插入后，Deque为第一个元素返回null

Question

当我向Deque添加一个元素然后立即尝试检查它时，它返回null。我确保进入的元素不是空的。这有什么问题：

已经尝试检查方法是否正在运行，add方法在DLL和Deque中运行。我也尝试过queueTail并重复进行排队，它仍然给我相同的NullPointerException

DoublyLinkedList类

public class DLL<T>{
    private static class Node<T>{
        T element;
        Node<T> next;
        Node<T> prev;
        public T getElement() {
            return element;
        }
        public void setElement(T element) {
            this.element = element;
        }
        public Node<T> getNext() {
            return next;
        }
        public void setNext(Node<T> next) {
            this.next = next;
        }
        public Node<T> getPrev() {
            return prev;
        }
        public void setPrev(Node<T> prev) {
            this.prev = prev;
        }
        public Node(T ele, Node<T> n, Node<T> p) {
            element = ele;
            next = n;
            prev = p;
        }
    }

    Node<T> header;
    Node<T> trailer;
    int size;

    public DLL() {
        size = 0;
        header = new Node<>(null,null,null);
        trailer = new Node<>(null,null,header);
        header.setNext(trailer);
    }

    public int size() {
        return size;
    }

    public boolean isEmpty() {
        return size == 0;
    }

    public T first() {
        if(isEmpty()) {
            return null;
        }
        return header.getNext().getElement();
    }

    public T last() {
        if(isEmpty()) {
            return null;
        }
        return trailer.getPrev().getElement();
    }

    private void addBetween(T ele,Node<T> before, Node<T> after) {
        Node<T> newN = new Node<T>(ele,after,before);
        before.setNext(newN);
        after.setPrev(newN);
        size++;
    }

    public void addFirst(T ele) {
        addBetween(ele,header.getNext(),header);
    }

    public void addLast(T ele) {
        addBetween(ele,trailer,trailer.getPrev());
    }

    private T remove(Node<T> rem) {
        Node<T> before = rem.getPrev();
        Node<T> after = rem.getNext();
        before.setNext(after);
        after.setPrev(before);
        size--;
        return rem.getElement();
    }

    public T removeFirst() {
        if(isEmpty()) {
            return null;
        }
        return remove(header.getNext());
    }

    public T removeLast() {
        if(isEmpty()) {
            return null;
        }
        return remove(trailer.getPrev());
    }

}

和类

public class Deque<T>{
    private DLL<T> list = new DLL<>();
    public Deque() {
    }

    public void queueHead(T ele) {
        list.addFirst(ele);
    }

    public void queueTail(T ele) {
        list.addLast(ele);
    }

    public T popHead() {
        return list.removeFirst();
    }

    public T popTail() {
        return list.removeLast();
    }

    public boolean isEmpty() {
        return list.isEmpty();
    }

    public int size() {
        return list.size();
    }

    public T peakHead() {
        return list.first();
    }

    public T peakTail() {
        return list.last();
    }
}

我的“Main”方法，运行并试图通过向Deque中添加Graph ADT对象来使用Deque然后尝试使用toString（）来显示它的部分

int population = Integer.parseInt((String) boxPopulation.getSelectedItem());
int perVacc = Integer.parseInt((String) boxVacc.getSelectedItem());
String disease = (String) boxDisease.getSelectedItem();
Disease d = null;
GraphMod grap = null;
com.graph.herdImmunity.Graph<Person> people;
if(disease.equalsIgnoreCase("Measles")) {
    d = new Disease("Measles",12,95,6.5);
    grap = new GraphMod(perVacc,d);
}
people = grap.initialise(population);//This just fills the graph with dummy data, this works I've tested

Deque<com.graph.herdImmunity.Graph<Person>> days = new Deque<>();

days.queueHead(people);

System.out.println(days.peakHead().toString());//this line gives me a nullpointerexception

Answer 1

你的addBetween是错的。它过于复杂，并且具有令人困惑的命名参数。它应该是这样的：

private void addAfter(T ele, Node<T> after) {
    Node<T> newN = new Node<T>(ele, after, after.getNext());
    after.getNext().setPrev(newN);
    after.setNext(newN);
    size++;
}

让after和before都会引入问题，你冒着混淆它们的风险（正如你所做的那样）并且你不能保证它们是一致的，如果我用一对不匹配的元素调用该方法会破坏整个名单。

然后，您需要通过以下方式调用该方法：

public void addFirst(T ele) {
    addAfter(ele, header);
}

public void addLast(T ele) {
    addAfter(ele, trailer.getPrev());
}

Answer 2

Java已经有一个Deque，可以很容易地与ArrayDeque一起使用。如果你坚持使用你所做的，你的问题其实很有趣。在

public void addFirst(T ele) {
        addBetween(ele,header.getNext(),header);
    }

你在getNext()参数中有before。

它应该是

public void addFirst(T ele) {
        addBetween(ele,header,header.getNext());
    }

此外，您应该将名称“DLL”更改为其他名称，因为它通常称为“动态链接库”。