Try #1 -- Without Interfaces

Elephant1

A minimal model of a lovely elephant
For brevity, it just has one ivar

#1public class Elephant1 {
	#2private int weight;

	public Elephant1(int aWeight) {
		this.weight = aWeight;
	}

	public int getWeight() {
		return weight;
	}
}

PopcornMachine1

A minimal model of a popcorn machine
For brevity, it just has one ivar

#1public class PopcornMachine1 {
	#2private int weight;

	public PopcornMachine1(int aWeight) {
		this.weight = aWeight;
	}

	public int getWeight() {
		return weight;
	}
}

Zoo1

Our zoo class.

Class header
List of objects at the zoo (elephants, desks, popcorn machines and many more types)

#1public class Zoo1 {
	private String name;
	#2private List<Object> objects;

Zoo1 Method 'getTotalWeight'

Our main software requirement is to compute the total weight of our objects. So we've coded a method "getTotalWeight".

Oops, a compiler error! 😕 Type "Object" does not understand message "getWeight".

public int getTotalWeight()  {
	int totalWeight = 0;
	for (Object eachObject: this.objects)
		totalWeight += eachObject.getWeight();
	return totalWeight;
}

Zoo is sending our specialized message "getWeight"
The receiver of the message is type "Object"

Type "Object" is very general and does not understand "getWeight" (in fact, it only knows a few general methods like "toString" and "equals") 😕

We need a specialized type that understands "getWeight" 👍🏽

Are you ready for Try 2?