How to declare compareTo() method and invoke it from the main method?

[Mahi Rahman]

Hi there!
I am trying to find out what would be correct syntex for declaring a compareTo() method and invoking it from the main program. I went through books but I didn't find anything related to compare two integers (but I found for String comparison). I also found suggestion for using interface but I'm not sure how to implement that. Can anyone please help me?

Mahi.

   
public class State {
private String name;
public int numberOfInhabitants;
private String currencyName;
//Part a
public State(String name, int numberOfInhabitants, 
                     String currencyName){
this.name = name;
this.numberOfInhabitants = numberOfInhabitants;
this.currencyName = currencyName;
}

//Part b compareName
public boolean compareName(State newName){
return (this.name == newName.name);
}
 
//Part c compareInhabitants

        public int compareTo(State s){
return numberOfInhabitants.compareTo(s.numberOfInhabitants);
/*Here I get an error.............
         What shoud be the correct syntex for returning an integer value
         of zero, less than zero or more than zero? */
        }
 

}
// The main method
public class TestState {
 
public static void main(String[] args) {
State London = new State("LONDON", 6000000, "GBP");
State Berlin = new State("BERLIN", 2700000, "EUR");
State Stockholm = new State("STOCKHOLM", 2300000, "SKR");
State Oslo = new State("OSLO", 1680000, "NKR");
State Helsinki = new State("HELSINKI", 2700000, "EUR");
//Comparing inhabitants 
System.out.println("London Inhabitants = Helsinki Inhabitants: ");
        System.out.println("Berlin Inhabitants = Helsinki Inhabitants: ");
        System.out.println("Oslo Inhabitants = Helsinki Inhabitants: ");
/*What would be the correct syntex to invoke the compareTo method and check if this integer are same.*/
}

[Garrett Rowe]

First off, if you are going to write a compareTo() method in your class, then you may as well declare to the world that instances of your class can now be used wherever a Comparable<State> is expected. You do that by declaring that your class implements Comparable<State>

public class State implements Comparable<State>
{
  //...
}

Now, as far as implementing a useful compareTo() method for your class, primitives (int, double, etc.) have no methods. You can not do something like this:

//This is not legal Java code
int valueOne = 1;
int valueTwo = 2;
int comparison = valueOne.compreTo(valueTwo); //<- Syntax error, int cannot be dereferenced

However the (<, >, <=, >=, ==) operators are defined for ints so you can do this.

int valueOne = 1;
int valueTwo = 2;
boolean valueOneIsGreater = valueOne > valueTwo;
System.out.println(valueOneIsGreater);

so in your compareTo() method, when comparing primitives you should do something similar.

public int compareTo(State other)
{
   if (numberOfInhabitants > other.numberOfInhabitants)
   {
      //.. do something
   }
   
   else
   {
      //.. do something else
   }
}

[ February 18, 2007: Message edited by: Garrett Rowe ]

[Mahi Rahman]

Hei Garrett!
Thanks a lot for your reply. I now understand what was the problem. Cheers!
Mahi.