for-each in Java trying to convert element types

Question

I have a problem in Java when using a for-each construct on an ArrayList. In one class, named Graph, I have the protected attribute

protected ArrayList<Edge<E>> edges

In my Edge class, I have the protected attribute

protected LinkedList<Connector<E>> pointers;

What I'm trying to do, in my Edge class, is to define a method to print on screen each pointer of each Edge of my Graph g. I tried using different approaches getting errors everytime, and then I ended up using the for-each loop (I thought it was the best choice since I know the type of the elements in the collection I'm trying to iterate on). My code is as follows:

public void printConnectors(Graph g){
    for (Edge<E> e: g.edges){
    System.out.println(e.iterator()); //I do have an iterator in my Edge class
    }
}

This is not working though, Eclipse gives me the following error:

Type mismatch: Cannot convert from element type Object to Edge

I know this may be a stupid error derived from something I'm not seeing in my code, but I can't get it to work. Can anyone help me and explain why the compiler tries to convert from Object to Edge even though edges should be an ArrayList of Edges?

score 4 · Accepted Answer · edited May 23 '17 at 12:05

4

You're using raw types. The code should be:

public void printConnectors(Graph<E> g) { // use the generic type Graph<E> here,
                                          // not the raw type Graph
    for (Edge<E> e: g.edges) {
        System.out.println(e.iterator()); 
    }
}

What is a raw type and why shouldn't we use it?

edited May 23 '17 at 12:05

Community

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answered May 31 '14 at 15:49

JB Nizet

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...and for those of you wondering why this fixes it, raw types strip off all generic info, effectively (but not exactly) treating the instance and all its fields/methods as if their type was `Object` – Bohemian May 31 '14 at 15:54

for-each in Java trying to convert element types

1 Answers1