super（）导致NullPointerException

Question

当我学习梁的书时，我陷入了困境，我不知道发生了什么。 错误的原因是MyArrayList类的构造函数。 作者警告我们不要调用super（object），但是他们没有解释原因。 现在，当我尝试运行代码时，作者是正确的-当我们调用super（object）时，我得到一个错误。 此错误的原因是什么？

MyArrayList.java

public class MyArrayList<E> extends MyAbstractList<E> {
  public static final int INITIAL_CAPACITY = 16;
  private E[] data = (E[])new Object[INITIAL_CAPACITY];

  /** Create a default list */
  public MyArrayList() {
  }

  /** Create a list from an array of objects */
  public MyArrayList(E[] objects) {
    /*for (int i = 0; i < objects.length; i++)
     add(objects[i]); // Warning: don't use super(objects)! */
     super(objects);  //!!! AUTHOR WARNS US ABOUT NOT INVOKING THIS LINE !!!
  }

  /** Add a new element at the specified index in this list */
  public void add(int index, E e) {
    ensureCapacity();

    // Move the elements to the right after the specified index
    for (int i = size - 1; i >= index; i--)
      data[i + 1] = data[i];

    // Insert new element to data[index]
    data[index] = e;

    // Increase size by 1
    size++;
  }

  /** Create a new larger array, double the current size */
  private void ensureCapacity() {
    if (size >= data.length) {
      E[] newData = (E[])(new Object[size * 2 + 1]);
      System.arraycopy(data, 0, newData, 0, size);
      data = newData;
    }
  }

  /** Clear the list */
  public void clear() {
    data = (E[])new Object[INITIAL_CAPACITY];
    size = 0;
  }

  /** Return true if this list contains the element */
  public boolean contains(E e) {
    for (int i = 0; i < size; i++)
      if (e.equals(data[i])) return true;

    return false;
  }

  /** Return the element from this list at the specified index */
  public E get(int index) {
    return data[index];
  }

  /** Return the index of the first matching element in this list.
   *  Return -1 if no match. */
  public int indexOf(E e) {
    for (int i = 0; i < size; i++)
      if (e.equals(data[i])) return i;

    return -1;
  }

  /** Return the index of the last matching element in this list
   *  Return -1 if no match. */
  public int lastIndexOf(E e) {
    for (int i = size - 1; i >= 0; i--)
      if (e.equals(data[i])) return i;

    return -1;
  }

  /** Remove the element at the specified position in this list
   *  Shift any subsequent elements to the left.
   *  Return the element that was removed from the list. */
  public E remove(int index) {
    E e = data[index];

    // Shift data to the left
    for (int j = index; j < size - 1; j++)
      data[j] = data[j + 1];

    data[size - 1] = null; // This element is now null

    // Decrement size
    size--;

    return e;
  }

  /** Replace the element at the specified position in this list
   *  with the specified element. */
  public E set(int index, E e) {
    E old = data[index];
    data[index] = e;
    return old;
  }

  /** Override toString() to return elements in the list */
  public String toString() {
    StringBuilder result = new StringBuilder("[");

    for (int i = 0; i < size; i++) {
      result.append(data[i]);
      if (i < size - 1) result.append(", ");
    }

    return result.toString() + "]";
  }

  /** Trims the capacity to current size */
  public void trimToSize() {
    if (size != data.length) { // If size == capacity, no need to trim
      E[] newData = (E[])(new Object[size]);
      System.arraycopy(data, 0, newData, 0, size);
      data = newData;
    }
  }
}

MyList.java

public interface MyList<E> {
  /** Add a new element at the end of this list */
  public void add(E e);

  /** Add a new element at the specified index in this list */
  public void add(int index, E e);

  /** Clear the list */
  public void clear();

  /** Return true if this list contains the element */
  public boolean contains(E e);

  /** Return the element from this list at the specified index */
  public E get(int index);

  /** Return the index of the first matching element in this list.
   *  Return -1 if no match. */
  public int indexOf(E e);

  /** Return true if this list contains no elements */
  public boolean isEmpty();

  /** Return the index of the last matching element in this list
   *  Return -1 if no match. */
  public int lastIndexOf(E e);

  /** Remove the first occurrence of the element o from this list.
   *  Shift any subsequent elements to the left.
   *  Return true if the element is removed. */
  public boolean remove(E e);

  /** Remove the element at the specified position in this list
   *  Shift any subsequent elements to the left.
   *  Return the element that was removed from the list. */
  public E remove(int index);

  /** Replace the element at the specified position in this list
   *  with the specified element and returns the new set. */
  public Object set(int index, E e);

  /** Return the number of elements in this list */
  public int size();
}

MyAbstractList.java

public abstract class MyAbstractList<E> implements MyList<E> {
  protected int size = 0; // The size of the list

  /** Create a default list */
  protected MyAbstractList() {
  }

  /** Create a list from an array of objects */
  protected MyAbstractList(E[] objects) {
    for (int i = 0; i < objects.length; i++)
      add(objects[i]);
  }

  /** Add a new element at the end of this list */
  public void add(E e) {
    add(size, e);
  }

  /** Return true if this list contains no elements */
  public boolean isEmpty() {
    return size == 0;
  }

  /** Return the number of elements in this list */
  public int size() {
    return size;
  }

  /** Remove the first occurrence of the element o from this list.
   *  Shift any subsequent elements to the left.
   *  Return true if the element is removed. */
  public boolean remove(E e) {
    if (indexOf(e) >= 0) {
      remove(indexOf(e));
      return true;
    }
    else
      return false;
  }
}

TestMyArrayList.java

public class TestMyArrayList {
    public static void main(String[] args)
    {

        String[] str = {"manisa","turkey","germany"};

        MyList<String> list = new MyArrayList<String>(str);

        list.add("America");
        list.add(0,"Canada");
        list.add(1,"England");
        System.out.println(list);
    }
}

这是错误代码：

Exception in thread "main" java.lang.NullPointerException
    at MyArrayList.ensureCapacity(MyArrayList.java:36)
    at MyArrayList.add(MyArrayList.java:21)
    at MyAbstractList.add(MyAbstractList.java:16)
    at MyAbstractList.<init>(MyAbstractList.java:11)
    at MyArrayList.<init>(MyArrayList.java:16)
    at TestMyArrayList.main(TestMyArrayList.java:8)

Answer 1

这里的问题是MyAbstractList中的构造函数在初始化data之前调用add 。

data字段在MyArrayList类中声明和初始化。 但是直到超类初始化完成后才进行初始化，并且在超类初始化期间进行add调用...当data仍为null 。

---

这里的一般问题是，构造函数调用可能被子类覆盖的方法是危险的。 可能在子类初始化发生之前调用重写方法。

在这种情况下， add(E)可能会被覆盖。 更糟糕的是， add(E)调用add(E, int) ，因为它在超类中是抽象的，所以肯定会被覆盖。

Answer 2

让我们将代码简化为基本要点：

public abstract class MyAbstractList<E> {
  protected int size = 0; // The size of the list

  protected MyAbstractList() {}

  protected MyAbstractList(E[] objects) {
    for (int i = 0; i < objects.length; i++)
      add(objects[i]);
}

---

public class MyArrayList<E> extends MyAbstractList<E> {
  public static final int INITIAL_CAPACITY = 16;
  private E[] data = (E[])new Object[INITIAL_CAPACITY];

  public MyArrayList(E[] objects) {
     super(objects); // this call to super() executes before data is initialized
  }
}

---

public static void main(String[] args) {
  String[] str = {"manisa","turkey","germany"};
  MyList<String> list = new MyArrayList<String>(str);
}

要了解的重要一点是，父类的构造函数在子类甚至尚未初始化之前就被调用（这就是为什么super()始终必须是构造函数中的第一个调用），这意味着在MyAbstractList的构造函数运行时， data仍然为null 。

用内容替换super()调用意味着在正确设置data 之后 ， 在 MyArrayList初始化时执行for循环。

本质上，问题在于MyAbstractList提供了一个构造函数，该构造函数调用将MyAbstractList类覆盖的方法，这是一种严重的反模式。 MyAbstractList不应该提供所有样式的构造函数。

有关更多信息，请参见有效的Java项目17，其中指出：

构造函数不得直接或间接调用可重写的方法 。 如果违反此规则，将导致程序失败。 超类构造函数在子类构造函数之前运行，因此子类中的重写方法将在子类构造函数运行之前被调用。 如果重写方法依赖于子类构造函数执行的任何初始化，则该方法将无法正常工作。