type;
}
@Override
public void setType(String type) {
this.type=type;
}
@Override
public int getSalary() {
return salary;
}
@Override
public void setSalary(int salary) {
this.salary=salary;
}
@Override
public String work() {
return"Build a Billing MicroService";
}
@Override
public String target() {
return"Has to be finshed in 14 PD";
}
@Override
public String toString() {
return "RefactorComputerEngineer [type=" + type + ", salary=" + salary
+ ", getType()=" + getType() + ", getSalary()=" + getSalary()
+ ", work()=" + work() + ", target()=" + target() + "]";
}
}
package com.example.codesmell.parallelinheritence;
public class ReFactorCivilEngineer implements EngineerMileStone{
private String type;
private int salary;
@Override
public String getType() {
return type;
}
@Override
public void setType(String type) {
this.type=type;
}
@Override
public int getSalary() {
return salary;
}
@Override
public void setSalary(int salary) {
this.salary=salary;
}
@Override
public String work() {
return "Create Twin Towers";
}
@Override
public String target() {
return "Has to be completed in 2 years";
}
@Override
public String toString() {
return "ReFactorCivilEngineer [type=" + type + ", salary=" + salary
+ ", getType()=" + getType() + ", getSalary()=" + getSalary()
+ ", work()=" + work() + ", target()=" + target() + "]";
}
}
package com.example.codesmell.parallelinheritence;
public class Manager {
public static void main(String[] args) throws InstantiationException, IllegalAccessException {
EngineerMileStone comp = new RefactorComputerEngineer();
comp.setType("Computer Engineer");
comp.setSalary(50000);
EngineerMileStone civil = new ReFactorCivilEngineer();
civil.setType("Civil Engineer");
civil.setSalary(60000);
System.out.println(comp);
System.out.println("********************");
System.out.println(civil);
}
}
Output
RefactorComputerEngineer [type=Computer Engineer, salary=50000, getType()=Computer Engineer, getSalary()=50000, work()=Build a Billing MicroService, target()=Has to be finshed in 14 PD]
********************
ReFactorCivilEngineer [type=Civil Engineer, salary=60000, getType()=Civil Engineer, getSalary()=60000, work()=Create Twin Towers, target()=Has to be completed in 2 years]
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