cannot convert list to list error in java generics

Question

Following is my simplified graph implementation

import java.util.ArrayList;
import java.util.List;

public class TreeNode<E extends Comparable<E>> {
    private E data;
    private List<TreeNode<E>> children;

    public TreeNode(E value) {
        data = value;
        children = new ArrayList<>();
    }

    public E getData() {
        return data;
    }

    public void setData(E data) {
        this.data = data;
    }

    public List<TreeNode<E>> getChildren() {
        return children;
    }

    public void setChildren(List<TreeNode<E>> children) {
        this.children = children;
    }

}

And I am writing code to find if 2 nodes are connected in a directed graph. I am getting compilation error

public static boolean findIfPathExists(TreeNode<? extends Comparable<?>> start, TreeNode<? extends Comparable<?>> end) {
    Set<TreeNode<? extends Comparable<?>>> visitedNodes = new HashSet<TreeNode<? extends Comparable<?>>>();
    return findIfPathExists(start, end, visitedNodes);
}

private static boolean findIfPathExists(TreeNode<? extends Comparable<?>> start, TreeNode<? extends Comparable<?>> end,
        Set<TreeNode<? extends Comparable<?>>> visitedNodes) {
    if(start == end) return true;
    visitedNodes.add(start);
    List<TreeNode<? extends Comparable<?>>> children = start.getChildren();
    for (TreeNode<? extends Comparable<?>> child : children) {
        if(visitedNodes.contains(child)) continue;
        if(findIfPathExists(child, end, visitedNodes)) return true;
    }
    return false;
}

I am getting error at line start.getchildren

 Type mismatch: cannot convert from List<TreeNode<capture #11 -of? extends 
Comparable<?>>> to List<TreeNode<? extends Comparable<?>>>

The problem is that wildcards could be anything and hence the compiler won't allow writing operations like visitedNodes.add(start); (which I guess is the line you're getting the error at - hint: don't make us guess). — Thomas
– Thomas, Commented Nov 23, 2016 at 16:39
@Thomas presumably, the one containing List<TreeNode<? extends Comparable<?>>>. — Andy Turner
– Andy Turner, Commented Nov 23, 2016 at 16:55

Andy Turner · Accepted Answer · 2016-11-23 16:40:12Z

6

Add a type variable to your method signatures:

public static <T extends Comparable<T>> boolean findIfPathExists(
    TreeNode<T> start, TreeNode<T> end) {

private static <T extends Comparable<T>> boolean findIfPathExists(
    TreeNode<T> start, TreeNode<T> end, Set<TreeNode<T>> visitedNodes) {

and then use T wherever you currently have ? extends Comparable<?>.

answered Nov 23, 2016 at 16:40

Andy Turner

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Comments

Jerry Sha · Accepted Answer · 2016-11-23 16:51:30Z

0

If you wanted to keep the wildcards, change the line from

List<TreeNode<? extends Comparable<?>>> children = start.getChildren()

to

List<? extends TreeNode<? extends Comparable<?>>> children = start.getChildren();

answered Nov 23, 2016 at 16:51

Jerry Sha

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