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Java Java Data Structures - Retired Organizing Data Interfaces

Derek Derek
Derek Derek
8,744 Points
Posted by Derek Derek
Derek Derek
8,744 Points

Help with overriding compareTo()

Hello,

I am trying to override compareTo() by lastNames (String). However, for some reason, I can't get it to work and keep getting NullPointException at line 15 of Information.java, which I think is caused by the compareTo() method in Contact.java.

Below are the files, Contact.java and Information.java respectively.

I would really appreciate if you could help me fix compareTo() in Contact.java. It is identical to Craig's, different only in variable names.

Thank you in advance!

public class Contact implements Comparable {

  private String mFirstName;
  private String mLastName;

  public Contact (String firstName, String lastName) {
    mFirstName = mFirstName;
    mLastName = mLastName;
  }

  public String getFirstName() {
    return mFirstName;
  }
  public String getLastName() {
    return mLastName;
  }

  public String getContact() {
    return mFirstName + " " + mLastName;
  }

  @Override
  public String toString() {
    return "["+getContact()+"]";
  }


  @Override
  public int compareTo(Object obj) {
    Contact other = (Contact) obj;
    if (equals(other)) {
      return 0;
    }
    int compareLastNames = mLastName.compareTo(other.mLastName);
    if (compareLastNames == 0) {
      return mLastName.compareTo(other.mLastName);
    }
    return compareLastNames;
  }



}

import java.util.Arrays;

public class Information {
    public static void main(String[] args) {
        Contact contact = new Contact("First1", "Last1");
        Contact secondContact = new Contact("First2", "Last2");

        System.out.println("This is name1: " + contact);
        System.out.println("This is name2: " + secondContact);

        Contact[] storage = {contact, secondContact};
        System.out.println("This is the original storage: " + storage);


        Arrays.sort(storage);

        for (Contact name : storage) {
            System.out.println(name);
        }

    }
}

2 Answers

Kourosh Raeen
Kourosh Raeen
23,733 Points
Kourosh Raeen
Kourosh Raeen
23,733 Points

You're getting the error because you're not setting the member fields correctly in the Contact constructor. It should look like this:

public Contact (String firstName, String lastName) {
    mFirstName = firstName;
    mLastName = lastName;
}
Derek Derek
Derek Derek
8,744 Points
Derek Derek
Derek Derek
8,744 Points

Wow, I can't believe I made that mistake and did catch it. Thank you!

coskun olcucu
coskun olcucu
5,340 Points
coskun olcucu
coskun olcucu
5,340 Points

what I see in your compareTo method that after equlas method ,you are supposed to compare fileds first name and last name,however,in your code you repeat comparing to first names.