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.