Question

I have seen that if I have interface named interfaceABC.

Example:

public class ABController extends AbstractCOntroller {


private interfaceABC inter;

I am confused that why we make object from interface not from class that implemented it.

Answer 1

private interfaceABC inter;

i am confused that why we make object from interface not from class that implemented it

We haven’t created an object/instance yet. We simply declared a variable to hold it. We don’t make objects from interfaces (you have to use a concrete class to do that), but we will often use interface types instead of the actual concrete class for variable declarations, method parameter types, and method return types.

Take this for exmaple:

List<Example> examples = new ArrayList<Example>();
...
public List<Example> getExamples() { return examples; }

Using the interface List here instead of the concrete class ArrayList follows a common best practice: to use interfaces instead of concrete classes whenever possible, e.g. in variable declarations, parameters types, and method return types. The reason this is considered a best practice is:

1. Using the interface for declarations and for return types hides an implementation detail, making it easier to modify in the future. For example, we may find that the code works better using a LinkedList rather than ArrayList. We can easily make this change in one place now, just where the list is instantiated. This practice is especially key for method parameter types and method return types, so that external users of the class won’t see this implementation detail of your class and are free to change it without affecting their code.

2. By using the interface, it may be clearer to a future maintainer that this class needs some kind of List, but it does not specifically need an ArrayList. If this class relied on some ArrayList-specific property, i.e. it needs to use an ArrayList method, than using ArrayList<Example> examples = ... instead of List<Example> examples = ... may be a hint that this code relies on something specific to an ArrayList.

3. It may simplify testing/mocking to use the more abstract List than to use the concrete class ArrayList.