SwapNode.java

public class SwapNode {

    // Function to swap nodes
    public static ListNode swapPairs(ListNode head) {

        ListNode dummy = new ListNode(0);
        dummy.next = head;

        ListNode prev = dummy;

        while (prev.next != null && prev.next.next != null) {

            ListNode first = prev.next;
            ListNode second = prev.next.next;

            // Swapping
            first.next = second.next;
            second.next = first;
            prev.next = second;

            prev = first;
        }

        return dummy.next;
    }

    // Print function
    public static void printList(ListNode head) {
        ListNode temp = head;
        while (temp != null) {
            System.out.print(temp.val + " -> ");
            temp = temp.next;
        }
        System.out.println("null");
    }

    // MAIN METHOD
    public static void main(String[] args) {

        // Create: 1 -> 2 -> 3 -> 4
        ListNode head = new ListNode(1);
        head.next = new ListNode(2);
        head.next.next = new ListNode(3);
        head.next.next.next = new ListNode(4);

        System.out.print("Before: ");
        printList(head);

        head = swapPairs(head);

        System.out.print("After: ");
        printList(head);
    }
}

// Separate class
class ListNode {
    int val;
    ListNode next;

    ListNode(int val) {
        this.val = val;
        this.next = null;
    }
}