在Java中将对象附加到ArrayList中

Question

I am working on an exercise and I ran into a problem.

In NodeList , create a static method Node arrayToNode(String[] arr) which converts a String array into a list. Your method should create a first Node , and then go through the rest of the array, creating a Node at each step, and using append to put the created Node at the end of the list. Test this method on the command line arguments. What happens if the array is empty?

Currently my code is like this

public static Node arrayToNode(String[] arr) {
    Node first = new Node(arr[0]);
    ArrayList<Node> list = new ArrayList<Node>();
    for(int i=1; i<arr.length;i++){
        list.add(new Node(arr[i]));
    }
}

as you can see there is no return statement YET. I am not sure if the person who wrote the exercise made a mistake by writing Node instead of void but I cannot ask him.

The append method is

public void append(Node fin){
    if(next==null)
        next=fin;
    else
        append(next);
}

and the instance variables and the constructor is as follows:

public String value;
public Node next;

public Node(String s){
    value =s;
    next=null;
}

I am quite unsure what it means to put the node at the end of the list as the ArrayList keeps on expanding. Also, I have questions about using the deploying the append method as in how to even use it in the TestNode class.

---

Thanks for the comment. I have now realized what the problem was and have made appropriate changes.

public static Node arrayToNode(String[] arr){
    Node first = new Node(arr[0]);
    for(int i=1; i<arr.length;i++){
        Node nd = new Node(arr[i]);
        nd.append(nd);
        first.next=nd;
    }

    return first;
}

could you see if this is correct?

Answer 1

Unless I'm missing something, your append method should be this:

public void append(Node fin) {
    if (next == null)
        next = fin;
    else
        next.append(fin); // <- this line changed
}

That will append fin down the line until it reaches the end of the list while the way you have it in your OP will give infinite recursion.

If it's supposed to be that way then creating the list is very simple. You can just append each value to the original one.

public class Node {
    public static void main(String[] args) {
        Node begin = arrToLL(new String[] {
            "hello 1", "hello 2", "hello 3", "hello 4", "hello 5"
        });

        while (begin != null) {
            System.out.println(begin.val);
            begin = begin.next;
        }
    }

    static Node arrToLL(String[] arr) {
        if (arr == null) {
            return null;
        } else if (arr.length == 0) {
            return new Node("null");
        }

        int ind = 0;
        Node begin = new Node(arr[ind++]);

        while (ind < arr.length) {
            begin.append(new Node(arr[ind++]));
        }

        return begin;
    }

    /* instance */

    String val;
    Node next;

    Node(String val) { this.val = val; }

    void append(Node ap) {
        if (next == null) {
            next = ap;
        } else {
            next.append(ap);
        }
    }
}

Output is:

hello 1
hello 2
hello 3
hello 4
hello 5

Inside the "to list" loop you could also "shuffle ahead" by assigning next to a variable like in my println loop. That way you aren't taking advantage of the "pass through".

public static Node arrayToNode(String[] arr){
    Node first = new Node(arr[0]);
    for(int i=1; i<arr.length;i++){
        Node nd = new Node(arr[i]);
        nd.append(nd); // <- appends nd to itself
        first.next=nd; // <- always assigns the new value to first
    }

    return first;
}

That's getting closer but I've commented the two lines that are in err. I think what you will end up with is:

• First Node with the first array element linked to
• A second Node with the last array element linked to
• Itself (the second Node)

You can do this without append but you need another variable to shuffle:

public static Node arrayToNode(String[] arr){
    Node first = new Node(arr[0]);
    Node current = first;

    for(int i=1; i<arr.length;i++){
        Node nd = new Node(arr[i]);

        current.next = nd; // <- append the new node to the last
        current = nd; // <- shuffle ahead to the new one
    }

    return first;
}

Otherwise if I'm correct in thinking append had an error you can do something closer to my main example (including the shuffle if you want and a for loop works just as well).