Linked List need help

[shivang sarawagi]

// A simple mailing list example.
import java.util.*;
class Address 
{
  private String name;  
  private String street;  
  private String city;
  private String state;
  private String code;
  Address(String n, String s, String c,String st, String cd) 
  {
    name = n;
    street = s;
    city = c;
    state = st;
    code = cd;
  }
  public String toString() 
  {
    return name + "\n" + street + "\n" + city + " " + state + " " + code;
  }
}
class MailList 
{
  public static void main(String args[]) 
  {
    LinkedList<Address> ml = new LinkedList<Address>();
    // Add elements to the linked list.
    ml.add(new Address("J.W. West", "11 Oak Ave","Urbana", "IL", "61801"));
    ml.add(new Address("Ralph Baker", "1142 Maple Lane","Mahomet", "IL", "61853"));
    ml.add(new Address("Tom Carlton", "867 Elm St","Champaign", "IL", "61820"));
    // Display the mailing list.
    for(Address element : ml)
    {
      System.out.println(element + "\n");  Printing Linked List of a class in another class
    }
    System.out.println();
  }
}

In the program above i have two doubts

A. What is the use of toString() method in Address class ??
B. Why can't i directly put object m1 in print statement in the MailList class ?? The program is not compiling by doing that, error is not able to identify variable m1.

Somebody please reply

[John Jai]

The toString() method is called when you intend to print an object. Since you have overridden the toString() in the Address class it prints nicely the name, street, city.. etc with line breaks.

This method will be called when you print the Address object directly or the List where you have added one or many Address objects.

But you have to provide the List reference correctly I fear your Address List is named ml and you try to print m1

[shivang sarawagi]

Thanks for the reply John, ya its was my fault putting ml as m1.

i have one more doubt earlier for an instance in the program given below there was no need of overriding toString() method
for displaying linked list. Directly the linked list object is given in the print statement. But for the prior code without overriding
toString() method,

"[Address@108c21a, Address@1ff6599, Address@676175]"

this is the output. Its not working. As far as i think the prior program should show the correct output without overriding
the toString() method.

// Demonstrate LinkedList.
import java.util.*;
class LinkedListDemo {
public static void main(String args[]) {
// Create a linked list.
LinkedList<String> ll = new LinkedList<String>();
// Add elements to the linked list.
ll.add("F");
ll.add("B");
ll.add("D");
ll.add("E");
ll.add("C");
ll.addLast("Z");
ll.addFirst("A");
ll.add(1, "A2");
System.out.println("Original contents of ll: " + ll);
// Remove elements from the linked list.
ll.remove("F");	
ll.remove(2);
System.out.println("Contents of ll after deletion: "
+ ll);

[John Jai]

Without overriding the toString() method in the Address class, the toString() method in the Object class gets called.

Read here what prints when the toString() method in Object class is called - http://download.oracle.com/javase/6/docs/api/java/lang/Object.html#toString()

Without overriding the toString() method, you cannot make your custom output (the address representation) printed.

[shivang sarawagi]

but John how come the output is fine in this code without overriding the toString() method, sorry for asking so many questions but i need to understand the concept

// Demonstrate LinkedList.  
import java.util.*;  
class LinkedListDemo {  
public static void main(String args[]) {  
// Create a linked list.  
LinkedList<String> ll = new LinkedList<String>();  
// Add elements to the linked list.  
ll.add("F");  
ll.add("B");  
ll.add("D");  
ll.add("E");  
ll.add("C");  
ll.addLast("Z");  
ll.addFirst("A");  
ll.add(1, "A2");  
System.out.println("Original contents of ll: " + ll);  
// Remove elements from the linked list.  
ll.remove("F");   
ll.remove(2);  
System.out.println("Contents of ll after deletion: "  
+ ll);  

This is the output which i am getting


Original contents of ll: [A, A2, F, B, D, E, C, Z]
Contents of ll after deletion: [A, A2, D, E, C, Z]

[John Jai]

shivang sarawagi wrote:without overriding the toString() method,

The Address class printed nice output since it overrides the toString() method. So check what is the class the List is holding... A String class?? So to make the String class print its content nicely what should have been done?? Any guess ?

sorry for asking so many questions but i need to understand the concept

You are always welcome to ask questions here

[Campbell Ritchie]

shivang sarawagi wrote: . . . without overriding
toString() method,

"[Address@108c21a, Address@1ff6599, Address@676175]"

this is the output. Its not working. . . .

Read the details of Object#toString() and you find that is normal behaviour. There is nothing wrong with it; only, as the link will tell you, you ought to override toString the way you want it to work, in all your classes.

[Campbell Ritchie]

shivang sarawagi wrote: . . . the output is fine in this code without overriding the toString() method, . . .

You are adding Strings. The String class does override the toString() method already. It is probably very simple, like this:

@Override public String toString() {
    return this;
}

[shivang sarawagi]

I got it. Thanks John & Campbell for your prompt replies

[Campbell Ritchie]

Well done