Constructor Understanding

[Michael Grünau]

Hello everyone,

back again on another project and with another questions.

Project includes a PhoneBook with different classes which I need to access.
The solution which should be implemented should work according to the code below.

Entry[] entries = new Entry[5];
entries[0] = new Entry("Agathe", "Ackermann", "555-11133322");
entries[1] = new Entry("Bernd", "Ackermann", "555-2251243");
entries[2] = new Entry("Winfried", "Becker", "555-225123");
entries[3] = new Entry("Alex", "Dieß", "555-343112");
entries[4] = new Entry("Beatrix", "Dieß",  "555-2123123");
PhoneBook phoneBook = new PhoneBook(entries);
// Hier sollte true ausgegeben werden
System.out.println(entries[0].getPhoneNumber().equals(phoneBook.find("Agathe", "Ackermann")));

Don't really understand the last PhoneBook line that's why am struggling at the moment...

So what's the question, I implemented a Class Entry and now want to have a class PhoneBook which contains an array entries.
The Array entries contains objects from the class entry (firstName, lastName etc.)

I know want to write a Constructor according to the code above that accepts arrays from Entry and initializes the Object.
How can I write the Constructor so he takes the array from Entry and puts it in the Phone Book?

Something like:

Entry [] entries; 

public PhoneBook()
{
  // Some Code that enters the new Arrays from entry to entries in general?
}

Thanks for any help!

[Campbell Ritchie]

You appear to have two separate questions.

If you are returning a phone number, is that going to be equal to a contact?

Then you are asking about writing a constructor. What do you know about constructors already? What does your book say about constructors? Search for defensive copies; there is a section about that in Effective Java by Joshua Bloch.

[Michael Grünau]

Well, I already wrote the Search Method for the lower Part,  my problem is that I don't really understand the Code from above.

What does the following line mean?

		PhoneBook phoneBook = new PhoneBook(entries);

That I have a some sort of PhoneBook from the beginning and that I creat a new one called "phoneBook" based on the PhoneBook Method/ Class and my Array Entries?

I can implement something like

	Entry[] entries;
	
	PhoneBook(Entry[] entries)
	{

	}

Where Entry stands for my Class Entry which should hand-over some Entries in a Array called "entries".
But what is the use of PhoneBook here?

[Campbell Ritchie]

Sorry, I was saying contact when I should have said entry.
Your phone book object contains an array of entries; it can be used to search the entries, add new entries, etc. At this point you will probably find that an array isn't as good a data structure as we thought at first, and classes from the Collections Framework would be much easier to use. But you can change the implementation as long as the field in line 1 has private access without anybody knowing anything about it. You can still pass an Entry[] as an argument to the constructor even if the implementation doesn't use an array.

[Michael Grünau]

Well , I have a another project including the same topic with three different classes and now it does not work again.

First of all I have my Animal Class

public class Animal extends MiniJava {
	private String name;
	private int foodCosts;
	
	Animal(String name, int foodCosts)
	{
		this.name = name;
		this.foodCosts = foodCosts;
	}
	
	//Getter
	public int getFoodCosts ()
	{
		return foodCosts;
	}
	
	public String getName()
	{
		return name;
	}
	
	//Methode 
	public String toString() {
		return "(name: " + name + ", foodCosts: " + foodCosts +")";
	}	
}

Afterwards the Vivarium Class

public class Vivarium extends MiniJava {
	private int area;
	private int constructionYear;
	Animal[] inhabitants;
	
	Vivarium(Animal[] inhabitants, int area, int constructionYear)
	{
		this.area = area;
		this.constructionYear = constructionYear;
		this.inhabitants = inhabitants;
	}
	
	//Methoden 
	public String toString() {
		return "[area: " + area + ", constructionYear: " + constructionYear + " animals: " + inhabitants +"]";
	}
	
	public int getCosts()
	{
		int fixCosts = 900 + (area * 100) + area * (2019 - constructionYear) * 5;
		int totalfoodCosts = 0;
		
		for (int i = 0; i < inhabitants.length; i++)
		{
			totalfoodCosts = totalfoodCosts + inhabitants[i].getFoodCosts();
		}
		
		return fixCosts + totalfoodCosts;
	}
}

And finaly a Zoo Class

public class Zoo extends MiniJava {
	Vivarium[] vivaria;
	
	Zoo(Vivarium[] vivaria)
	{
		
	}
	
	public String toString()
	{
		return "{" + vivaria + "}";
	}
	
	//Methode
	
	public int getCosts()
	{
		int ZooCosts = 0;
		
		for (int i = 0; i < vivaria.length; i++)
		{
			ZooCosts = ZooCosts + vivaria[i].getCosts();
		}
		
		return ZooCosts;	
	}
}

Why does my code work for Vivarium but when I want to use the same style for my Zoo like:
Vivarium[] vivaria; it shows me that Vivarium cannot be resolved to a Type?

[Knute Snortum]

When you're asking about a error message, please include the entire error message.  It may contain clues that will help us.

What does your project's directory layout look like?  Can Zoo access Vivarium?  You may need an include statement to tell Zoo where Vivarium is.

[Michael Grünau]

Thanks for the quick reply.

The Error Message is only "Vivarium" cannot be resolved to a type"

The idea behind the classes is basically that you creat an animal which gets listed in an array for Vivarium where you have informations about the vivarium and than the class Zoo contains a array containing all Vivarium with their animals.
I thought the Arrays can simply be moved by something like:

Animals [] a Name;

and

Vivarium [] a Name;

but it is only working inside the Vivarium Class and not longer in Zoo...
Why is that?

[Knute Snortum]

I wrote:What does your project's directory layout look like?  Can Zoo access Vivarium?  You may need an include statement to tell Zoo where Vivarium is.

[Michael Grünau]

Sorry but I don't really understand what you mean/ meant...

Zoo needs to access Vivarium to get the created Vivarium and calculate the costs for each Vivaria to Sum up the total costs of the Zoo.

[Knute Snortum]

Are Zoo and Vivarium in different directories?  How are you running the project?  From a command line?  From an IDE?

[Campbell Ritchie]

What's a MiniJava? Can you really say that an Animal IS‑A MiniJava? Can you really say that a Zoo IS‑A MiniJava? Can you really say that a Vivarium IS‑A MiniJava?
Unless you can confidently answer YES to my three questions, are you right to use the extends keyword for those three classes? Or is MiniJava some sort of general type, rather like java.lang.Object?