Java - declaring from Interface type instead of Class

In my quest to correctly grasp Interface best practices, I have noticed declarations such as:

List<String> myList = new ArrayList<String>();

instead of

ArrayList<String> myList = new ArrayList<String>();

-To my understanding the reason is because it allows flexibility in case one day you do not want to implement an ArrayList but maybe another type of list.

With this logic, I set up an example:

public class InterfaceTest {

    public static void main(String[] args) {

        PetInterface p = new Cat();
        p.talk();

    }

}

interface PetInterface {                

    public void talk();

}

class Dog implements PetInterface {

    @Override
    public void talk() {
        System.out.println("Bark!");
    }

}

class Cat implements PetInterface {

    @Override
    public void talk() {
        System.out.println("Meow!");
    }

    public void batheSelf() {
        System.out.println("Cat bathing");
    }

}

My question is, I cannot access the batheSelf() method because it only exists for Cat. That leads me to believe that I should only declare from an Interface if I am only going to use methods declared in the Interface (and not extra methods from the subclass), otherwise I should declare from the Class directly (in this case Cat). Am I correct in this assumption?

When there is a choice between referring to an object by their interface or a class, the former should be preferred, but only if an appropriate type exists.

Consider StringimplementsCharSequence as an example. You should not just blindly use CharSequence in preferrence to String for all cases, because that would deny you simple operations like trim(), toUpperCase(), etc.

However, a method that takes a String only to care about its sequence of char values should use CharSequence instead, because that is the appropriate type in this case. This is in fact the case with replace(CharSequence target, CharSequence replacement) in the String class.

Another example is java.util.regex.Pattern and its Matcher matcher(CharSequence) method. This lets a Matcher be created from Pattern for not just String, but also for all other CharSequence there are out there.

A great example in the library of where an interface should've been used, but unfortunately wasn't, can also be found in Matcher: its appendReplacement and appendTail methods accept only StringBuffer. This class has largely been replaced by its faster cousin StringBuilder since 1.5.

A StringBuilder is not a StringBuffer, so we can not use the former with the append… methods in Matcher. However, both of them implementsAppendable (also introduced in 1.5). Ideally Matcher's append… method should accept any Appendable, and we would then be able to use StringBuilder, as well as all other Appendable available!

So we can see how when an appropriate type exists referring to objects by their interfaces can be a powerful abstraction, but only if those types exist. If the type does not exist, then you may consider defining one of your own if it makes sense. In this Cat example, you may define interface SelfBathable, for example. Then instead of referring to a Cat, you can accept any SelfBathable object (e.g. a Parakeet)

If it does not make sense to create a new type, then by all means you can refer to it by its class.

See also

• Effective Java 2nd Edition, Item 52: Refer to objects by their interfaces

  If appropriate interface types exist, then parameters, return values, and fields should all be declared using interface types. If you get into the habit of using interface types, your program will be much more flexible. It is entirely appropriate to refer to an object by a class if no appropriate interface exists.

Related links

• Bug ID: 5066679 - java.util.regex.Matcher should make more use of Appendable

Yes, you are correct. You should declare as the most general type providing the methods you use.

This is the concept of polymorphism.

Your are correct, but you can cast from the interface to the desired pet if you need. For example:

PetInterface p = new Cat();
((Cat)p).batheSelf();

Of course if you try to cast your pet to a dog you cannot call the batheSelf() method. It would not even compile. So, to avoid problems, you could have a method like this:

public void bathe(PetInterface p){
    if (p instanceof Cat) {
        Cat c = (Cat) p;
        c.batheSelf();
    }
}

When using instanceof, you make sure you will not try to make a dog bathe himself during runtime. Which would throw an error.