Question on collections and specifically using the comparable interface

[Kaiser Nahom]

Currently going through an exercise on collections. I need some help answering because I've studied how the Comparable and Comparator interfaces work, but am struggling to see what I need to do. The question is as following:

Given below is a class for street addresses in a town that is meant to allow sorting.

public class Address implements Comparable<Address> {
public String street; // street name
public Integer number; // house number
public Address(String street, Integer number) {
this.street = street; this.number = number;
}

public int compareTo(Address other) {
int streetCheck = this.street.compareTo(other.street);
if (streetCheck == 0)
return this.number.compareTo(other.number);
else
return streetCheck;
}

public boolean equals(Address other) {
return this.street.equals(other.street)
&& this.number.equals(other.number);
}
}


Define a class called MailingList for storing street addresses and names for a group
of residents. It should have the structure:


public class MailingList {
private SortedMap<Address, String> map;
public MailingList(ArrayList<Address> addresses,
ArrayList<String> residents) {
[b]... // to be filled in[/b]
}
public void print() {
[b]... // to be filled in[/b]
}
}

The constructor should create a map and add all the given street addresses along
with the corresponding names of residents. The print method should print the
street addresses and resident names of all the people in the mailing list in the
ascending order of street address.

[Campbell Ritchie]

Welcome to the Ranch

Were you given that code exactly as you wrote it? That is pretty dreadful style; the lack of indentation will mean you can easily get lost, and the lack of {} in lines 11‑13 can cause more confusion. The old Sun Style Guide will tell you there is a better way to write that sort of code anyway. And public fields (lines 2‑3) is poor design.
Anyway, you have code which will work, however unreliably, and you know all about Comparable<T> and Comparator<T>? Remind yourself about object ordering in the Java™ Tutorials. If you click the “previous” link, you will find it tells you about Maps. That shou‍ld tell you all you need to know about adding things to Maps. Remind yourself about how a SortedMap works, and you are going to find what I sometimes say, that these things are actually easier to implement than they look at first sight.
I presume the details of the Map mean that the “K”s represent addresses and the “V”s the names of the occupants.

[Campbell Ritchie]

Kaiser Nahom wrote:. . .public MailingList(ArrayList<Address> addresses, ArrayList<String> residents) . . .

Are you really given parallel Lists? That is another bit of poor design.

[Kaiser Nahom]

Thanks Rob. I've attached an image of the problem just so it's easier on the eye, but yes lines 11 to 13 really do miss out any swirly brackets.
I have looked at most of the stuff in the collections interface, sortedmap included as well as Comparator and Comparable. I think I understand them, that comparable is used to define a way of naturally ordering a set, whilst the comparator allows us to implement natural ordering for the basic classes (string and int and that) but I learned about it in a slightly different context and using a different print method.

I went through a video tutorial on comparable and natural ordering here https://www.youtube.com/watch?v=OlMw3ecL1ow

this is the code from the video, with my own commenting (feel free to point out any misunderstandings)

import java.util.ArrayList;
import java.util.Collection;
import java.util.Collections;
import java.util.List;
import java.util.SortedSet;
import java.util.TreeSet;


/*
 * Learning about how we can establish our own way of naturally sorting items in a collection.
 */

class Individual implements Comparable<Individual>{ //we pass into comparable the thing that we want to compare our class to, in this case, other objects of the same class
	private String name;
	
	public Individual (String name){
		this.name = name;
	}
	
	public String toString(){
		return name;
	}
	
	// need to define hashcode and equals methods to add objects to a set

	 /*
	  * compareTo method works the same as compare does in the Comparator interface
	  * The object, which we can access with this. , is gonna be compared with another object of the same type.
	  * If the object is here^^ is less than person, we wanna return -1, greater than, we wanna return +1 and zero if they're equal
	  */
	@Override
	public int compareTo(Individual guy) {
		// TODO Auto-generated method stub
		int len1 = name.length();
		int len2 = guy.name.length(); //the one being passed in
		
		if(len1>len2){ // if the length of the string is longer, sort later
			return 1;
		}
		else if(len1<len2){ //if the length of the string is shorter, sort earlier
			return -1;
		}
		else {
			return name.compareTo(guy.name);
		}
	}

	@Override
	public int hashCode() { //returns an ID that is different if the names are different
		final int prime = 31;
		int result = 1;
		result = prime * result + ((name == null) ? 0 : name.hashCode());
		return result;
	}

	@Override
	public boolean equals(Object obj) { //equals will only be true if the names are equal
		if (this == obj)
			return true;
		if (obj == null)
			return false;
		if (getClass() != obj.getClass())
			return false;
		Individual other = (Individual) obj;
		if (name == null) {
			if (other.name != null)
				return false;
		} else if (!name.equals(other.name))
			return false;
		return true;
	}
}
public class OurComparable {
	
	public static void main (String [] args){
		List<Individual> list = new ArrayList<Individual>(); //creating arraylist called list, which is full of Individuals
		SortedSet<Individual> set = new TreeSet<Individual>(); //creating array TreeSet called set, which is full of Individuals 
		 
		addElements(list);
		addElements(set);
		
		Collections.sort(list);
		
		showElements(list);
		System.out.println();
		showElements(set);
	} 
	
	// we want to add the same elements to both of the collections, list and set, so we define this method
	// this method will add a bunch of Individuals to whatever collection we pass in
	
	/*
	 * if we want to pass a list or set to a method, without knowing if it's a list or a set beforehand,
	 * they both implement the collection interface, which is a super interface of list and set.
	 * the collection interface is pretty limited but we can use it to add and iterate through the collection we want.
	 */
	
	private static void addElements(Collection<Individual> col) { //col is a parameter of the type collection (parameterised with Individual here)
		col.add(new Individual("Joe"));
		col.add(new Individual("Sue"));
		col.add(new Individual("Juliet"));
		col.add(new Individual("Clare"));
		col.add(new Individual("Mike"));
	}
	
	//method to iterate through the items in the collection and display them using a print method
	private static void showElements(Collection<Individual> col) { 
		for (Individual element: col){
			System.out.println(element);
		}
	}

}

[Campbell Ritchie]

Kaiser Nahom wrote:Thanks Rob. . . .

Who's Rob?

I went through a video tutorial on comparable and natural ordering here . . .

Quite a good tutorial, but there is one bad bit of style and two potential errors in the code.
How far have you got with your assignment, which is probably very easy to complete?

[Kaiser Nahom]

Ahem, sorry, my bad Campbell.
I understand from the description of the exercise what needs to be done, however I'm still unsure of what actually needs to be done.
It states that:

The constructor should create a map and add all the given street addresses along
with the corresponding names of residents. The print method should print the
street addresses and resident names of all the people in the mailing list in the
ascending order of street address.

However I don't know what this would look like.
Then in the print method, I'm confused as to why there is nothing being passed into the method. So much confusion.

[Kaiser Nahom]

*unsure of how to exactly implement a constructor in this setting, and use this sort of method to print

[Campbell Ritchie]

One thing at a time. Start by creating a constructor at all. Then we can add more to it later.

[Kaiser Nahom]

Objects of the MailingList class are gonna be instantiated with addresses of the class Address(that has been already made) and residents of the class String.
If I had to say then I would think it would be

this.map = <adresses,residents>;

[Campbell Ritchie]

That syntax looks as if you had been guessing. Don't guess; the compiler won't like it, for a start.
How would you usually create a new object and assign it to a field? Hint: I would call the left half of that statement correct, providing the map field has already been declared.

[Kaiser Nahom]

We've already specified what type of object we want to pass into the map, <Address>, and a map consists of a key and value pair, which is why I put the addresses as the key, and the residents as the value. But we've only specified that they have they are of the type Object and would need to clarify that they come in an ArrayList. So I would think it would be

public class MailingList {
private SortedMap<Address, String> map;
public MailingList(ArrayList<Address> addresses,
ArrayList<String> residents) {
this.map = ArrayList addresses, ArrayList residents;
}

but if not then it would be this (which I don't think it is, since Address is the type of object being passed in)

public class MailingList {
private SortedMap<Address, String> map;
public MailingList(ArrayList<Address> addresses,
ArrayList<String> residents) {
this.map = <Address> addresses, <Address>residents;
}

[Kaiser Nahom]

Oh wait, we have to apply the same logic to the left hand side then too?

public class MailingList {
private SortedMap<Address, String> map;
public MailingList(ArrayList<Address> addresses,
ArrayList<String> residents) {
this.SortedMap map = ArrayList addresses, ArrayList residents;
}

[Campbell Ritchie]

No.

All that new code (line 5 now) is wrong. It won't compile. You are still guessing. At the moment it is what is to the right of the = sign that is wrong.
Start by formatting every line of your code correctly so you can actually read it. If I have difficulty with it, you having less experience, will have even more difficulty reading what you have written. Go back to the original code from your first post. Change it bit by bit. Go back to your original lecture notes about how to create an object. Remind yourself about Maps.

[Kaiser Nahom]

Oh sorry, I'm not sure if I mentioned, but this was from a written question in a previous paper, so I cant't change any code..

public class MailingList {
private SortedMap<Address, String> map;
public MailingList(ArrayList<Address> addresses,
ArrayList<String> residents) {
this.map = <Address> addresses, <Address>residents; //apart from inserting in this line
}
public void print() {
... // and this line over here
}
}

could you please send me what the code should look like? I can then go through the code myself? It's how I learn best, because I prefer going through established examples, if I have any difficulties then I will ask.

[Campbell Ritchie]

I thought that from reading the screenshot you posted. You must use the code provided, even though some of it is not very good, but you are not filling in the gaps correctly.
You appear to have forgotten what you were taught about how to create an object. You have already declared the map. Please go to the SortedMap documentation and remind yourself what things there might be in an implementing class that allow you to create an object.

We don't hand out complete solutions.