Thread: Sorted list and compareTo method

Hey, so I need to write a program that simulates a phonebook. The program should contain the options to lookup a person,delete a person,insert a person, and too check the size(amount of persons inside the phonebook) now we're given a class Person, and an interface Phonebook. We're susposed to implement the interface in our class and use the methods insert,delete,lookup in order to do these tasks. This is what I've got so far;

 
class PhoneBookList implements PhoneBook {
	private int size = 0;
	Node head = null;
 
	Person lookup(String name) {
		Node p = head;
		int pos = 0;
 
		while( p!=null) {
			int comparison = p.compareTo(name);
 
		if(comparison == 0) 
			return name;
		else Out.println("Person does not exist");
		    return -1;
		}
	}
 
	boolean insert(Person person) {
		Node p = head;
		Node prev = null;
 
		while( p!=null) {
 
		Node n = new Node(person);
		n.next = p;
 
		if(prev == null) {
			head = n;
		}else{
			prev.next = n;
	}
		size++;
}
 
	boolean delete(String name) {
		Node p = head;
		Node prev = null;
 
		while( p!=null) {
			prev = p;
			p = p.next;
			int comparison = p.compareTo(name);
 
		if(comparison == 0) {
			prev.next = p.next;
			return true;
		}else {
			Out.println("The person does not exist");
			return false;
		}
		size--;
	}
}	
	int size() {
		Out.println(size);
	}
 
	static final class Node {
		final Person person;
		Node next;
 
		Node(Person person) {
			this.person = person;
 
		}
	}
 
}
}

Now I'm not sure if I am doing this right, we are also suspossed to use this compareTo method to check if the person does exist,if yes ok if not give out an error.The same analogy applies for the methods delete and insert. Am I using these methods right and if not what am I doing wrong? Thanks!

Aight I'll try that, thanks!

Okay, so after trying for 2 days straight I'm quite lost in this program.I cannot get it to work and I need some assistance. I put comments on parts of the code that I'm not sure how to implement and what the idea behind the code is.

 
class PhoneBookList implements PhoneBook {
	private int size = 0;
	Node head = null;
 
	private static final class Node {
		final Person person;
		Node next;
 
		Node(Person person) {
			this.person = person;
		}
	}
 
	public Person lookup(String name) {
		Node p = head, prev = null;
 
		while(p!=null) {
		//Not really sure how to implement this method, idea is to compare the first element p with an string that the user puts in.
			int comparison = p.compareTo(name);
 
		  if(comparison == 0){
			return //Not sure what my return value is,it is susposed to be a person with the correspoding name,how do I execute that?
		  }
		    else if(comparison != 0) {
		      prev = p;
			  p = p.next;
			}
		   //Idea behind this is to keep moving on in the list,if the first element isnt the one we looked for.
		     else {
				 return null;
				 //If nothing found return null
		      }
 
	public boolean insert(Person person) {
		 Node p = head, prev = null;
 
		 while(p!=null) {
		   int comparison = p.compareTo(person);
 
		   if(comparison != 0) {
			   return true;
			   size++;
		   }
		   //This person doesnt exist, return true and increase the size by one.
		  else  if(comparison == 0) {
		      prev = p;
		      p = p.next;
		  }
 
			  //The same idea as with lookup, maybe it isn't the first element keep looking.
			  else {
				  return false;
				//Nothing found,return false.
		 }
 
		 Node n = new Node(person);
 
		 if (prev == null){
			 head = n;
		 }else{ 
		    prev.next = n;
            n.next = p;
		 }
 
 }
	public boolean delete(String name) {
		Node p = head, prev = null; 
 
		while(!p=null) {
			int comparison = p.compareTo(name);
 
			if(comparison == 0) {
				return true;
				size--;
			}
			//This person exists,we found it and we are going to delete it.
			  else if(comparison != 0) {
			   prev = p;
		       p = p.next;
			}
 
			  else {
				  return false;
			  }
 
		     if (prev == null) {
				 head = p.next; 
			} 
			  else {
				  prev.next = p.next;
			}
		}
	}

Maybe some help? An example on how to implement one of these methods or some guidance would be really nice! Thanks.

"I think the description of that code should be: compare the name of the person in the Node object with the string entered by the user."

Yes that is exactly what i want the compare to method to do. How would I implement that, we haven't covered this compareTo method in class so I'm kinda going of my intuition per say.PhoneBookList.java:23: error: illegal start of expression
}
^
PhoneBookList.java:34: error: illegal start of expression
public boolean insert(Person person) {
^
PhoneBookList.java:66: error: illegal start of expression
public boolean delete(String name) {
^
3 errors

These are the errors I am getting when i add the last method, which just to print out the size of the phone book.

Aight I'll do that

--- Update ---

Okay after some trying I've decided to take the program step by step, in order to make it easier.The logical way implies to start with the method insert and I've ran into an issue here. The method is boolean so it needs to return values true or false. In which part of the code do I return the values. After what checks?

 
class PhoneBookList implements PhoneBook {
	private int size = 0;
	Node head = null;
 
	private static final class Node {
		final Person person;
		Node next;
 
		Node(Person person) {
			this.person = person;
		}
	}
 
	public boolean insert(Person person) {
		Node p = head, prev = null; 
 
		while(p!=null) {
			int comparison = person.compareTo(person);
 
		  if(comparison != 0) {
			return true;
			size++;
		 }
		    if(comparison == 0) {
			prev = p;
            p = p.next;
		  }
	  }
 
	Node n = new Node(person);
	  if (prev == null){
		  head = n;
	  }
      else{
		  prev.next = n;
          n.next = p;
	  }
	}
}

Now I think the part where you need to return the true value is quite obvious. After checking that the person doesn't exist return true and increase the size by 1. But where should I implement the return false method? Thanks.

Yes if the entry that we are trying to insert alredy exists the code should return false.Insert is susposed to create a new Person object(insert it in the list). The person contains a name,adress and phone number. On the part with that valuetoReturn. Can you show me an example of what you mean? Maybe just a super short and basic code (few lines) so that I can visualise the concept. Thanks buddy!

Hmmm okay gimme a few minutes to try to implement that.

--- Update ---

Aight I've implemented your suggestion like this;

 
class PhoneBookList implements PhoneBook {
	private int size = 0;
	Node head = null;
 
	private static final class Node {
		final Person person;
		Node next;
 
		Node(Person person) {
			this.person = person;
		}
	}
 
	public boolean insert(Person person) {
		Node p = head, prev = null; 
		boolean value = true;
 
		while(p!=null) {
			int comparison = person.name.compareTo(person.name);
 
		  if(comparison != 0) {
			size++;
		 }
		    if(comparison == 0) {
			prev = p;
            p = p.next;
		    value = false;
		  }
	  }
 
	Node n = new Node(person);
	  if (prev == null){
		  head = n;
	  }
      else{
		  prev.next = n;
          n.next = p;
	  }
	  return value;
	}
}

The only error I'm getting is that I have not implemented all of the methods from the interface PhoneBook, which is true,sooo it should be working?

Well in the method insert no; That is if I've done everything and that includes; checking if a person with that name exists, if yes inserting that person if not returning false. And What do you mean empty methods. How does that look like?

--- Update ---

Yes I've ajusted that, I've realised that as well

PhoneBookList.java:19: error: cannot find symbol
int comparison = p.name.compareTo(person.name);
^
symbol: variable name
location: variable p of type Node
1 error

This is the error I'm getting.

The variable name is defined in an class Person. Sure i'll use something a bit more descriptive.

So how would I use this method to tell it compare the person in the list with the one the user inputs?

Hmmmm okay I'll give that a shot

--- Update ---

Okay so I've come up with something

 
class PhoneBookList implements PhoneBook {
	private int size = 0;
	Node head;
 
	private static final class Node { 
		final Person person;
		Node next;
 
		Node(Person person) {
			this.person = person;
		}
	}
 
	public boolean insert(Person person) {
		Node n = new Node(person); //die Person die sagen wir vor der Tür steht.
		Node p = head,prev = null;// den Head auf p zuweisen.
 
 
		if(p == null) {//Niemand ist in dem raum
		    head = n;//also soll die neu Person die erste sein die rein geht.
			return true;
		}
 
		int comparison = p.name.compareTo(n.name);//den Namen von der ersten person mit dem Namen der neuen person vergleichen.
 
		if(comparison < 0) {
			n.next = p;
			p = n;
			size++;// den wert size um 1 zu erhohen, brauche ich dass überhaupt?
			return true;
		}
 
		if(comparison == 0) {
			return false;
 
		}
 
		Node current = p;
		while(current.next !=null && current.next.person < person) {
			current = current.next;
		}
		if(current.next !=null && current.next.person == person) {
			return false;
 
		}
 
		n.next = current.next;
        current.next = n;
        return true;
	}

Only thing left is to correctly implement the compareTo method. I will try what you suggested (ignore the comments its in german) but I think I've implemented all of the other parts of the method

Is there a way to implement that method without chaning the Person class?