07-20-2023, 10:57 AM
The advantage is [Dependency Injection (DI)][1]. It means outsourcing the task of object creation.Let me explain with an example.
public interface Lunch
{
public void eat();
}
public class Buffet implements Lunch
{
public void eat()
{
// Eat as much as you can
}
}
public class Plated implements Lunch
{
public void eat()
{
// Eat a limited portion
}
}
Now in my code I have a class LunchDecide as follows:
public class LunchDecide {
private Lunch todaysLunch;
public LunchDecide(){
this.todaysLunch = new Buffet(); // choose Buffet -> eat as much as you want
//this.todaysLunch = new Plated(); // choose Plated -> eat a limited portion
}
}
In the above class, depending on our mood, we pick Buffet() or Plated(). However this system is tightly coupled. Every time we need a different type of Object, we need to change the code. In this case, commenting out a line ! Imagine there are 50 different classes used by 50 different people. It would be a hell of a mess. In this case, we need to Decouple the system. Let's rewrite the LunchDecide class.
public class LunchDecide {
private Lunch todaysLunch;
public LunchDecide(Lunch todaysLunch){
this.todaysLunch = todaysLunch
}
}
Notice that instead of creating an object using new keyword we passed the reference to an object of Lunch Type as a parameter to our constructor. Here, object creation is outsourced. This code can be wired either using Xml config file (legacy) or Java Annotations (modern). Either way, the decision on which Type of object would be created would be done there during runtime. An object would be injected by Xml into our code - Our Code is dependent on Xml for that job. Hence, Dependency Injection (DI).
DI not only helps in making our system loosely coupled, it simplifies writing of Unit tests since it allows dependencies to be mocked. Last but not the least, DI streamlines Aspect Oriented Programming (AOP) which leads to further decoupling and increase of modularity.
Also note that above DI is Constructor Injection. DI can be done by Setter Injection as well - same plain old setter method from encapsulation.
[1]:
public interface Lunch
{
public void eat();
}
public class Buffet implements Lunch
{
public void eat()
{
// Eat as much as you can
}
}
public class Plated implements Lunch
{
public void eat()
{
// Eat a limited portion
}
}
Now in my code I have a class LunchDecide as follows:
public class LunchDecide {
private Lunch todaysLunch;
public LunchDecide(){
this.todaysLunch = new Buffet(); // choose Buffet -> eat as much as you want
//this.todaysLunch = new Plated(); // choose Plated -> eat a limited portion
}
}
In the above class, depending on our mood, we pick Buffet() or Plated(). However this system is tightly coupled. Every time we need a different type of Object, we need to change the code. In this case, commenting out a line ! Imagine there are 50 different classes used by 50 different people. It would be a hell of a mess. In this case, we need to Decouple the system. Let's rewrite the LunchDecide class.
public class LunchDecide {
private Lunch todaysLunch;
public LunchDecide(Lunch todaysLunch){
this.todaysLunch = todaysLunch
}
}
Notice that instead of creating an object using new keyword we passed the reference to an object of Lunch Type as a parameter to our constructor. Here, object creation is outsourced. This code can be wired either using Xml config file (legacy) or Java Annotations (modern). Either way, the decision on which Type of object would be created would be done there during runtime. An object would be injected by Xml into our code - Our Code is dependent on Xml for that job. Hence, Dependency Injection (DI).
DI not only helps in making our system loosely coupled, it simplifies writing of Unit tests since it allows dependencies to be mocked. Last but not the least, DI streamlines Aspect Oriented Programming (AOP) which leads to further decoupling and increase of modularity.
Also note that above DI is Constructor Injection. DI can be done by Setter Injection as well - same plain old setter method from encapsulation.
[1]:
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