Up until now, we had a look at the Single Responsibility, Open/Closed, Liskov Substitution, and Interface Segregation principles.

Dependency Inversion is one of the last principles we are going to look at. The principle states that:

1. High-level modules should not depend on low-level modules. Both should depend on abstractions.

2. Abstractions should not depend on details. Details should depend on abstractions.

Let’s get started with some code that violates that principle.

Say you are working as part of a software team. We need to implement a project. For now, the software team consists of:

A BackEnd Developer

package com.gkatzioura.solid.di;

public class BackEndDeveloper {

    public void writeJava() {
    }
}

And a FrontEnd developer:

package com.gkatzioura.solid.di;

public class FrontEndDeveloper {

    public void writeJavascript() {
    }

}

And our project uses both throughout the development process:

package com.gkatzioura.solid.di;

public class Project {

    private BackEndDeveloper backEndDeveloper = new BackEndDeveloper();
    private FrontEndDeveloper frontEndDeveloper = new FrontEndDeveloper();

    public void implement() {

        backEndDeveloper.writeJava();
        frontEndDeveloper.writeJavascript();
    }

}

So as we can see, the Project class is a high-level module, and it depends on low-level modules such as BackEndDeveloper and FrontEndDeveloper. We are actually violating the first part of the dependency inversion principle.

Also, by inspecting the implement function of Project.class, we realize that the methods writeJava and writeJavascript are methods bound to the corresponding classes. Regarding the project scope, those are details since, in both cases, they are forms of development. Thus, the second part of the dependency inversion principle is violated.

In order to tackle this problem, we shall implement an interface called the Developer interface:

package com.gkatzioura.solid.di;

public interface Developer {

    void develop();
}

Therefore, we introduce an abstraction.

The BackEndDeveloper shall be refactored to:

package com.gkatzioura.solid.di;

public class BackEndDeveloper implements Developer {

    @Override
    public void develop() {
        writeJava();
    }

    private void writeJava() {
    }

}

And the FrontEndDeveloper shall be refactored to:

package com.gkatzioura.solid.di;

public class FrontEndDeveloper implements Developer {

    @Override
    public void develop() {
        writeJavascript();
    }

    public void writeJavascript() {
    }

}

The next step, in order to tackle the violation of the first part, would be to refactor the Project class so that it will not depend on the FrontEndDeveloper and the BackendDeveloper classes.

package com.gkatzioura.solid.di;

import java.util.List;

public class Project {

    private List<Developer> developers;

    public Project(List<Developer> developers) {

        this.developers = developers;
    }

    public void implement() {

        developers.forEach(d->d.develop());
    }

}

The outcome is that the Project class does not depend on lower level modules, but rather abstractions. Also, low-level modules and their details depend on abstractions.

You can find the source code on GitHub.