public static void addHandler(PtrUIHandlerHolder head, PtrUIHandler handler) {

        if (null == handler) {
            return;
        }
        if (head == null) {
            return;
        }
        if (null == head.mHandler) {
            head.mHandler = handler;
            return;
        }

        PtrUIHandlerHolder current = head;
        for (; ; current = current.mNext) {

            // duplicated
            if (current.contains(handler)) {
                return;
            }
            if (current.mNext == null) {
                break;
            }
        }

        PtrUIHandlerHolder newHolder = new PtrUIHandlerHolder();
        newHolder.mHandler = handler;
        current.mNext = newHolder;
    }
A single linked list to wrap PtrUIHandler
ptr-lib/src/in/srain/cube/views/ptr/PtrUIHandlerHolder.java