避免代码重复的良好做法 Java

Question

I have an specification interface lets call it Spec and two classes that implement it lets call them ImpA and ImpB . It worked great until I realized that few methods from ImpA and impB share the same implementation. I have tried to avoid it by declaring methods in Spec with default and implementing them right in it, but I'm sure that there is a better way to do it. Maybe some design pattern?

Answer 1

If the code that's duplicated doesn't use the state of the class (the instance variables), putting the code in a default method in the interface is one solution.

For more complex situations where you need to access class instance variables, you can create an additional abstract class with the common code. Then ImpA and ImpB would extend the abstract class.

Collections in Java use a bit of both. For example, ArrayList and LinkedList extend AbstractList , which implements List , which contains a few default methods.

Answer 2

While you could make them default methods as you've proposed, another potential solution is to use the Delegation design pattern. This means having a fourth class that is entirely for the purpose of separating the common logic. If that logic needs values to operate then they should be injected into the delegate to allow it to operate (and be tested) independently.

Personally, I find code that prefers delegation over composition to be more elegant, more testable and much more 'refactorable'. But, like many other design questions, there are many varied and valid opinions on this.

interface Spec { 
    void method();
}

class Imp1 implements Spec {
    private final Delegate delegate;

    public Imp1() {
        this.delegate = new Delegate(...);
    }

    public void method() {
        delegate.setContext(...);
        delegate.method();
    }
}

Answer 3

You can make Spec a superclass and have ImpA and ImpB the subclasses. You can find more information on the JavaDoc