Programming to an interface

[Thomas Hauck]

For OCA Java SE 7, Mala Gupta does not mention "Programming to an Interface" in her book.
Edward Finegan mentions the following points on page 312 of his book.

Programming to an interface is the concept that

1. Code should interact based on a defined set of functionality instead of an explicitly defined object type.
2. It is better for the public interfaces of objects to use data types that are defined as interfaces as opposed to a particular class when possible.
3. When an object is implementing an interface, it is declaring that it has a certain set of functionalities.
4. Many different classes can implement the same interface and provide its functionality.
5. When a method uses an interface as its argument type, it allows any object, regardless of its type, to be used as long as it implements the interface.
6. Programming to an interface is also known as "design by contract."
7. Programming to an interface allows the developer to specify the capabilities or behaviors that are expected, instead of strictly defining an expected object type.

Example of Programming to an interface:

/*
For the method feed within the class Species,  it accepts an argument of type Animal.
Any class that implements the Animal interface can be passed as an argument.
*/

interface Animal {
	void eat();
}
/* The class Species accepts any Class that implements Animal */
class Species{ 
	void feed(Animal a){ 
		a.eat();
	}
} 

class Mammal implements Animal {
	public void eat() {
		System.out.println("Mammal eats");
	}
}


public class Human extends Mammal{

	public static void main(String[] args) {
		Mammal m1 = new Human();
		Animal a1 = new Mammal();
		Species s1 = new Species();
		s1.feed(m1);
		s1.feed(a1);
	

	}
	public void eat() {
		System.out.println("Human eats");
	}
}
/* Program output 

Human eats
Mammal eats
*/

Question: Besides the points listed for "Programming to an Interface",
what additional items should one be aware of with respect to this topic?

Any additional examples of "Programming to an Interface" would be appreciated?

[Roel De Nijs]

Programming to an interface, not an implementation is a good design practice!

Here you'll find some more info about this topic.

[Thomas Hauck]

One of the techniques used to program to an interface
is to write a method that contains a parameter of type interface.

Question:
If a method contains a parameter that is of an interface type,
is this technique automatically considered "programming to an interface?"

interface LivingThing {
	void breath();
}
class Species{
	void breath(LivingThing liv){
		liv.breath();
	}
}

class Plant implements LivingThing{
	public void breath() {
		System.out.println("Plant breaths");		
	}	
}
class Animal implements LivingThing{
	public void breath() {
		System.out.println("Animal breaths");		
	}
}
public class Test {
	public static void main(String[] args) {
		Plant p1 = new Plant();
		Animal a1 = new Animal();
		Species s1 = new Species();
		s1.breath(p1);
		s1.breath(a1);
	}
}
/* Program output 
Plant breaths
Animal breaths
*/

[Roel De Nijs]

Thomas Hauck wrote:If a method contains a parameter that is of an interface type, is this technique automatically considered "programming to an interface?"

Of course!