How do I add spaces between elements in a doubly linked list when concentrating them into a string in Java?

I'm creating a toString() function that should return a String created by concatenating each Nodes toString(). A single space: ‘ ’ should be inserted between each Nodes toString(). No trailing space should be inserted.

For example, if the list contained 3 Node objects, the toString() return value would be [1 2 3], but not [123] or [1 2 3 ].

This is what I've tried so far and but it does not put a space between the elements (I get [01] when I need [0 1] for example):

public String toString() {
        
        String str = "";
        
        Node temp = head;
        
        while (temp != null) {
        
                str += temp.getString() + "";
                temp = temp.next;
        }
        
        return str;

    }

Here is the doubly linked list class I have with the add method:

public class DSEList {
    
    public Node head;
    private Node tail;

    public DSEList() {
        head = null;
        tail = null;
        
    }
    
    public DSEList(Node head_) {
        
        head = head_;
    }

public boolean add(String obj) {
        
            Node newNode = new Node(null, null, new String(obj));
            
            Boolean success = false;
        
            if(head == null) {  
                head = tail = newNode;  
                head.prev = null;  
                tail.next = null;
                success = true;
            } 

            else {  
                
                tail.next = newNode;  
                newNode.prev = tail;  
                tail = newNode;  
                tail.next = null;
                success = true;
            }  
            
            return success;

        }  
    

Here is the node class I have where the getString() comes from:

public class Node {

    public Node next;
    public Node prev;
    
    private String t;

    public Node(Node next, Node prev, String token) {
        this.next = next;
        this.prev = prev;
        this.t = token;
    }

    public String getString() {
        return t;
    }

    @Override
    public boolean equals(Object other) {
        if (this == other)
            return true;
        if (other == null)
            return false;
        if (!(other instanceof Node))
            return false;

        return t.equals(((Node)other).getString());
    }

    @Override
    public int hashCode() {
        if ( t == null )
            return 0;
        return t.hashCode();
    }

}

Here is an example of how it would work:

DSEList l = new DSEList();
l.add(new String(""+0));
l.toString() //this should return [0]
l.add(new String(""+1));
l.toString() //this should return [0 1]
l.add(new String(""+2));
l.toString() //this should return [0 1 2]

Thank you!

Use a StringJoiner or a StringBuilder rather than repeatedly appending strings in a loop. If using a StringBuilder, prepend the space if and only if the StringBuilder is non-empty.

Example with StringJoiner:

@Override
public String toString() {
    StringJoiner joiner = new StringJoiner(" ");
    
    Node node = head;
    while (node != null) {
        joiner.append(node.getString());
        node = node.next;
    }
    
    return joiner.toString();
}

Example with StringBuilder:

@Override
public String toString() {
    StringBuilder sb = new StringBuilder();
    
    Node node = head;
    while (node != null) {
        if (!sb.isEmpty()) {
            sb.append(' ');
        }
        sb.append(node.getString());
        node = node.next;
    }
    
    return sb.toString();
}

Note: temp is a terrible variable name for anything other than temp files and the intermediate value used in a swap.