Java object constructor returning "null"

Question

New to Java! I have created a class that has constructors for various fields. However, when I try and print the fields, I recieve "null" as the output. Please help me understand why. Here is the class:`

 public class User {
   //All relevant information to identify each user.
      private String FirstName;
      private String LastName;
      private String Email;
      private long PhoneNum;
      private long CardNum;

   //Build Constructors for User info.

public User(String Fname, String Lname, String Mail, long num, long card)
{
   Fname=FirstName;
    Lname=LastName;
    Mail=Email;
    num=PhoneNum;
    card=CardNum;
}

  //Method to set FirstName.  
 public void setFirstName (String Fname) 
{
    FirstName=Fname;
}
//Method to get FirstName.
public String getFirstName()
{
    return FirstName;
}
 //Method to set LastName. 
public void setLastName (String Lname)
{
    LastName=Lname;
}


 //Method to get Lastname. 
    public String getLastname()
    {
        return LastName;
    }
    //Method to set email.
    public void setEmail (String Mail)
    {
        Email=Mail;
    }
     //Method to get email.
    public String getEmail()
    {
        return Email;
    }
    //Method to set phonenumber.
    public void setPhoneNum(long num)
    {
        PhoneNum=num;
    }
   //Method to get phonenumber.
    public long getPhoneNum()
    {
        return PhoneNum;
    }
   //Method to set credit card number.
        public void setCardNum(long card)
    `enter code here`{
        CardNum=card;
    }
     // Method to get credit card number.
         public long getCardNum()
    {
        return CardNum;
    }

Now when I run this code, I receive "null":

public class UserDemo {
public static void main(String[] args)
{
    String first="Matt";
    String last="Burns";
    String email="[email protected]";
   long phone=333;
   long card=222;
    User Matt=new User(first,last,email,phone,card);
    System.out.print(Matt.getLastname());
}

What am I doing wrong? Thank you in advance!

`

Pay attention to the compiler warning messages. You should have gotten lots of "variable may be uninitialized" warnings. — Jim Garrison
– Jim Garrison, Commented May 6, 2016 at 3:58

Titus · Accepted Answer · 2016-05-06 03:49:37Z

1

Is the other way around, instead of:

public User(String Fname, String Lname, String Mail, long num, long card){
    Fname=FirstName;
    Lname=LastName;
    Mail=Email;
    num=PhoneNum;
    card=CardNum;
}

it should be:

public User(String Fname, String Lname, String Mail, long num, long card){
    FirstName = Fname;
    LastName = Lname;
    Email = Mail;
    PhoneNum = num;
    CardNum = card;
}

To avoid this kind of thing you can use the this keyword. Here is an example:

String firstName;
public Constructor(String firstName){
    this.firstName = firstName;
}

answered May 6, 2016 at 3:49

Titus

22.6k1 gold badge26 silver badges36 bronze badges

Sign up to request clarification or add additional context in comments.

Comments

Michael Markidis · Accepted Answer · 2016-05-06 03:51:07Z

0

You have your constructor assignments the wrong way:

public User(String Fname, String Lname, String Mail, long num, long card)
{
    // this refers to the current object being constructed
    this.FirstName = Fname;
    this.LastName = Lname;
    this.Email = Mail;
    this.PhoneNum = num;
    this.CardNum = card;
}

answered May 6, 2016 at 3:51

Michael Markidis

4,1911 gold badge16 silver badges21 bronze badges

Comments

Pranav A. · Accepted Answer · 2017-02-03 13:33:29Z

0

Java constructor objects should be created like so:

public User(String Fname, String Lname)
{
    this.firstName = Fname; 
    //keyword 'this' refers to the new object, so the new object's Fname would equal the firstName place below
    this.lastName = Lname; //same as above
}

To create a new User, use this code:

User Matt = new("Matt", "Burns"); //Creates a new User Matt, with firstName "Matt" and lastName "Burns"
System.out.println(Matt.getfirstName()); //prints User Matt's firstName to the console.

edited Feb 3, 2017 at 13:33

answered Feb 3, 2017 at 2:47

Pranav A.

4601 gold badge5 silver badges18 bronze badges

1 Comment

xlm Over a year ago

This solves the user's problem but it would be useful if you could explain why/what they did wrong too.

Collectives™ on Stack Overflow

Java object constructor returning "null"

3 Answers 3

Comments

Comments

1 Comment

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

3 Answers 3

Comments

Comments

1 Comment

Your Answer

Sign up or log in

Post as a guest

Related