jdk源码_List

李明华

2020-10-08 (Updated: 2021-05-12)

List

继承了Collection，Collection继承了Iterable

文档简述：
有序的集合，也被称为序列，此接口的使用者可以对插入List中的每个元素进行精准的控制，可以通过下标搜索或访问List中的每一个元素。
与set不同，List允许重复的元素。
索引从零开始。
ListIterator：更强大的迭代器，继承于Iterator接口,只能用于各种List类型的访问。
List接口提供了两个方法来搜索指定的对象，但在很多实现中，它们将执行高开销的线性搜索。
List接口提供了两种方法来有效地插入，并在该列表中的任意点移除多个元件。
虽然List允许把自身做为元素包含,但equals方法和hashCode方法在这样的List上将不具备很好的支持性

/**
 * An ordered collection (also known as a <i>sequence</i>).  The user of this
 * interface has precise control over where in the list each element is
 * inserted.  The user can access elements by their integer index (position in
 * the list), and search for elements in the list.<p>
 * <p>
 * Unlike sets, lists typically allow duplicate elements.  More formally,
 * lists typically allow pairs of elements <tt>e1</tt> and <tt>e2</tt>
 * such that <tt>e1.equals(e2)</tt>, and they typically allow multiple
 * null elements if they allow null elements at all.  It is not inconceivable
 * that someone might wish to implement a list that prohibits duplicates, by
 * throwing runtime exceptions when the user attempts to insert them, but we
 * expect this usage to be rare.<p>
 * <p>
 * The <tt>List</tt> interface places additional stipulations, beyond those
 * specified in the <tt>Collection</tt> interface, on the contracts of the
 * <tt>iterator</tt>, <tt>add</tt>, <tt>remove</tt>, <tt>equals</tt>, and
 * <tt>hashCode</tt> methods.  Declarations for other inherited methods are
 * also included here for convenience.<p>
 * <p>
 * The <tt>List</tt> interface provides four methods for positional (indexed)
 * access to list elements.  Lists (like Java arrays) are zero based.  Note
 * that these operations may execute in time proportional to the index value
 * for some implementations (the <tt>LinkedList</tt> class, for
 * example). Thus, iterating over the elements in a list is typically
 * preferable to indexing through it if the caller does not know the
 * implementation.<p>
 * <p>
 * The <tt>List</tt> interface provides a special iterator, called a
 * <tt>ListIterator</tt>, that allows element insertion and replacement, and
 * bidirectional access in addition to the normal operations that the
 * <tt>Iterator</tt> interface provides.  A method is provided to obtain a
 * list iterator that starts at a specified position in the list.<p>
 * <p>
 * The <tt>List</tt> interface provides two methods to search for a specified
 * object.  From a performance standpoint, these methods should be used with
 * caution.  In many implementations they will perform costly linear
 * searches.<p>
 * <p>
 * The <tt>List</tt> interface provides two methods to efficiently insert and
 * remove multiple elements at an arbitrary point in the list.<p>
 * <p>
 * Note: While it is permissible for lists to contain themselves as elements,
 * extreme caution is advised: the <tt>equals</tt> and <tt>hashCode</tt>
 * methods are no longer well defined on such a list.
 *
 * <p>Some list implementations have restrictions on the elements that
 * they may contain.  For example, some implementations prohibit null elements,
 * and some have restrictions on the types of their elements.  Attempting to
 * add an ineligible element throws an unchecked exception, typically
 * <tt>NullPointerException</tt> or <tt>ClassCastException</tt>.  Attempting
 * to query the presence of an ineligible element may throw an exception,
 * or it may simply return false; some implementations will exhibit the former
 * behavior and some will exhibit the latter.  More generally, attempting an
 * operation on an ineligible element whose completion would not result in
 * the insertion of an ineligible element into the list may throw an
 * exception or it may succeed, at the option of the implementation.
 * Such exceptions are marked as "optional" in the specification for this
 * interface.
 *
 * <p>This interface is a member of the
 * <a href="{@docRoot}/../technotes/guides/collections/index.html">
 * Java Collections Framework</a>.
 *
 * @param <E> the type of elements in this list
 * @author Josh Bloch
 * @author Neal Gafter
 * @see Collection
 * @see Set
 * @see ArrayList
 * @see LinkedList
 * @see Vector
 * @see Arrays#asList(Object[])
 * @see Collections#nCopies(int, Object)
 * @see Collections#EMPTY_LIST
 * @see AbstractList
 * @see AbstractSequentialList
 * @since 1.2
 */

public interface List<E> extends Collection<E> {
	...
}

AbstractList

1
2
3

public abstract class AbstractList<E> extends AbstractCollection<E> implements List<E> {
	...
}

modConut

/**
 * The number of times this list has been <i>structurally modified</i>.
 * Structural modifications are those that change the size of the
 * list, or otherwise perturb it in such a fashion that iterations in
 * progress may yield incorrect results.
 *
 * <p>This field is used by the iterator and list iterator implementation
 * returned by the {@code iterator} and {@code listIterator} methods.
 * If the value of this field changes unexpectedly, the iterator (or list
 * iterator) will throw a {@code ConcurrentModificationException} in
 * response to the {@code next}, {@code remove}, {@code previous},
 * {@code set} or {@code add} operations.  This provides
 * <i>fail-fast</i> behavior, rather than non-deterministic behavior in
 * the face of concurrent modification during iteration.
 *
 * <p><b>Use of this field by subclasses is optional.</b> If a subclass
 * wishes to provide fail-fast iterators (and list iterators), then it
 * merely has to increment this field in its {@code add(int, E)} and
 * {@code remove(int)} methods (and any other methods that it overrides
 * that result in structural modifications to the list).  A single call to
 * {@code add(int, E)} or {@code remove(int)} must add no more than
 * one to this field, or the iterators (and list iterators) will throw
 * bogus {@code ConcurrentModificationExceptions}.  If an implementation
 * does not wish to provide fail-fast iterators, this field may be
 * ignored.
 */
protected transient int modCount = 0;

Itr

ListItr

AbstractSequentialList

有序List

1
2
3

public abstract class AbstractSequentialList<E> extends AbstractList<E> {
	...
}

get() – AbstractObjectList

public Object get(int index) {
    if (index >= 0 && index < getLength()) {
        return item(index);
    }
    throw new IndexOutOfBoundsException("Index: " + index);
}

get() – AbstractSequentialList

/**
 * Returns the element at the specified position in this list.
 *
 * <p>This implementation first gets a list iterator pointing to the
 * indexed element (with <tt>listIterator(index)</tt>).  Then, it gets
 * the element using <tt>ListIterator.next</tt> and returns it.
 *
 * @throws IndexOutOfBoundsException {@inheritDoc}
 */
public E get(int index) {
    try {
        return listIterator(index).next();
    } catch (NoSuchElementException exc) {
        throw new IndexOutOfBoundsException("Index: " + index);
    }
}

listIterator(index)

/**
 * Returns a list-iterator of the elements in this list (in proper
 * sequence), starting at the specified position in the list.
 * Obeys the general contract of {@code List.listIterator(int)}.<p>
 * <p>
 * The list-iterator is <i>fail-fast</i>: if the list is structurally
 * modified at any time after the Iterator is created, in any way except
 * through the list-iterator's own {@code remove} or {@code add}
 * methods, the list-iterator will throw a
 * {@code ConcurrentModificationException}.  Thus, in the face of
 * concurrent modification, the iterator fails quickly and cleanly, rather
 * than risking arbitrary, non-deterministic behavior at an undetermined
 * time in the future.
 *
 * @param index index of the first element to be returned from the
 *              list-iterator (by a call to {@code next})
 * @return a ListIterator of the elements in this list (in proper
 * sequence), starting at the specified position in the list
 * @throws IndexOutOfBoundsException {@inheritDoc}
 * @see List#listIterator(int)
 */
public ListIterator<E> listIterator(int index) {
    checkPositionIndex(index);
    return new ListItr(index);
}

ListIterator

/**
 * An iterator for lists that allows the programmer
 * to traverse the list in either direction, modify
 * the list during iteration, and obtain the iterator's
 * current position in the list. A {@code ListIterator}
 * has no current element; its <I>cursor position</I> always
 * lies between the element that would be returned by a call
 * to {@code previous()} and the element that would be
 * returned by a call to {@code next()}.
 * An iterator for a list of length {@code n} has {@code n+1} possible
 * cursor positions, as illustrated by the carets ({@code ^}) below:
 * <PRE>
 *                      Element(0)   Element(1)   Element(2)   ... Element(n-1)
 * cursor positions:  ^            ^            ^            ^                  ^
 * </PRE>
 * Note that the {@link #remove} and {@link #set(Object)} methods are
 * <i>not</i> defined in terms of the cursor position;  they are defined to
 * operate on the last element returned by a call to {@link #next} or
 * {@link #previous()}.
 *
 * <p>This interface is a member of the
 * <a href="{@docRoot}/../technotes/guides/collections/index.html">
 * Java Collections Framework</a>.
 *
 * @author  Josh Bloch
 * @see Collection
 * @see List
 * @see Iterator
 * @see Enumeration
 * @see List#listIterator()
 * @since   1.2
 */
public interface ListIterator<E> extends Iterator<E> {
	...
}

next()

/**
 * Returns the next element in the list and advances the cursor position.
 * This method may be called repeatedly to iterate through the list,
 * or intermixed with calls to {@link #previous} to go back and forth.
 * (Note that alternating calls to {@code next} and {@code previous}
 * will return the same element repeatedly.)
 *
 * @return the next element in the list
 * @throws NoSuchElementException if the iteration has no next element
 */
E next();

previous()

/**
 * Returns the previous element in the list and moves the cursor
 * position backwards.  This method may be called repeatedly to
 * iterate through the list backwards, or intermixed with calls to
 * {@link #next} to go back and forth.  (Note that alternating calls
 * to {@code next} and {@code previous} will return the same
 * element repeatedly.)
 *
 * @return the previous element in the list
 * @throws NoSuchElementException if the iteration has no previous
 *         element
 */
E previous();

CopyOnWriteArrayList

线程安全的ArrayList结构

/**
 * A thread-safe variant of {@link java.util.ArrayList} in which all mutative
 * operations ({@code add}, {@code set}, and so on) are implemented by
 * making a fresh copy of the underlying array.
 *
 * <p>This is ordinarily too costly, but may be <em>more</em> efficient
 * than alternatives when traversal operations vastly outnumber
 * mutations, and is useful when you cannot or don't want to
 * synchronize traversals, yet need to preclude interference among
 * concurrent threads.  The "snapshot" style iterator method uses a
 * reference to the state of the array at the point that the iterator
 * was created. This array never changes during the lifetime of the
 * iterator, so interference is impossible and the iterator is
 * guaranteed not to throw {@code ConcurrentModificationException}.
 * The iterator will not reflect additions, removals, or changes to
 * the list since the iterator was created.  Element-changing
 * operations on iterators themselves ({@code remove}, {@code set}, and
 * {@code add}) are not supported. These methods throw
 * {@code UnsupportedOperationException}.
 *
 * <p>All elements are permitted, including {@code null}.
 *
 * <p>Memory consistency effects: As with other concurrent
 * collections, actions in a thread prior to placing an object into a
 * {@code CopyOnWriteArrayList}
 * <a href="package-summary.html#MemoryVisibility"><i>happen-before</i></a>
 * actions subsequent to the access or removal of that element from
 * the {@code CopyOnWriteArrayList} in another thread.
 *
 * <p>This class is a member of the
 * <a href="{@docRoot}/../technotes/guides/collections/index.html">
 * Java Collections Framework</a>.
 *
 * @param <E> the type of elements held in this collection
 * @author Doug Lea
 * @since 1.5
 */
public class CopyOnWriteArrayList<E>
        implements List<E>, RandomAccess, Cloneable, java.io.Serializable {
	...
}

COWIterator

1
2
3

static final class COWIterator<E> implements ListIterator<E> {
	...
}

ArrayList

get() – arrayList

public E get(int index) {
    rangeCheck(index);
		
    return elementData(index);
}

检查是否越界，返回对应下标元素

elementData

/**
    * The array buffer into which the elements of the ArrayList are stored.
    * The capacity of the ArrayList is the length of this array buffer. Any
    * empty ArrayList with elementData == DEFAULTCAPACITY_EMPTY_ELEMENTDATA
    * will be expanded to DEFAULT_CAPACITY when the first element is added.
    */
   transient Object[] elementData; // non-private to simplify nested class access

存储ArrayList中的元素

1	private static final Object[] EMPTY_ELEMENTDATA = {};

空ArrayList

1	private static final int DEFAULT_CAPACITY = 10;

1	private static final Object[] DEFAULTCAPACITY_EMPTY_ELEMENTDATA = {};

1	private static final Object[] EMPTY_ELEMENTDATA = {};

初始化时的默认大小
当初始化ArrayList实例时，若为实例赋大小，则使用 EMPTY_ELEMENTDATA，若不赋大小，则使用 DEFAULTCAPACITY_EMPTY_ELEMENTDATA
区别在当向空的ArrayList添加元素时

   public ArrayList(int initialCapacity) {
       if (initialCapacity > 0) {
           this.elementData = new Object[initialCapacity];
       } else if (initialCapacity == 0) {
           this.elementData = EMPTY_ELEMENTDATA;
       } else {
           throw new IllegalArgumentException("Illegal Capacity: " +
                   initialCapacity);
       }
   }
   public ArrayList() {
       this.elementData = DEFAULTCAPACITY_EMPTY_ELEMENTDATA;
   }

...

   private static int calculateCapacity(Object[] elementData, int minCapacity) {
       if (elementData == DEFAULTCAPACITY_EMPTY_ELEMENTDATA) {
           return Math.max(DEFAULT_CAPACITY, minCapacity);
       }
       return minCapacity;
   }

   private void ensureCapacityInternal(int minCapacity) {
       ensureExplicitCapacity(calculateCapacity(elementData, minCapacity));
   }

   private void ensureExplicitCapacity(int minCapacity) {
       modCount++;

       // overflow-conscious code
       if (minCapacity - elementData.length > 0)
           grow(minCapacity);
   }

...

   public boolean add(E e) {
       ensureCapacityInternal(size + 1);  // Increments modCount!!
       elementData[size++] = e;
       return true;
   }

   private void grow(int minCapacity) {
       // overflow-conscious code
       int oldCapacity = elementData.length;
       int newCapacity = oldCapacity + (oldCapacity >> 1);
       if (newCapacity - minCapacity < 0)
           newCapacity = minCapacity;
       if (newCapacity - MAX_ARRAY_SIZE > 0)
           newCapacity = hugeCapacity(minCapacity);
       // minCapacity is usually close to size, so this is a win:
       elementData = Arrays.copyOf(elementData, newCapacity);
   }

next()

public E next() {
    checkForComodification();
    int i = cursor;
    if (i >= size)
        throw new NoSuchElementException();
    Object[] elementData = ArrayList.this.elementData;
    if (i >= elementData.length)
        throw new ConcurrentModificationException();
    cursor = i + 1;
    return (E) elementData[lastRet = i];
}

previous()

public E previous() {
    checkForComodification();
    int i = cursor - 1;
    if (i < 0)
        throw new NoSuchElementException();
    Object[] elementData = ArrayList.this.elementData;
    if (i >= elementData.length)
        throw new ConcurrentModificationException();
    cursor = i;
    return (E) elementData[lastRet = i];
}

LinkedList

next()

public E next() {
    checkForComodification();
    if (!hasNext())
        throw new NoSuchElementException();

    lastReturned = next;
    next = next.next;
    nextIndex++;
    return lastReturned.item;
}

previous()

public E previous() {
    checkForComodification();
    if (!hasPrevious())
        throw new NoSuchElementException();

    lastReturned = next = (next == null) ? last : next.prev;
    nextIndex--;
    return lastReturned.item;
}