Java classes and ArrayList

Question

At the moment I'm trying to write a program to make a MinHeap, although I can't seem to get the ArrayList to work properly as one of the classes seem to not connect with another class. The problem occurs when one class is trying to add Elements to the ArrayList. Classes are separated by ----

public interface QUEUE {
public Element getMin();
public void add(Element e);
}

public class Element {

public int key;
public Object data;

public Element(int i, Object o){
this.key = i;
this.data = o;
    }
}

class TestProject {

public static void main(String[] args) {

System.out.println();
QUEUE q = new QUEUEHeap(10);

System.out.println("  5, 1, 2, 33, -1, 3, 1, 2, 23, 13");
System.out.println();

q.insert(new Element(5,new Integer(5)));
q.insert(new Element(1,new Integer(1)));
q.insert(new Element(2,new Integer(2)));
}

import java.util.ArrayList;

public class QUEUEHeap implements QUEUE {

private ArrayList<Integer> q;

public PQHeap(int maxElements) {
    q = new ArrayList<>(maxElements);
    System.out.println("Element at index 1: " + q);
}

public Element getMin() {


}


public void insert(Element e) {
q.add(e.key,e.data); //e.key = i, e.data = o

}
}

New error:

no suitable method found for add(int,Obje
q.add(e.key,e.data); //e.key = i, e.data = o
  ^
method List.add(int,Integer) is not applicable
  (argument mismatch; Object cannot be converted to Integer)
method AbstractList.add(int,Integer) is not applicable
  (argument mismatch; Object cannot be converted to Integer)
method ArrayList.add(int,Integer) is not applicable
  (argument mismatch; Object cannot be converted to Integer)

The error is that pq from the insert method is not defined. Although I have no idea how to fix this. It seems that the insert method can't access the created ArrayList? Please help. — Colour
– Colour, Commented Apr 3, 2015 at 17:25
@Prashant Then there is no such method!! What do you mean to be exact? And is there anyway to make it work like that? — Colour
– Colour, Commented Apr 3, 2015 at 17:29
@Prashant ArrayList#add(int, E) exists. It's to place the element at a specific index — Vince
– Vince, Commented Apr 3, 2015 at 17:33
@VinceEmigh Would I be able to use that: ArrayList#add(int, E) instead of pq.add(e.key, e.data); ? — Colour
– Colour, Commented Apr 3, 2015 at 17:56
No, ArrayList#add(int, E) is the format used to talk about instance methods. It's not actual syntax — Vince
– Vince, Commented Apr 3, 2015 at 17:58

Vince · Accepted Answer · 2015-04-03 22:05:06Z

4

Declare the ArrayList as a field variable, rather than a variable local to the constructor:

class MiniHeap {
    private ArrayList<Integer> pq;

    public MiniHeap(int maxElements) {
        pq = new ArrayList<>(maxElements);
    }

    //your methods..
}

Keep in mind that the constructor argument for ArrayList is not a maximum size; its an initial size. Lists don't have maximum sizes.

Element#data is of type Object. Since your ArrayList only accepts Integer, you are getting that error, hence the:

Object cannot be converted to Integer

Element#data should be the type as the ArrayList in your heap class. To allow variety in type, I'd personally give Element a type parameter:

class Element<T> {
   private int key;
   private T data;

   public Element(int key, T data) {
       this.key = key;
       this.data = data;
   }

   public T getData() {
       return data;
   }

   public int getKey() {
       return key;
   }
}

Then create a heap that allows a specific type of data (assign your list to that type):

class MiniHeap<T> {
   private ArrayList<T> pq;

   public MiniHeap(int initialAmount) {
       pq = new ArrayList<>(initialAmount);
   }

   public void insert(Element<T> element) {
       pq.add(element.getKey(), element.getData());
   }
}

Just simply declare the Element and MiniHeap with the same type argument:

public class Main {
    public static void main(String[] args) {
        Element<Integer> element = new Element<>(0, 10);
        MiniHeap<Integer> heap = new MiniHeap<>(20);

        heap.insert(element);
    }
}

Beware of auto-boxing. Primitive types cannot be used for generic type arguments, so primitive wrapper classes are used instead, which wraps the primitive value in an object. When uoi declare new Element<>(0, 10), the 10 is boxed into an object, since the constructor's parameters is (int, Integer) and not (int, int). To prevent auto-boxing, use new Integer(10) instead of just 10. Auto-unboxing could still be an issue though, depending on your situation.

edited Apr 3, 2015 at 22:05

answered Apr 3, 2015 at 17:29

Vince

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4 Comments

Vince Over a year ago

@Colour Why don't you try it? The problem with your code is you declare ArrayList in a scope only the constructor can access. You need to declare it in the field so all the methods can access it aswell

Colour Over a year ago

I did try it out, but now it returns the error: PQHeap.java:19: error: no suitable method found for add(int,Obje pq.add(e.key,e.data); //e.key = i, e.data = o ^ method List.add(int,Integer) is not applicable (argument mismatch; Object cannot be converted to Integer) method AbstractList.add(int,Integer) is not applicable (argument mismatch; Object cannot be converted to Integer) method ArrayList.add(int,Integer) is not applicable (argument mismatch; Object cannot be converted to Integer)

Vince Over a year ago

@Colour Edit your error and your new code into your question

Colour Over a year ago

Thanks a lot for your time! Your solution didn't fit directly, although I understood the concept of it :-). Thank you sir!

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