Try #2 -- With Interfaces

Failure Point

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

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

As coders, the nice thing is when we need a new class to do something, we can simply code it up.

public void printFullReport(PrintStream report)  {
	/*Compile error, "printOn not understood by Object
	for (Object each: this.list)
		each.printOn(report);		//<---------- compile error here
	*/
	report.println("College: " + this.name);
	report.println("Apologies, printing of list is not yet supported");
	report.println("Objects do not understand \"printOn\"");
}

public void printFullReport(PrintStream report)  {
	/*Compile error, "printOn not understood by Object
	for (Object each: this.list)
		each.printOn(report);		//<---------- compile error here
	*/
	report.println("College: " + this.name);
	report.println("Apologies, printing of list is not yet supported");
	report.println("Objects do not understand \"printOn\"");
}

public void printFullReport(PrintStream stream)  {
	//printOn is understood by Printable dudes, so we're good to go
	stream.println("College: " + this.name);
	stream.println("I have a list of entities of size: " + this.list.size());
	stream.printf("%nHere is my list:%n%n");
	for (Printable each: this.list)
		each.printOn(stream);
}

Printable Interface

Here is the code for the "Printable" 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's simple and we like simple.

public interface Printable {
	public void printOn(PrintStream report);
}

Classroom2

We simply add "implements Printable" to the class header
We must have a method with a method header matching that in the interface.

Now:

Classroom2 "is-a" Printable
Classroom2 understands the method "printOn"

We do similar for Professor2 as shown here...

#1public class Classroom2 implements Printable {
	private int roomNumber;

	public Classroom2(int aRoomNumber) {
		this.roomNumber = aRoomNumber;
	}

	public int getRoomNumber() {
		return roomNumber;
	}

	#2public void printOn(PrintStream report) {
		report.println("I am Classroom: " + getRoomNumber());
	}
}

College2

Now back to class "College2".

This is the method where the bug bit us in try #1.
It now works! 😊 The method "printOn" is understood by Printable.

#1public void printFullReport(PrintStream stream)  {
	//printOn is understood by Printable dudes, so we're good to go
	stream.println("College: " + this.name);
	stream.println("I have a list of entities of size: " + this.list.size());
	stream.printf("%nHere is my list:%n%n");
	for (Printable each: this.list)
		#2each.printOn(stream);
}