Btukfyl

switch(menuChoice) {



case 1: 

    System.out.println("Enter your contact's first name:n");

    String fname = scnr.next();

    System.out.println("Enter your contact's last name:n");

    String lname = scnr.next();

    Necronomicon.addContact(new Person(fname, lname));

    break;



// main truncated here for readability

import java.util.ArrayList;



public class AddressBook {



   ArrayList<Person> ArrayOfContacts= new ArrayList<Person>();



   public void addContact(Person p) {

      ArrayOfContacts.add(p);



 /* 

    for(int i = 0;  i < ArrayOfContacts.size(); i++) {

       if(ArrayOfContacts.get(i).getID() != p.getID())

           ArrayOfContacts.add(p);



    else 

       System.out.println("Sorry this contact already exists.");

    }       

  */



    }

}

public class Person {



   private String fName = null;

   private String lName = null;

   private static int ID = 1000;



   public Person(String fName, String lName) {       // Constructor I'm using to try and increment the ID each time a Person object is created starting at 1001.



     this.fName = fName;

     this.lName = lName;

     ID = ID + 1;

   }

}

I am trying to create an address book whereby each contact has a first name, last name and a unique ID.

My question is how to I prevent a user from entering in duplicate contacts with the same first and last name? Should I implement some kind of check in the addContact method or right in main? How?

Here is the code for keeping out duplicate ID's.

public void addContact(Person p) {



    for(int i = 0;  i < ArrayOfContacts.size(); i++) {

        Person contact = ArrayOfContacts.get(i);

        if(contact.getID() == p.getID()) {

            System.out.println("Sorry this contact already exists.");

            return; // the id exists, so we exit the method. 

        }

    }



    // Otherwise... you've checked all the elements, and have not found a duplicate

    ArrayOfContacts.add(p);



}

If you would like to change this code to keep out duplicate names, then do something like this

public void addContact(Person p) {

    String pName = p.getFname() + p.getLname();

    for(int i = 0;  i < ArrayOfContacts.size(); i++) {

        Person contact = ArrayOfContacts.get(i);

        String contactName =  contact.getFname() + contact.getLname(); 

        if(contactName.equals(pName)) {

            System.out.println("Sorry this contact already exists.");

            return; // the name exists, so we exit the method. 

        }

    }



    // Otherwise... you've checked all the elements, and have not found a duplicate

    ArrayOfContacts.add(p);



}

You could simply use HashSet and avoid any kind of loops to test it. HashSet is in charge of this function.

import java.util.Set;

import java.util.HashSet;



public class AddressBook {



   Set<Person> listOfContacts = new HashSet<>();



   public void addContact(Person p) {

      if (!listOfContacts.add(p))

         System.out.println("Sorry this contact already exists.");    

   }

}            

To increment ID property you should have 2 properties, 1 static and another one, and increment it in constructor. Look:

public class Person {

   private final int ID;

   private static int id = 1000;

   private String fName;

   private String lName;



   public Person(String fName, String lName) {       // Constructor I'm using to try and increment the ID each time a Person object is created starting at 1001.

      this.ID= ++id;

      this.fName = fName;

      this.lName = lName;   

   } 

To make the HashSet not accept duplicate objects you should set which properties should not be duplicated in the class (Person in your case). Follows an example:

   @Override

   public int hashCode() {

      int hash = 7;

      hash = 61 * hash + Objects.hashCode(this.fName);

      hash = 61 * hash + Objects.hashCode(this.lName);

      return hash;

   }



   @Override

   public boolean equals(Object obj) {

      if (obj == null || getClass() != obj.getClass())

         return false;



      final Person other = (Person) obj;

      if (!Objects.equals(this.fName, other.fName))

         return false;



      return Objects.equals(this.lName, other.lName);

   }

}

By the way, you can generate equals and hashCode methods using your IDE (Eclipse, NetBeans, etc.)

EDIT

Since you aren't able to use HashSet, I will show a version with ArrayList. By the way, you have to use HashCode and equals as I said to make it work well

import java.util.List;

import java.util.ArrayList;



public class AddressBook {



   List<Person> arrayOfContacts = new ArrayList<>();



   public void addContact(Person p) {

      if (listOfContacts.contains(p))

         System.out.println("Sorry this contact already exists.");



      else

         arrayOfContacts.add(p);    

   }

}

Use a HashSet. For every user's name, add it to the HashSet.

For example:

HashSet<String> namesUsed = new HashSet<String>();



if (namesUsed.contains(userName)) {

    //do what you want here, if this is entered it means there is a duplicate

} else {

    namesUsed.add(userName); //add it to the list of used names

}

You should Override equals inside your Person class

public boolean equals(Object obj) {

    if(fName.equals(obj.getfName()) && lName.equals(obj.getlName)) {

        return true;

    }

    return false;

}

Then just call:

if(!(person1.equals(person2))) {

//not a duplicate

}

And of course substitute the variables/objects with whatever objects you want.
You also should add getters and setters for last and first name.
Hope this helps!