Static or Non Static (that is the question).

[Owen Martin]

Hey guys, I am just trying out 'simple' Java techniques for practise and I am already stuck.

Don't worry about the code itself in terms of efficiency etc, I just want to know why this doesn't work.

I know I can change the 'add' method to static but I done that in a previous program and nearly all my methods were made static and running it etc started to confuse me.

I have one class, and a test class.

  import java.util.*;

public class Students {

    public ArrayList<String>helpqueue = new ArrayList<String>();
    
    public  boolean add(String s)
    {    
        String studentname = s;
        
        if (helpqueue.contains(studentname)== false)
            {
            helpqueue.add(studentname);
            return true;
            }
          else return false;
    }
    
}  

   public class Test {

    public static void main(String[] args) {
        Test tester  = new Test();
          Students.add("tom");
    }
} 

[marc weber]

Static methods are associated with the class. Instance (non-static) methods are associated with an instance of the class.

You cannot call an instance method using the class...

Students.add("Tom"); //won't work because add is not static

Instead, you need to create an instance of the class, and call the method using that instance...

Students myStudents = new Students(); //create an instance (object) of the class
myStudents.add("Tom"); //call instance method

[Campbell Ritchie]

You may be getting confused because you have strangely-named classes. You have a class named Students which appears really to be a queue. You ought to have a Student class and a HelpQueue class. Then you create a HelpQueue object, and add Students to the Queue (or remove them).

You would do better to use a class which implements the Queue<T> interface rather than a straight List<T>. ArrayDeque would be a good choice. Note there are methods named differently for Queues, eg offer and poll.

Never never say == false or == true. Apart from that being very bad style, you can get all sorts of really nasty errors if you accidentally write = instead of ==. It's not

     if (helpqueue.contains(studentname)== false) . . .

but

     if (!helpqueue.contains(studentname)) . . .