Thread: Need some help with Generics

Originally Posted by bankston13

...I believe the easiest way to do union and intersect is to have the methods go through their parameters and then return a third arrayList with the results.

Give it a try

Originally Posted by bankston13

Can anyone give me some pointers or tips on a good starting point to do the rest of the methods.

One at a time. Get one method working and tested before moving on.

Please post the error message, your screen is hard to read from here.
Also post the relevant code with the error.

I can't even run the code, because of the syntax issue. here si the relevent code:

public static <T> Set<T> intersect(Set<T> first, Set<T> second )
        {
 
 
            for (int i = 0; i < first.size(); i++){
                for (int j = 0; j < second.size(); j++){
                    if (first.get(i) == second.get(i))               //****This is the error area****//
 
                } 
            }        
	}

I pretty much know what I want to do after it as far as adding the results to an arraylist. The problem is that while I can do it rather easily with a predefined type, I can't seem to figure out how to do it when it involves generics. I'm not sure if I have to make a new arraylist with the T type or do something else.

Originally Posted by bankston13

... or do something else.

When I am faced with an error, the first thing I do is to gather as much information as I can from the error. See if I can understand what the error is saying. Error messages usually describe what is not quite correct. After you understand what is wrong, you can start working on making it right.

Originally Posted by jps

Please post the error message, your screen is hard to read from here.

After some more though, I think I've completed most of the code, aside from the union and intersect methods. What I'm trying to do is make the two arraylists that are passed into the two methods call the toArray method, that way they are converted to Arrays and somewhat easier to compare. I'm going off of Pseudo code that my teacher provided us for these two methods, and they both use Arrays and not arraylists to compare, so thats why I'm trying to convert. I understand the pseudo code for the most part, but I'm getting an error when I try to convert the arraylists to arrays. The syntax error I'm getting says this:

incompatable types
required: T[]
found: Object[]
where T is a type variable


I can't figure out where to go from here. i understand that its an incompatability type, but I'm not sure on how to fix it when making my arrays. here is the Pseudo code I have if anyone needs it. I also have my revised MySet class as well.

Intercept:

C empty set
for i 0 to n-1
for j 0 to m-1
if A[i] = B[j]
add B[j] to C
break out the j loop
end if
end for j
end for i



Union:

C A //copy A to C
for i 0 to m-1
add true
for j 0 to n-1 //determine if C already has B[i]
if C[j] = B[i]
add false
break out of j loop
end if
end for j
if add
add B[i] to C
end if
end for i

import java.util.*;
public class MySet<T> implements Set<T>
{
	private ArrayList<T> set;
        private int size;
 
	public MySet()
	{
		set = new ArrayList<T>();
	}
 
	public void clear()
	{
		int i;
                for (i = 0; i < set.size(); i++){
                    set.remove(i);
                }
	}
 
	public boolean add(T e)
	{
		boolean exist = contains(e);
 
                if (exist == false){
                    set.add(e);
                    return true;
                }
                else
                    return false;
 
 
 
	}
 
	public boolean contains(Object o)
	{
		int i;
                int result = 0;
                for (i = 0; i < set.size(); i++){
                    if (set.get(i) == o)
                        result = i;
                    else
                        result = -1;
                }
 
                if (result == -1)
                    return false;
                else
                    return true;
 
	}
 
	public boolean isEmpty()
        {
                if (set.size() == 0)
                    return true;
                else
                    return false;
	}
 
	public boolean remove(Object o)
	{
 
	}
 
	public int size()
	{
		return set.size();
	}
 
 
	public Object[] toArray()
	{
                ArrayList<T> temp = (ArrayList<T>) set.clone();
		T[] array = (T[]) new Object[set.size()];
		int i;
		for (i=0; i<set.size(); i++)
				array[i] = (T) temp.get(i);
		return array;		
	}
 
	public static <T> Set<T> union(Set<T> first, Set<T> second ) 
	{
 
	}
 
	public static <T> Set<T> intersect(Set<T> first, Set<T> second )
        {
            T[] one = first.toArray();
            T[] two = second.toArray();                                                            ****//This method is one I need help with//****
	}
 
	public void printAll()
	{
		int i;
                for (i = 0; i < set.size(); i++) {
                System.out.println(set.get(i) + " ");
            }
	}
 
 
}

I'm particularly looking for help with the Intersect and Union methods, but any pointers would be appreciated.

Alright, so i figured out what was wrong with my toArray calls. I needed to make object arrays instead of T arrays. However, I'm having a hard time adding to the arraylist that I made to hold the common values in the Intersect method. The problem happens at my line that adds the value of the second array[j] to the arraylist. My error is basicaly saying that there is no method for add(object). I don't understand whats happening. here is the specific part of the code that I am having problems with.

public static <T> Set<T> intersect(Set<T> first, Set<T> second )
        {
            Object[] one = first.toArray();
            Object[] two = second.toArray();
            ArrayList<T> third = new ArrayList<T>();;
            int i, j;
 
            for (i = 0; i < one.length - 1; i++ ){
                for (j = 0; j < two.length - 1; j++){
                    if (one[i] == two[j]){
                        third.add(two[j]);                                             ****//This is line I am having trouble with//****
                    }
                }
            }
	}