Are you using the default method of the interface properly?

Introduction

The default method of the interface was introduced in Java 8, and from Java 9, private methods can also be defined in the interface. It's a useful feature, but misuse can reduce the maintainability of your application. So what kind of usage is desirable?

Bad example

public interface BadUsage {

    void someMethod();

    // (1)Utility-like things for mixins and implementation inheritance
    default String normalize(String str) {
        return str.toUpperCase().trim();
    }

    // (2)Processing that depends on other objects
    default int getSomeValue() {
        return Singleton.getInstance().getValue();
    }
}

(1) of the above sample code uses the default method for the purpose of adding functions as a mixin to the class that implements the interface, or providing utility functions that can be commonly used on the implementation class side. I will. Transfer rather than inheritance is an object-oriented rule of thumb. It is best to avoid this usage and use utility classes and helper classes. (2) is similar to (1) in terms of usage, but what is more problematic is that it describes processing that depends on other objects. In this example, the singleton object is accessed, but the following processing can be mentioned as an example.

• IO such as files and networks
• Access to threads
• Access to DI container (such as Spring ApplicationContext)

If you write such a process, unit testing will be very difficult. The interface should not depend on the concrete class in the first place.

How to identify bad usage

The default method of the interface is public, which is an operation that is exposed to the user like an abstract method. So the default method should also be part of the responsibility of the interface. In object-oriented, object is a collection of data and behavior, so the instance method that represents the behavior should give access to the object's data (instance variables).

Of course, the interface itself doesn't have instance variables, so the default method indirectly accesses the instance variables of the implementation class by calling other abstract methods. Conversely, you should question the default method, which does not call other abstract methods.

Correct usage example

Suppose you have the following interface: In the interface of the Composite pattern, there are Composite and Leaf in the concrete class.

Component.java

public interface Component {

    String getName();

    int getPoint();

    List<Component> getChildren();

    boolean isLeaf();

}

Let's say you want to add a find method to this interface that returns a list of components that meet your criteria. Since this method can be described as a common process that does not depend on the concrete class, implement it as the default method.

Component.java

    default List<Component> find(Predicate<Component> p) {
        return getChildren().stream().filter(p).collect(Collectors.toList());
    }

Thus, regardless of what the concrete class really is, the default method should implement common behavior that would be universal if the Component interface was implemented correctly.

Exceptional case

There are exceptional cases where it is acceptable (not to call other abstract methods). That's the case where you only need an interface instead of creating an abstract base class that provides a default implementation. Let's take a concrete example. Suppose you want to introduce a Visitor pattern that scans the composite structure.

Visitor.java

public interface Visitor {

    void visit(Composite composite);

    void visit(Leaf leaf);
}

Component.java

public interface Component {
    // ...
    void accept(Visitor visitor);
}

The following is an example of a concrete Visitor implementation class.

LeafCounterVisitor.java

public class LeafCounterVisitor implements Visitor {

    private int count = 0;

    @Override
    public void visit(Composite composite) {
        //I'm not interested in Composite nodes, so an empty implementation
    }

    @Override
    public void visit(Leaf leaf) {
        count ++;
    }

    public int getCount() {
        return count;
    }
}

Visits to nodes that are not of interest to the visitor are empty implementations as described above. There are only two types of nodes this time, but if there are ten, overriding all visit methods and putting in an empty implementation can be a daunting task. In that case, there is a way to prepare an abstract base class with an empty implementation prepared in advance and inherit this class as shown below. (In derived classes, you only need to override the visit method of interest)

BaseVisitor.java

public abstract class BaseVisitor implements Visitor {

    @Override
    public void visit(Composite composite) {
        //Empty implementation
    }

    @Override
    public void visit(Leaf leaf) {
        //Empty implementation
    }
}

The interface's default method eliminates the need for the above abstract base class.

Visitor.java(Modified version)

public interface Visitor {

    default void visit(Composite composite) {}

    default void visit(Leaf leaf) {}
}

I think this usage does not deviate from the original intention of the default method in the sense that it provides a default implementation.

Summary

The default method of the interface should be treated like any other abstract method and used like an object-oriented design, except that it has a default implementation.