Interface and implements

Hello guys ,

I am new to java bassically I used to C++ and C#

I stuck with a little problem here :

I have to make a new interface class Comparable
to make a Student class that imlements comparable
and to make an alghorithm that will recieve an array of Comparale (s) (that will be a students ) and sort them


Comparable need to have only one function

int compareto

can someone please help me how do I make this interface
and student classes that the alghoritm will sort students ??

Comparable is an existing interface. Look it up.

Yes i know , But I was told to build it myself
(I'am learning at the university, and this is what they want )
I googled some hours to find its source but I found not :(

actually the comparable I have to write takes 2 arguments ..

pleasse can you make a little example how can i make it myself ?

What Is an Interface? (The Java™ Tutorials > Learning the Java Language > Object-Oriented Programming Concepts)

Sounds pretty straight forward, Make an interface that only has one method. An example would be something similar to this

Code:

---

public interface Shape {
       
        public void draw(Graphics g);

}

---

The one i tried to do is
public interface Comparable
{
// int compareTo(Student student1 , Student student2);
int comareTo(Comparable student1,Comparable student2);
}
public class Student implements Comparable , Printable {

private int id;
private String name;

Student ()//constructor
{
int tempid;
String tempname;

Scanner scan = new Scanner(System.in);
System.out.println("Please enter ID");
tempid = scan.nextInt();
System.out.println("Enter Name");
tempname = scan.nextLine();
setId(tempid);
setName(tempname);
}
public void setId(int id) {
this.id = id;
}
public int getId() {
return id;
}
public void setName(String name) {
this.name = name;
}
public String getName() {
return name;
}
//@Override
public int comareTo(Comparable student1, Comparable student2) {
return (student1).getName().compareTo((student2).getName( )) ;
}
}




but it doesn't work for me :(

I guess i missing something (I don't get how do I connect student to be comparable)

hey guys please I really need a little help here .. just point me to the problem

Hi,

First of all: remember to add [code][/code] tags around source code. By doing that the code stays formatted and does not left allign.

Second, I think there are some design flaws in your code.

1. Your comparable interface should be usable for many, if not any, classes. Using a specific class in the compareTo method might not be the best thing to do.
2. Do you need 2 arguments in the compareTo method? Inside the implementation you compare two students but you do nothing with the student data of the object self.
3. Are you sure you want to ask for the data of a student in its constructor? Consider this: what happens if the student class is used in a GUI application? Does it still work then?

Good luck,
Erik

Oh Ok sorry for code /code

1- yes and i don't understand how to do that

2) yes i need 2 ... i know it is ridicolous but that what they want

Code:

---

comparable1.compareTo(comparable1,comparable2);

---

3- no we work in console only it is our first java homework so ... I know a much about it .

This is your first java homework? And they immediately start with interfaces... strange (IMHO).

Never the less,

1. Do you know the root of all Java classes? I suggest you use that class in the method of your interface.
2. I really cannot believe they want that! I should stick with student1.compareTo(student2).
3. Okay, so you are only using the console. But even then it is not a very good idea to put the interaction in the constructor. Why don't you create a constructor with two arguments: name and id. Create a new class StudentApp for example that contains the main entry, reads the data and creates the Student objects?

Good luck,
I'm off to bed so I won't be of any help today :(

They want two arguments because that way you can't simply copy the existing Comparable interface. It's not much, but I suspect that's the idea. There's no point telling the OP to do something different to the requirements in a uni project. The best they can do is question it with the tutor.

Anyway, to the OP. What errors are you getting? You haven't actually told us.
I think I can see them, but I want you to actually tell us.

at student class I have a problem with ;

Code:

---


  public int comareTo(Comparable student1, Comparable student2) {
        return (student1).getName().compareTo((student2).getName( )) ;

---

It says comparable doesn't have a function getname()


The question is how do I make Student to be Comparable ?

Quote:

---

Originally Posted by Bored2

at student class I have a problem with ;

Code:

---


  public int comareTo(Comparable student1, Comparable student2) {
        return (student1).getName().compareTo((student2).getName( )) ;

---

It says comparable doesn't have a function getname()


The question is how do I make Student to be Comparable ?

---

Aren't you confusing the Comparable<T> interface (one argument compareTo( ... ) method) with the Comparator<T> interface (two arguments compareTo( ... ) method)?

Normally the Comparable<T> interface is implemented by classes that need to compare objects with object of the same class while the Comparator<T> interface is implemented by a class that needs to compare classes (objects) of possibly another type.

kind regards,

Jos

No, I'm not confusing Comparable to Comperator .


Normally the Comparable<T> interface is implemented by classes that need to compare objects with object of the same class => this is what I need

I just can't make Comparable to be student .. how should I do that ?

?? Does my Comparable interface OK ? does it missing something ??

Code:

---


public interface Comparable
{
        int comareTo(Comparable student1,Comparable student2);       
}

public class Student implements Comparable , Printable {

        private int id;
        private String name;
       
        Student ()//constructor
        {
          int tempid;
          String tempname;
         
          Scanner scan = new Scanner(System.in);
          System.out.println("Please enter ID");
          tempid = scan.nextInt(); 
          System.out.println("Enter Name");
          tempname =  scan.nextLine();
         
          setId(tempid);
          setName(tempname);
         
        }
       
        public void setId(int id) {
                this.id = id;
        }
        public int getId() {
                return id;
        }
        public void setName(String name) {
                this.name = name;
        }
        public String getName() {
                return name;
        }
        //@Override
        public int comareTo(Comparable student1, Comparable student2) {
                return        (student1).getName().compareTo((student2).getName()) ;
        }

}

---

[QUOTE=venerik;155220]This is your first java homework? And they immediately start with interfaces... strange (IMHO).

Never the less,

1. Do you know the root of all Java classes? I suggest you use that class in the method of your interface.

Object class ?

you suggest

Code:

---


    interface Comparable {
     
      int compareTo(Object O1, Object O2)

---

actually i tried
but then when I tried to sort them:

Code:

---

      if(comparables[i].comparaeTo(comparables[i],comparables[i+1])

---

it Gave me an error that compareTo doesn't get 2 comparables

it says

There is nothing wrong with your interface and implementation thereof per se but note that your Comparable interface has nothing to do with the 'standard' Comparable interface. Your implementation class(es) will not considered to be implementations of the (standard) interface though.

Your interface demands code like this:

Code:

---

Student s1= new Student();
Student s2= new Student();
Student s3= new Student();
int s1s2= s1.compareTo(s1, s2); // compares s1 and s2
int s1s2s3= s3.compareTo(s1, s2); // also compares s1 and s2

---

Note that last line: it looks funny but that's because any Student object can compare any other two Student objects.

kind regards,

Jos

Jos


student class

Code:

---

        //@Override
        public int comareTo(Comparable student1, Comparable student2) {
                return        (student1).getName().compareTo((student2).getName()) ;

---

I reciving an error here ... it tells that student1 && student2 doesn't have any getName etc

Quote:

---

Originally Posted by Bored2

Jos


student class

Code:

---

        //@Override
        public int comareTo(Comparable student1, Comparable student2) {
                return        (student1).getName().compareTo((student2).getName()) ;

---

I reciving an error here ... it tells that student1 && student2 doesn't have any getName etc

---

The compiler only knows that student1 and student2 are of type Comparable; it doesn't know that that particular interface is implemented by your Student class. Cast both parameters to type Student (because they actually are). You have to do it, the compiler is completely ignorant here:

Code:

---

        public int comareTo(Comparable student1, Comparable student2) {
                return        ((Student)student1).getName().compareTo(((Student)student2).getName()) ;

---

kind regards,

Jos

:-O and I breaking my head for 2 days what is wrong there ..

WoW thanks you !!

Quote:

---

Originally Posted by Bored2

:-O and I breaking my head for 2 days what is wrong there ..

WoW thanks you !!

---

You're welcome of course; don't think too much of the 'intelligence' of the compiler; it can't assume anything, all that it knows is that your Student class implements your Comparable interface so it implements the method 'comareTo' that takes to Comparables; it doesn't know that these Comparables are always Students (no matter the fact that this class implements that interface). Also, consider implementing the 'standard' Comparable interface (with a one argument compareTo( ... ) method), it avoids the silly use I showed in one of my previous replies.

kind regards,

Jos