如何从队列Java

Question

这是我在运行我编写的JUnit测试时删除队列中第一个元素的代码，它在E上提供了nullPointerException。retVal = head.getElement（）; 这是构造函数

/** Create a new (empty) instance of this class. */
    public ProperQueue() {
        head = null;
        tail = null;
        size = 0;
    }



public E remove() {
        if (head == null) {
            tail = null;
        }
        E retVal = head.getElement();
         if( head.getNext()==null) {
             head=null;
             tail=null;
             size-=1;
             return retVal;
         }
        head = head.getNext();
        return retVal;
    }

这是我的Junit测试

@Test
    public void testRemove() {
        ProperQueue testRemove = new ProperQueue();
        testRemove.add(7);
        testRemove.add(6);
        testRemove.remove();
       assertEquals(7 ,testRemove.remove());
    }