Linked List prepend and toString methods

I am trying to output the list, I have prepended 3 strings to the list and it all outputs null. The code seems to prepend and output correctly, however it just doesn't output the data. The methods I am focusing on are prepend and toString.

``
public class StringList {

/* ------------------------------------------------------- *
 * Inner classes                                           *
 * ------------------------------------------------------- */
/**
 * The node used by StringList.
 */
private class StrNode {

    String data;
    StrNode next;
}

/* ------------------------------------------------------- *
 * Instance variables                                      *
 * ------------------------------------------------------- */
private StrNode head;   // the head of the singly-linked list.

/* ------------------------------------------------------- *
 * Instance methods                                        *
 * ------------------------------------------------------- */
/**
 * No-argument constructor.
 */
public StringList() {
    head = null;
}

/**
 * Adds an item to the start of the list.
 *
 * @param s the item to add
 */
public void prepend(String s) {
    var newNode = new StrNode();
    // TODO: Adds an item to the start of the list.		
    if(head == null) {
    	head = newNode;
    }
    else {
    newNode.next = head.next;
	head.next = newNode;
    }
    
}

/**
 * Adds an item to the end of the list.
 *
 * @param s the item to add
 */
public void append(String s) {
    // TODO: 	Adds an item to the end of the list.
	
}

/**
 * Inserts an item after the first instance of a key if the key exists.
 *
 * @param s the item to insert
 * @param key the item in the list to insert after
 * @return whether the insertion was successful
 */
public boolean insertAfter(String s, String key) {
    // TODO: 	Inserts an item after the first instance of a key if the key exists.
	
	return false;
}

/**
 * Deletes the first instance of an item from the list.
 *
 * @param key the value of the item to delete from the list.
 * @return whether the deletion was successful.
 */
public boolean delete(String key) {
    // TODO:	Deletes the first instance of an item from the list.
	
	return false;
}

/**
 * Returns the value of the nth item in the list.
 * 
 * @param n the zero-based index of the item to return
 * @return the value of the nth item
 */
public String get(int n) {
    // TODO: 	Returns the value of the nth item in the list.
    // Note: if n is out of bounds, raise an IndexOutOfBoundsException.
	
	
	return null;
}

/**
 * Returns the number of items in the list.
 *
 * @return the number of items in the list
 */
public int length() {
    // TODO: 	Returns the number of items in the list.
	int length = 0;
	StrNode current = head;
	
	while(current != null) {
		length++;
		current = current.next;
	}
	
	return length;
}

/**
 * Returns a string of all the items in the list separated by a space.
 *
 * The last item will have a space after it too.
 *
 * @return list of the list's values
 */
@Override
public String toString() {
    // TODO: 	Returns a string of all the items in the list separated by a space.
	String result = "{";
	StrNode current = this.head;
	
	while(current != null) {
		result += current.data + " ";
		current = current.next;
	}
 
    return result + "}";

}

}
``

Driver:
``

public class Main {
/**
* @param args the command line arguments
*/
public static void main(String[] args) {

	StringList s = new StringList();
	
	s.prepend("one");
	s.prepend("two");
	s.prepend("three");
	System.out.println(s);      
    
}

}

``

The error is in your prepend(String s) method. Have a close look -you never use s, i.e., the string which you pass to this method is never stored. You simply have to add newNode.data = s; as the second line. However, there is also a second error as the StrNode objects are not linked correctly.

Solution

public void prepend(String s) {
    var newNode = new StrNode();
    newNode.data = s;
    if (head == null) {
        head = newNode;
    } else {
        // First, we set the current head as the successor of the new newNode
        newNode.next = head;
        // Then, we set the new newNode as head (as we prepend)
        head = newNode;
    }
}

Then the output is as aspected:

{three two one }

Additional Comment

Your toString() method concats strings in a loop with +. That is inefficient and should be replaced by a StringBuilder. The StringBuilder also allows easily to delete the last space after "one":

@Override
public String toString() {
    StringBuilder result = new StringBuilder("{");
    StrNode current = this.head;

    while (current != null) {
        result.append(current.data).append(" ");
        current = current.next;
    }
    result.setLength(result.length() - 1);

    return result + "}";
}