节点未正确添加到LinkedList

Question

I know I shouldn't be trying to reinvent the wheel, however I'm required to for this assignment. I'm struggling to get any output at all from my LinkedList, I believe my addFirst() method is not properly adding the node or my toString() method is not properly displaying the LinkedList. Any idea why? I've been stumped on this for a bit and not sure where else to look for help.

I'm not getting any errors, just zero output.

Main:

public static void main(String args[]) {

    LinkList theList = new LinkList();

    theList.addFirst("One");
    theList.getFirst();
    theList.size();

    theList.toString();

}   // end main()

LinkList.java:

public class LinkList {

private Node first;
private Node current;

public LinkList() {
    first = null;
    current = first;
}

public boolean isEmpty() {
    return (first == null);
}

public void removeFirst() {
    try {
        if (!isEmpty()) {
            Node temp = first;
            first = first.getNext();
        }
    } catch (NullPointerException e) {
    }
}

public int size() {
    int count = 0;

    if (isEmpty()) {
        return 0;
    }

    while (current != null) {
        current = current.getNext();
        count++;
    }
    return count;
}

@Override
public String toString() {
    String str;
    int size = size();
    str = "Size: " + size;
    str += "\nElements:";
    while (current != null) {
        for (int i = 0; i < size; i++) {
            str += "\nNode: " + current.getNext();
        }
    }
    return str;
}

public String getFirst() {
    if (first == null) {
        return null;
    }
    return first.getData();
}

public void addFirst(String data) {
    Node newNode = new Node(data);
    newNode.getNext();
    first = newNode;
}

public boolean contains(String target) {
    while (current != null) {
        if (current.getData().equals(target)) {
            return true;
        }
        current.getNext();
    }
    return false;
}

Node.java

public class Node {

private String data;
private Node next;

public Node(String data) {
    this.data = data;
}

public String getData() {
    return data;
}

public Node getNext() {
    return next;
}

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

public void setNext(Node next) {
    this.next = next;
}

public String toString() {
    return "Node: " + getData();
}

}

Answer 1

You should surround calls you want to see output of in the console with System.out.println(); , eg:

System.out.println(theList.size());
System.out.println(theList.toString());

Note that you don't need to call toString() method explicitly in the second line, System.out.println(theList); calls theList 's toString() method automatically and it's equivalent to System.out.println(theList.toString()); .

Answer 2

In your addFirst() method, you should:

• Create the new node
• Set the current first node as the next node of the new node
• Set the new node as first

You're missing one of the above steps.

Additionally, all of the methods that include iterating the list ( toString() , size() etc.) would need to initialize with current = first; before the iteration starts. Otherwise current can be null and the iteration won't start.