Error: cannot find symbol

[silvia]

Please i need help.

Code:

class Node
{

public Object data;
public Node next;

public Node()
{
this.data = null;
this.next = null;
}

public Node(Object a)
{
this.data = a;
this.next = null;
}

public Node(Object a, Node b)
{
this.data = a;
this.next = b;
}

}

Code:

public class List
{
private Node head;
private Node tail;
public List()
{
this.head = null;
this.tail = null;
}
public void insertAtFront(Object obj)
{
if (this.size()==0)
{
Node n = new Node(obj);
this.head = n;
this.tail = n;
}
else
{
Node n = new Node(obj);
n.next = this.head;
this.head = n;
}
}
public void insertAtBack(Object obj)
{
if (this.size()==0)
{
Node n = new Node(obj);
this.head = n;
this.tail = n;
}
else
{
Node n = new Node(obj);
this.tail.next = n;
this.tail = n;
}
}
public Object removeFromFront()
{
if (this.size()==0)
{
System.out.println("No data to remove.");
return null;
}
else if (this.size()==1)
{
Object a = this.head.data;
this.head = null;
this.tail = null;
return a;
}
else
{
Object a = this.head.data;
this.head = this.head.next;
return a;
}
}

Code:

public void insertAtSorted(String s)
{
if (this.size()==0)
{
insertAtFront(s);
return;
}
else if (s<=this.head)// stuck on these type of statements
{
insertAtFront(s);
return;
}
else if (s>=this.tail) // how to make so this works? <----------
{
insertAtBack(s);
return;
}
else
{
Node ntlnode = this.head;
while (ntlnode.next <= s)
{
ntlnode = ntlnode.next;
Node blah = new Node(s);
blah.next = ntlnode;
this.next = blah;
}
}
}

Code:

public Object removeFromBack()
{
if (this.size()==0)
{
System.out.println("No data to remove.");
return null;
}
else if (this.size()==1)
{
Object a = this.tail.data;
this.head = null;
this.tail = null;
return a;
}
else
{
Object a = this.tail.data;
Node ntlnode = this.head;
while (ntlnode.next != this.tail)
{
ntlnode = ntlnode.next;
}
this.tail = ntlnode;
this.tail.next = null;
return a;
}
}
public boolean isEmpty()
{
return this.head == null;
}
public void print()
{
if (this.size()==0)
{
System.out.println("Empty list!");
return;
}
Node curr = this.head;
while (curr != null)
{
System.out.println(curr.data.toString());
curr = curr.next; //move forward to the next data item.
}
System.out.println();
}
public int size()
{
int count = 0;
Node curr = this.head;
while (curr != null)
{
count++;
curr = curr.next;
}
return count;
}
}

The error is:

Code:

List.java:100: operator <= cannot be applied to java.lang.String,java.lang.Object
else if (s<=this.head.data) //not sure about this format of object<---------------
^
List.java:105: operator >= cannot be applied to java.lang.String,java.lang.Object
else if (s>=this.tail.data)
^
List.java:113: operator <= cannot be applied to Node,java.lang.String
while (ntlnode.next <= s)
^
List.java:118: cannot find symbol
symbol : variable next
location: class List
this.next = blah;
^
4 errors

Thanks

[barney]

Code:

java.lang.String,java.lang.Object
else if (s<=this.head.data) //not sure about this format of object<---------------

Ok, I'm rusty on my Java but I'm pretty sure you can't use the operators on strings. As well, when comparing two things, they must be of the same type. So we can't compare a string to an object. So let's try something else...

//This converts the Object data to a string and then compares it to s

Code:

elseif(s.compareTo(this.head.data) <= 0)

//This does the same as the above to my knowledge, but explicitly casts the Object to a string

Code:

elseif(s.compareTo((String)this.head.data) <= 0)

Thus, for the other one, change head to tail and <= to >=
Greetings.