Try #2 -- With Interfaces

Failure Point

Recall: "Object" does not understand "getWeight".

We simply need something that does understand "getWeight". That would solve this problem.

As coders, the nice thing is when we need a new type to do something, we can simply code it up a new class. Interfaces give us yet another approach to add new types.

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

Weighable Interface

Here is the code for the "Weighable" interface. Please note the similarity to a "class" except:

The word "interface" is used instead of "class"
Method code (body) is not present (just the method header).

It is like a shell of a class. It is nice and lightweight.

public interface Weighable {
	public int getWeight();
}

Elephant2

We simply add "implements Weighable" to the class header
We must have a method with a method header matching all methods in the interface (which we do)

Now:

Elephant2 "is-a" Weighable
Elephant2 understands the method "getTotalWeight"

We do similar for PopcornMachine2 as shown here...

#1public class Elephant2 implements Weighable {
	private int weight;

	public Elephant2(int aWeight) {
		this.weight = aWeight;
	}

	#2public int getWeight() {
		return weight;
	}
}

Zoo2

Only three tweaks here. We change our ivar "objects" to use the "Weighable" type. And then we change "Object" to "Weighable" in the code as needed (two places shown).

public class Zoo2 {
	private String name;
	private List<Weighable> objects;

	public Zoo2(String aName) {
		this.name = aName;
		this.objects = new ArrayList<>();
	}

	public void add(Weighable aObject) {
		this.objects.add(aObject);
	}

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

	public String toString() {
		return String.format("%s -- Total Weight of Objects = %d",
								this.name, getTotalWeight());
	}

}

Bug Check

Please recall that we previously were bitten by a bug when we sent "getWeight". The bug is gone! 😊 Celebrate good times.

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

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

The specialized type "Weighable" does understand "getWeight". 😊

The change we made was very small as we'll see in the next diagram