How to create an instance of a parent node from Java DisjointSet

Question

Here is the link to the data structure: http://git.eclipse.org/c/platform/eclipse.platform.ui.git/plain/bundles/org.eclipse.ui.ide/src/org/eclipse/ui/internal/ide/misc/DisjointSet.java

Here is what I tried in the main method. (A parent node points to itself and has a rank of 0.)

 public static void main(String[] args) {
    DisjointSet x = new DisjointSet();
    **Node<T> parent = new Node<T>(parent, 0);**
 }

Here are my error messages:

Error: non-static type variable T cannot be referenced from a static context

Error: non-static type variable T cannot be referenced from a static context

Looks like one error for T on each side of the equal sign.

Answer 1

The error is because you are accessing a non-static instance variable from within a static method

All member fields of an interface are by default public, static and final.

Since inner interface is static by default, you can't refer to T from static fields or methods.

T is actually associated with an instance of a class, if it were associated with a static field or method which is associated with class then it wouldn't make any sense

You can create one class with informations:

public class MyTreeNode<T>{
    public T data = null;
    public MyTreeNode parent = null;

    public MyTreeNode(T data) {
        this.data = data;
    }

    public void addChild(MyTreeNode child) {
        child.setParent(this);
        this.children.add(child);
    }

    public void addChild(T data) {
        MyTreeNode<T> newChild = new MyTreeNode<>(data);
        newChild.setParent(this);
        children.add(newChild);
    }

    public void addChildren(List<MyTreeNode> children) {
        for(MyTreeNode t : children) {
            t.setParent(this);
        }
        this.children.addAll(children);
    }


    public T getData() {
        return data;
    }

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

and main with examples:

MyTreeNode<String> root = new MyTreeNode<>("Root");

MyTreeNode<String> child1 = new MyTreeNode<>("Child1");
child1.addChild("Grandchild1");
child1.addChild("Grandchild2");

MyTreeNode<String> child2 = new MyTreeNode<>("Child2");
child2.addChild("Grandchild3");

root.addChild(child1);
root.addChild(child2);
root.addChild("Child3");

root.addChildren(Arrays.asList(
        new MyTreeNode<>("Child4"),
        new MyTreeNode<>("Child5"),
        new MyTreeNode<>("Child6")
));

for(MyTreeNode node : root.getChildren()) {
    System.out.println(node.getData());
}

Reference: here and here