Java source code learning (JDK 11)-java.lang.Collection

AbstractCollection : abstract class
- AbstractSet : abstract class
  - HashSet : implement the Set interface
    - LinkedHashSet : implement the Set interface
  - TreeSet : implements the NavigableSet interface
- AbstractList : abstract class
  - ArrayList : implements the List interface
  - Vector : implements the List interface
    - Stack
  - AbstractSequentialList : abstract class
    - LinkedList : implements List and Deque interfaces
Set: interface
- SortedSet: interface
  - NavigableSet: interface
List: Interface
Queue: interface
- Deque: interface

Collection

public interface Collection<E> extends Iterable<E> {
    // Query Operations
    int size();
    boolean isEmpty();
    boolean contains(Object o);
    Iterator<E> iterator();
    Object[] toArray();
    <T> T[] toArray(T[] a);

    default <T> T[] toArray(IntFunction<T[]> generator) {
        return toArray(generator.apply(0));
    }

    // Modification Operations
    boolean add(E e);
    boolean remove(Object o);


    // Bulk Operations
    boolean containsAll(Collection<?> c);
    boolean addAll(Collection<? extends E> c);
    boolean removeAll(Collection<?> c);

    default boolean removeIf(Predicate<? super E> filter) {
        Objects.requireNonNull(filter);
        boolean removed = false;
        final Iterator<E> each = iterator();
        while (each.hasNext()) {
            if (filter.test(each.next())) {
                each.remove();
                removed = true;
            }
        }
        return removed;
    }

    boolean retainAll(Collection<?> c);
    void clear();


    // Comparison and hashing
    boolean equals(Object o);
    int hashCode();

    @Override
    default Spliterator<E> spliterator() {
        return Spliterators.spliterator(this, 0);
    }

    default Stream<E> stream() {
        return StreamSupport.stream(spliterator(), false);
    }

    default Stream<E> parallelStream() {
        return StreamSupport.stream(spliterator(), true);
    }
}

Collection implements Iterable interface, with iterator () method, which can be iterated.

AbstractCollection

public abstract class AbstractCollection<E> implements Collection<E> {
    protected AbstractCollection() {
    }

    // Query Operations
    public abstract Iterator<E> iterator();
    public abstract int size();

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

    public boolean contains(Object o) {
        Iterator<E> it = iterator();
        if (o==null) {
            while (it.hasNext())
                if (it.next()==null)
                    return true;
        } else {
            while (it.hasNext())
                if (o.equals(it.next()))
                    return true;
        }
        return false;
    }

    public Object[] toArray() {
        // Estimate size of array; be prepared to see more or fewer elements
        Object[] r = new Object[size()];
        Iterator<E> it = iterator();
        for (int i = 0; i < r.length; i++) {
            if (! it.hasNext()) // fewer elements than expected
                return Arrays.copyOf(r, i);
            r[i] = it.next();
        }
        return it.hasNext() ? finishToArray(r, it) : r;
    }

    @SuppressWarnings("unchecked")
    public <T> T[] toArray(T[] a) { /*...*/ }


    private static final int MAX_ARRAY_SIZE = Integer.MAX_VALUE - 8;

    // 重新分配数组空间 供 toArray 方法使用
    @SuppressWarnings("unchecked")
    private static <T> T[] finishToArray(T[] r, Iterator<?> it) {
        int i = r.length;
        while (it.hasNext()) {
            int cap = r.length;
            if (i == cap) {
                int newCap = cap + (cap >> 1) + 1;	// 扩容大小 1.5 倍 + 1
                // overflow-conscious code
                if (newCap - MAX_ARRAY_SIZE > 0)
                    newCap = hugeCapacity(cap + 1);
                r = Arrays.copyOf(r, newCap);
            }
            r[i++] = (T)it.next();
        }
        // trim if overallocated
        return (i == r.length) ? r : Arrays.copyOf(r, i);
    }

    private static int hugeCapacity(int minCapacity) {
        if (minCapacity < 0) // overflow
            throw new OutOfMemoryError("Required array size too large");
        return (minCapacity > MAX_ARRAY_SIZE) ? Integer.MAX_VALUE : MAX_ARRAY_SIZE;
    }

    // Modification Operations

    public boolean add(E e) {	// 支持 add 的 Collection 覆盖该方法 不支持的直接抛出异常
        throw new UnsupportedOperationException();
    }

    public boolean remove(Object o) {
        Iterator<E> it = iterator();
        if (o==null) {
            while (it.hasNext()) {
                if (it.next()==null) {
                    it.remove();
                    return true;
                }
            }
        } else {
            while (it.hasNext()) {
                if (o.equals(it.next())) {
                    it.remove();
                    return true;
                }
            }
        }
        return false;
    }


    // Bulk Operations
    public boolean containsAll(Collection<?> c) { /*...*/ }

    public boolean addAll(Collection<? extends E> c) { /*...*/ }

    public boolean removeAll(Collection<?> c) { /*...*/ }

    public boolean retainAll(Collection<?> c) { /*...*/ }

    public void clear() {
        Iterator<E> it = iterator();
        while (it.hasNext()) {
            it.next();
            it.remove();
        }
    }

    //  String conversion
    public String toString() { /*...*/ }
}

AbstractCollection provides the implementation of almost all methods. Various operations are based on the iterator () abstract method, so subclasses only need to implement the iterator () and size () methods, and the modifiable Collection needs to provide the implementation of add.

AbstractSet

public abstract class AbstractSet<E> extends AbstractCollection<E> implements Set<E> {

    protected AbstractSet() {
    }

    // Comparison and hashing

    public boolean equals(Object o) {
        if (o == this)
            return true;

        if (!(o instanceof Set))
            return false;
        Collection<?> c = (Collection<?>) o;
        if (c.size() != size())
            return false;
        try {
            return containsAll(c);
        } catch (ClassCastException | NullPointerException unused) {
            return false;
        }
    }

    public int hashCode() { /*...*/ }

    public boolean removeAll(Collection<?> c) {
        Objects.requireNonNull(c);
        boolean modified = false;

        if (size() > c.size()) {
            for (Object e : c)
                modified |= remove(e);
        } else {
            for (Iterator<?> i = iterator(); i.hasNext(); ) {
                if (c.contains(i.next())) {
                    i.remove();
                    modified = true;
                }
            }
        }
        return modified;
    }
}

AbstractSet covers the hashCode and equals methods, and equals is implemented using containsAll.
AbstractSet overrides the removeAll method and does not require an iterator for larger capacity cases.

HashSet

public class HashSet<E>
    extends AbstractSet<E>
    implements Set<E>, Cloneable, java.io.Serializable
{
    static final long serialVersionUID = -5024744406713321676L;

    // HashSet 内部利用 HashMap 的 keySet() 来实现
    private transient HashMap<E,Object> map;

    // Dummy value to associate with an Object in the backing Map
    // 作为 Map 键值对里的 value
    private static final Object PRESENT = new Object();

    // default initial capacity (16) and load factor (0.75).
    public HashSet() {
        map = new HashMap<>();
    }

    public HashSet(Collection<? extends E> c) {
        map = new HashMap<>(Math.max((int) (c.size()/.75f) + 1, 16));
        addAll(c);
    }

    public HashSet(int initialCapacity, float loadFactor) {
        map = new HashMap<>(initialCapacity, loadFactor);
    }

    public HashSet(int initialCapacity) {
        map = new HashMap<>(initialCapacity);
    }

    // 提供给 LinkedHashSet 使用
    HashSet(int initialCapacity, float loadFactor, boolean dummy) {
        map = new LinkedHashMap<>(initialCapacity, loadFactor);
    }

    public Iterator<E> iterator() {
        return map.keySet().iterator();
    }

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

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

    public boolean contains(Object o) {
        return map.containsKey(o);
    }

    public boolean add(E e) {
        return map.put(e, PRESENT)==null;
    }

    public boolean remove(Object o) {
        return map.remove(o)==PRESENT;
    }

    public void clear() {
        map.clear();
    }

    @SuppressWarnings("unchecked")
    public Object clone() { /*...*/ }

    private void writeObject(java.io.ObjectOutputStream s) throws java.io.IOException { /*...*/ }

    private void readObject(java.io.ObjectInputStream s) throws java.io.IOException, ClassNotFoundException { /*...*/ }

    public Spliterator<E> spliterator() {
        return new HashMap.KeySpliterator<>(map, 0, -1, 0, 0);
    }
}

HashSet is implemented internally based on HashMap , and covers add, remove and other methods to improve efficiency.

LinkedHashSet

public class LinkedHashSet<E>
    extends HashSet<E>
    implements Set<E>, Cloneable, java.io.Serializable {

    // 构造函数借助于 HashSet 中的 HashSet(int initialCapacity, float loadFactor, boolean dummy) 方法
    // 该方法实际创建了一个 LinkedHashMap

    public LinkedHashSet(Collection<? extends E> c) {
        super(Math.max(2*c.size(), 11), .75f, true);
        addAll(c);
    }

    @Override
    public Spliterator<E> spliterator() {
        return Spliterators.spliterator(this, Spliterator.DISTINCT | Spliterator.ORDERED);
    }
}

LinkedHashSet is almost exactly the same as HashSet, but LinkedHashMap is used internally .
LinkedHashSet maintain the insertion order .

TreeSet

public class TreeSet<E> extends AbstractSet<E>
    implements NavigableSet<E>, Cloneable, java.io.Serializable
{

    private transient NavigableMap<E,Object> m;

    private static final Object PRESENT = new Object();

    TreeSet(NavigableMap<E,Object> m) {
        this.m = m;
    }

    // 内部基于 TreeMap
    public TreeSet() {
        this(new TreeMap<>());
    }

    // 构造器

    public Iterator<E> iterator() {
        return m.navigableKeySet().iterator();
    }

    public Iterator<E> descendingIterator() {
        return m.descendingKeySet().iterator();
    }

    public NavigableSet<E> descendingSet() {
        return new TreeSet<>(m.descendingMap());
    }

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

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

    public boolean contains(Object o) {
        return m.containsKey(o);
    }

    public boolean add(E e) {
        return m.put(e, PRESENT)==null;
    }

    public boolean remove(Object o) {
        return m.remove(o)==PRESENT;
    }

    public void clear() {
        m.clear();
    }

    public  boolean addAll(Collection<? extends E> c) {
        // Use linear-time version if applicable
        if (m.size()==0 && c.size() > 0 &&
            c instanceof SortedSet &&
            m instanceof TreeMap) {
            SortedSet<? extends E> set = (SortedSet<? extends E>) c;
            TreeMap<E,Object> map = (TreeMap<E, Object>) m;
            Comparator<?> cc = set.comparator();
            Comparator<? super E> mc = map.comparator();
            if (cc==mc || (cc != null && cc.equals(mc))) {
                map.addAllForTreeSet(set, PRESENT);
                return true;
            }
        }
        return super.addAll(c);
    }

    public NavigableSet<E> subSet(E fromElement, boolean fromInclusive,  E toElement,   boolean toInclusive) {
        return new TreeSet<>(m.subMap(fromElement, fromInclusive, toElement, toInclusive));
    }

    public NavigableSet<E> headSet(E toElement, boolean inclusive) {
        return new TreeSet<>(m.headMap(toElement, inclusive));
    }

    public NavigableSet<E> tailSet(E fromElement, boolean inclusive) {
        return new TreeSet<>(m.tailMap(fromElement, inclusive));
    }

    public SortedSet<E> subSet(E fromElement, E toElement) { return subSet(fromElement, true, toElement, false); }

    public SortedSet<E> headSet(E toElement) { return headSet(toElement, false); }

    public SortedSet<E> tailSet(E fromElement) { return tailSet(fromElement, true); }

    public Comparator<? super E> comparator() { return m.comparator(); }

    public E first() { return m.firstKey(); }

    public E last() { return m.lastKey(); }

    // NavigableSet API methods

    public E lower(E e) { return m.lowerKey(e); }

    public E floor(E e) { return m.floorKey(e); }

    public E ceiling(E e) { return m.ceilingKey(e); }

    public E higher(E e) { return m.higherKey(e); }

    public E pollFirst() {
        Map.Entry<E,?> e = m.pollFirstEntry();
        return (e == null) ? null : e.getKey();
    }

    public E pollLast() {
        Map.Entry<E,?> e = m.pollLastEntry();
        return (e == null) ? null : e.getKey();
    }

    @SuppressWarnings("unchecked")
    public Object clone() { /*...*/ }

    private void writeObject(java.io.ObjectOutputStream s) throws java.io.IOException { /*...*/ }

    private void readObject(java.io.ObjectInputStream s) throws java.io.IOException, ClassNotFoundException { /*...*/ }

    public Spliterator<E> spliterator() {
        return TreeMap.keySpliteratorFor(m);
    }

    private static final long serialVersionUID = -2479143000061671589L;
}

The implementation of TreeMap and HashMap is not much different. It depends on TreeMap internally and implements the NavigableSet interface, providing a series of orderly operations.
TreeMap covers the addAll method. If the comparators of two ordered sets are the same, they can be completed in linear time.

AbstractList

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

    protected AbstractList() {
    }

    public boolean add(E e) {
        add(size(), e);	// 在末尾插入
        return true;
    }

    public abstract E get(int index);

    public E set(int index, E element) {
        throw new UnsupportedOperationException();
    }

    public void add(int index, E element) {
        throw new UnsupportedOperationException();
    }

    public E remove(int index) {
        throw new UnsupportedOperationException();
    }


    // Search Operations

    public int indexOf(Object o) {
        ListIterator<E> it = listIterator();
        if (o==null) {
            while (it.hasNext())
                if (it.next()==null)
                    return it.previousIndex();
        } else {
            while (it.hasNext())
                if (o.equals(it.next()))
                    return it.previousIndex();
        }
        return -1;
    }

    public int lastIndexOf(Object o) { /*...*/ }	// 与 indexOf 类似 从后向前遍历


    // Bulk Operations

    public void clear() {
        removeRange(0, size());
    }

    public boolean addAll(int index, Collection<? extends E> c) {
        rangeCheckForAdd(index);
        boolean modified = false;
        for (E e : c) {
            add(index++, e);
            modified = true;
        }
        return modified;
    }


    // Iterators
    // 借助内部类来实现
    public Iterator<E> iterator() { return new Itr(); }

    public ListIterator<E> listIterator() { return listIterator(0); }

    public ListIterator<E> listIterator(final int index) {
        rangeCheckForAdd(index);
        return new ListItr(index);
    }

    private class Itr implements Iterator<E> {
        /**
         * Index of element to be returned by subsequent call to next.
         */
        int cursor = 0;

        /**
         * Index of element returned by most recent call to next or
         * previous.  Reset to -1 if this element is deleted by a call
         * to remove.
         */
        int lastRet = -1;

        /**
         * The modCount value that the iterator believes that the backing
         * List should have.  If this expectation is violated, the iterator
         * has detected concurrent modification.
         */
        int expectedModCount = modCount;

        public boolean hasNext() {
            return cursor != size();
        }

        public E next() {
            checkForComodification();
            try {
                int i = cursor;
                E next = get(i);
                lastRet = i;
                cursor = i + 1;
                return next;
            } catch (IndexOutOfBoundsException e) {
                checkForComodification();
                throw new NoSuchElementException();
            }
        }

        public void remove() {
            if (lastRet < 0)
                throw new IllegalStateException();
            checkForComodification();

            try {
                AbstractList.this.remove(lastRet);
                if (lastRet < cursor)
                    cursor--;
                lastRet = -1;
                expectedModCount = modCount;
            } catch (IndexOutOfBoundsException e) {
                throw new ConcurrentModificationException();
            }
        }

        final void checkForComodification() {
            if (modCount != expectedModCount)
                throw new ConcurrentModificationException();
        }
    }

    private class ListItr extends Itr implements ListIterator<E> {
        ListItr(int index) {
            cursor = index;
        }

        public boolean hasPrevious() {
            return cursor != 0;
        }

        public E previous() {
            checkForComodification();
            try {
                int i = cursor - 1;
                E previous = get(i);
                lastRet = cursor = i;
                return previous;
            } catch (IndexOutOfBoundsException e) {
                checkForComodification();
                throw new NoSuchElementException();
            }
        }

        public int nextIndex() {
            return cursor;
        }

        public int previousIndex() {
            return cursor-1;
        }

        public void set(E e) {
            if (lastRet < 0)
                throw new IllegalStateException();
            checkForComodification();

            try {
                AbstractList.this.set(lastRet, e);
                expectedModCount = modCount;
            } catch (IndexOutOfBoundsException ex) {
                throw new ConcurrentModificationException();
            }
        }

        public void add(E e) {
            checkForComodification();

            try {
                int i = cursor;
                AbstractList.this.add(i, e);
                lastRet = -1;
                cursor = i + 1;
                expectedModCount = modCount;
            } catch (IndexOutOfBoundsException ex) {
                throw new ConcurrentModificationException();
            }
        }
    }

    public List<E> subList(int fromIndex, int toIndex) { // 使用静态内部类实现
        subListRangeCheck(fromIndex, toIndex, size());
        return (this instanceof RandomAccess ? new RandomAccessSubList<>(this, fromIndex, toIndex) : new SubList<>(this, fromIndex, toIndex));
    }

    static void subListRangeCheck(int fromIndex, int toIndex, int size) {
        if (fromIndex < 0)
            throw new IndexOutOfBoundsException("fromIndex = " + fromIndex);
        if (toIndex > size)
            throw new IndexOutOfBoundsException("toIndex = " + toIndex);
        if (fromIndex > toIndex)
            throw new IllegalArgumentException("fromIndex(" + fromIndex +
                                               ") > toIndex(" + toIndex + ")");
    }

    // Comparison and hashing

    public boolean equals(Object o) {
        if (o == this)
            return true;
        if (!(o instanceof List))
            return false;

        ListIterator<E> e1 = listIterator();
        ListIterator<?> e2 = ((List<?>) o).listIterator();
        while (e1.hasNext() && e2.hasNext()) {
            E o1 = e1.next();
            Object o2 = e2.next();
            if (!(o1==null ? o2==null : o1.equals(o2)))
                return false;
        }
        return !(e1.hasNext() || e2.hasNext());
    }

    public int hashCode() { /*...*/ }

    protected void removeRange(int fromIndex, int toIndex) {
        ListIterator<E> it = listIterator(fromIndex);
        for (int i=0, n=toIndex-fromIndex; i<n; i++) {
            it.next();
            it.remove();
        }
    }

    // iterator 使用 modCount 值来判断在遍历的过程中是否发生了结构修改
    protected transient int modCount = 0;

    private void rangeCheckForAdd(int index) {
        if (index < 0 || index > size())
            throw new IndexOutOfBoundsException(outOfBoundsMsg(index));
    }

    private String outOfBoundsMsg(int index) {
        return "Index: "+index+", Size: "+size();
    }

    static final class RandomAccessSpliterator<E> implements Spliterator<E> { /*...*/ }

    private static class SubList<E> extends AbstractList<E> { /*...*/ }

    private static class RandomAccessSubList<E> extends SubList<E> implements RandomAccess { /*...*/ }
}

AbstractList internally maintains modCount to record the number of times the structure has changed. During the traversal, the iterator will determine whether the value of modCount is consistent. Inconsistency indicates that there is a concurrent modification problem.
AbstractList has two internal classes, Itr, and ListItr, which inherits Itr, corresponding to the iterator () and listIterator () methods, respectively.
Itr maintains 3 integer variables internally
- cursor: record the current subscript for traversal operation
- lastRet: the last traversed subscript used for the delete operation
- expectedModCount: The expected modCount value, used to determine concurrent modification problems
Itr's traversal operation is achieved by get ().
ListItr inherits Itr and provides bidirectional traversal and traversal starting from a certain index.

ArrayList

public class ArrayList<E> extends AbstractList<E>
        implements List<E>, RandomAccess, Cloneable, java.io.Serializable
{
    private static final long serialVersionUID = 8683452581122892189L;

    // 默认初始容量为 10
    private static final int DEFAULT_CAPACITY = 10;

    // 空 ArrayList 共享使用
    private static final Object[] EMPTY_ELEMENTDATA = {};

    // 使用默认构造器时使用
    private static final Object[] DEFAULTCAPACITY_EMPTY_ELEMENTDATA = {};

    // 使用数组存储对象
    transient Object[] elementData; // non-private to simplify nested class access

    private int size; // 列表当前容量

    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);
        }
    }

    // 默认构造器 数组初始长度为 0 当添加第一个元素时 将数组容量扩充为 10
    public ArrayList() {
        this.elementData = DEFAULTCAPACITY_EMPTY_ELEMENTDATA;
    }

    public ArrayList(Collection<? extends E> c) { /*...*/ }

    public void trimToSize() {
        modCount++;
        if (size < elementData.length) {
            elementData = (size == 0) ? EMPTY_ELEMENTDATA : Arrays.copyOf(elementData, size);
        }
    }

    // 确保数组容量不小于 minCapacity
    // 用于在添加元素前使用 减少添加过程中数组扩容消耗的时间
    public void ensureCapacity(int minCapacity) {
        if (minCapacity > elementData.length
            && !(elementData == DEFAULTCAPACITY_EMPTY_ELEMENTDATA && minCapacity <= DEFAULT_CAPACITY)) {
        	// 当前为默认构造器构造的空列表 且 minCapacity <= 10 不会扩容
        	// 因为该情况下 加入第一个元素时 容量就会扩大到 10
            modCount++;
            grow(minCapacity);
        }
    }

    private static final int MAX_ARRAY_SIZE = Integer.MAX_VALUE - 8;

    // 扩容

    private Object[] grow(int minCapacity) {
        return elementData = Arrays.copyOf(elementData, newCapacity(minCapacity));
    }

    private Object[] grow() {
        return grow(size + 1);
    }

    // minCapacity >= 1.5 * size 扩容到 minCapacity
    // minCapacity <  1.5 * size 扩容 1.5 倍
    // 最少扩容 50%
    private int newCapacity(int minCapacity) {
        // overflow-conscious code
        int oldCapacity = elementData.length;
        int newCapacity = oldCapacity + (oldCapacity >> 1);
        if (newCapacity - minCapacity <= 0) {
            if (elementData == DEFAULTCAPACITY_EMPTY_ELEMENTDATA)	// 当前为默认构造器构造的空列表
                return Math.max(DEFAULT_CAPACITY, minCapacity);		// 至少扩容到 10
            if (minCapacity < 0) // overflow
                throw new OutOfMemoryError();
            return minCapacity;
        }
        return (newCapacity - MAX_ARRAY_SIZE <= 0) ? newCapacity : hugeCapacity(minCapacity);
    }

    private static int hugeCapacity(int minCapacity) {
        if (minCapacity < 0) // overflow
            throw new OutOfMemoryError();
        return (minCapacity > MAX_ARRAY_SIZE) ? Integer.MAX_VALUE : MAX_ARRAY_SIZE;
    }

    // Positional Access Operations

    @SuppressWarnings("unchecked")
    E elementData(int index) {
        return (E) elementData[index];
    }

    @SuppressWarnings("unchecked")
    static <E> E elementAt(Object[] es, int index) {
        return (E) es[index];
    }

    public E get(int index) {
        Objects.checkIndex(index, size);
        return elementData(index);
    }

    public E set(int index, E element) {
        Objects.checkIndex(index, size);
        E oldValue = elementData(index);
        elementData[index] = element;
        return oldValue;
    }

    private void add(E e, Object[] elementData, int s) {
        if (s == elementData.length)
            elementData = grow(); // 等价于 grow(size+1) minCapacity = size+1
        elementData[s] = e;
        size = s + 1;
    }

    public boolean add(E e) {
        modCount++;
        add(e, elementData, size);
        return true;
    }

    public void add(int index, E element) {
        rangeCheckForAdd(index);
        modCount++;
        final int s;
        Object[] elementData;
        if ((s = size) == (elementData = this.elementData).length)
            elementData = grow();
        System.arraycopy(elementData, index, elementData, index + 1, s - index);
        elementData[index] = element;
        size = s + 1;
    }

    public E remove(int index) {
        Objects.checkIndex(index, size);
        final Object[] es = elementData;

        @SuppressWarnings("unchecked") E oldValue = (E) es[index];
        fastRemove(es, index);

        return oldValue;
    }

    /**
     * Private remove method that skips bounds checking and does not
     * return the value removed.
     */
    private void fastRemove(Object[] es, int i) {
        modCount++;
        final int newSize;
        if ((newSize = size - 1) > i)
            System.arraycopy(es, i + 1, es, i, newSize - i);
        es[size = newSize] = null;
    }

    // 数组长度不变
    public void clear() {
        modCount++;
        final Object[] es = elementData;
        for (int to = size, i = size = 0; i < to; i++)
            es[i] = null;
    }

    /**
     * An optimized version of AbstractList.Itr
     */
    private class Itr implements Iterator<E> { /*...*/ }

    /**
     * An optimized version of AbstractList.ListItr
     */
    private class ListItr extends Itr implements ListIterator<E> { /*...*/ }

    private static class SubList<E> extends AbstractList<E> implements RandomAccess { /*...*/ }

    @Override
    public void forEach(Consumer<? super E> action) {
        Objects.requireNonNull(action);
        final int expectedModCount = modCount;
        final Object[] es = elementData;
        final int size = this.size;
        for (int i = 0; modCount == expectedModCount && i < size; i++)
            action.accept(elementAt(es, i));
        if (modCount != expectedModCount)
            throw new ConcurrentModificationException();
    }

    @Override
    public Spliterator<E> spliterator() {
        return new ArrayListSpliterator(0, -1, 0);
    }

    /** Index-based split-by-two, lazily initialized Spliterator */
    final class ArrayListSpliterator implements Spliterator<E> { /*...*/ }
}

ArrayList expansion mechanism: array copy Arrays.copyOf (T [] original, int newLength) method When
adding elements, the current capacity is equal to the length of the array:
- minCapacity = size + 1 call grow (minCapacity) to ensure that the capacity is at least 1 greater after expansion
  - minCapacity <1.5 * size 1.5 times expansion
  - minCapacity >= 1.5 * size
    - The empty list constructed by the default constructor is expanded to max (minCapacity, 10)
    - Expand to minCapacity in other cases
That is, an empty list constructed by a default constructor. When the element is added for the first time, the length of the array is changed from 0 to 10 , and then expanded by at least 1.5 times.
A list that specifies the initial capacity is expanded at least 1.5 times each time an element is added.
ArrayList internal classes Itr and ListItr are implemented based on array operations.

Vector


public class Vector<E>
    extends AbstractList<E>
    implements List<E>, RandomAccess, Cloneable, java.io.Serializable
{

    protected Object[] elementData;

    /**
     * The number of valid components in this {@code Vector} object.
     * Components {@code elementData[0]} through
     * {@code elementData[elementCount-1]} are the actual items.
     *
     * @serial
     */
    protected int elementCount;

    /**
     * The amount by which the capacity of the vector is automatically
     * incremented when its size becomes greater than its capacity.  If
     * the capacity increment is less than or equal to zero, the capacity
     * of the vector is doubled each time it needs to grow.
     *
     * @serial
     */
    protected int capacityIncrement;

    public Vector(int initialCapacity, int capacityIncrement) {
        super();
        if (initialCapacity < 0)
            throw new IllegalArgumentException("Illegal Capacity: "+
                                               initialCapacity);
        this.elementData = new Object[initialCapacity];
        this.capacityIncrement = capacityIncrement;
    }

    public Vector(int initialCapacity) {
        this(initialCapacity, 0);
    }

    public Vector() {
        this(10);
    }


    private void grow(int minCapacity) {
        // overflow-conscious code
        int oldCapacity = elementData.length;
        int newCapacity = oldCapacity + ((capacityIncrement > 0) ?
                                         capacityIncrement : oldCapacity);
        if (newCapacity - minCapacity < 0)
            newCapacity = minCapacity;
        if (newCapacity - MAX_ARRAY_SIZE > 0)
            newCapacity = hugeCapacity(minCapacity);
        elementData = Arrays.copyOf(elementData, newCapacity);
    }
}

Vector and ArrayList very similar, are thread-safe version of the ArrayList, the method is synchronizedmodified.
When Vector does not specify the capacity increment, capacity expansion is to double the capacity, and the default initial capacity is also 10.

Stack

public class Stack<E> extends Vector<E> {

    public Stack() {
    }

    public E push(E item) {
        addElement(item);

        return item;
    }

    public synchronized E pop() {
        E       obj;
        int     len = size();

        obj = peek();
        removeElementAt(len - 1);

        return obj;
    }

    public synchronized E peek() {
        int     len = size();

        if (len == 0)
            throw new EmptyStackException();
        return elementAt(len - 1);
    }

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

    public synchronized int search(Object o) {
        int i = lastIndexOf(o);

        if (i >= 0) {
            return size() - i;
        }
        return -1;
    }

    private static final long serialVersionUID = 1224463164541339165L;
}

Stack inherits from Vector and is also thread-safe.

AbstractSequentialList

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

    protected AbstractSequentialList() {
    }

    // 通过 ListIterator 实现 ArrayList 是直接取数组元素
    public E get(int index) {
        try {
            return listIterator(index).next();
        } catch (NoSuchElementException exc) {
            throw new IndexOutOfBoundsException("Index: "+index);
        }
    }

    public E set(int index, E element) {
        try {
            ListIterator<E> e = listIterator(index);
            E oldVal = e.next();
            e.set(element);
            return oldVal;
        } catch (NoSuchElementException exc) {
            throw new IndexOutOfBoundsException("Index: "+index);
        }
    }

    public void add(int index, E element) {
        try {
            listIterator(index).add(element);
        } catch (NoSuchElementException exc) {
            throw new IndexOutOfBoundsException("Index: "+index);
        }
    }

    public E remove(int index) {
        try {
            ListIterator<E> e = listIterator(index);
            E outCast = e.next();
            e.remove();
            return outCast;
        } catch (NoSuchElementException exc) {
            throw new IndexOutOfBoundsException("Index: "+index);
        }
    }


    // Bulk Operations

    public boolean addAll(int index, Collection<? extends E> c) { /*...*/ }


    // Iterators

    // 返回的是 listIterator
    public Iterator<E> iterator() {
        return listIterator();
    }

    public abstract ListIterator<E> listIterator(int index);
}

AbstractSequentialList implements basic add, remove and other methods, all based on ListIterator.

linkdedlist

public class LinkedList<E>
    extends AbstractSequentialList<E>
    implements List<E>, Deque<E>, Cloneable, java.io.Serializable
{
    transient int size = 0;

    /**
     * Pointer to first node.
     * Invariant: (first == null && last == null) ||
     *            (first.prev == null && first.item != null)
     */
    transient Node<E> first;

    /**
     * Pointer to last node.
     * Invariant: (first == null && last == null) ||
     *            (last.next == null && last.item != null)
     */
    transient Node<E> last;

    /**
     * Constructs an empty list.
     */
    public LinkedList() {
    }

    public LinkedList(Collection<? extends E> c) {
        this();
        addAll(c);
    }

    /**
     * Links e as first element.
     */
    private void linkFirst(E e) {
        final Node<E> f = first;
        final Node<E> newNode = new Node<>(null, e, f);
        first = newNode;
        if (f == null)
            last = newNode;
        else
            f.prev = newNode;
        size++;
        modCount++;
    }

    /**
     * Links e as last element.
     */
    void linkLast(E e) { /*...*/ }

    /**
     * Inserts element e before non-null Node succ.
     */
    void linkBefore(E e, Node<E> succ) {
        // assert succ != null;
        final Node<E> pred = succ.prev;
        final Node<E> newNode = new Node<>(pred, e, succ);
        succ.prev = newNode;
        if (pred == null)
            first = newNode;
        else
            pred.next = newNode;
        size++;
        modCount++;
    }

    /**
     * Unlinks non-null first node f.
     */
    private E unlinkFirst(Node<E> f) {
        // assert f == first && f != null;
        final E element = f.item;
        final Node<E> next = f.next;
        f.item = null;
        f.next = null; // help GC
        first = next;
        if (next == null)
            last = null;
        else
            next.prev = null;
        size--;
        modCount++;
        return element;
    }

    /**
     * Unlinks non-null last node l.
     */
    private E unlinkLast(Node<E> l) { /*...*/ }

    /**
     * Unlinks non-null node x.
     */
    E unlink(Node<E> x) {
        // assert x != null;
        final E element = x.item;
        final Node<E> next = x.next;
        final Node<E> prev = x.prev;

        if (prev == null) {
            first = next;
        } else {
            prev.next = next;
            x.prev = null;
        }

        if (next == null) {
            last = prev;
        } else {
            next.prev = prev;
            x.next = null;
        }

        x.item = null;
        size--;
        modCount++;
        return element;
    }

    public E getFirst() {
        final Node<E> f = first;
        if (f == null)
            throw new NoSuchElementException();
        return f.item;
    }

    public E getLast() { /*...*/ }

    public E removeFirst() { /*...*/ }

    public E removeLast() { /*...*/ }

    public void addFirst(E e) { /*...*/ }

    public void addLast(E e) { /*...*/ }

    public boolean contains(Object o) {
        return indexOf(o) != -1;
    }

    public int size() { /*...*/ }

    // 尾插
    public boolean add(E e) {
        linkLast(e);
        return true;
    }

    public boolean remove(Object o) {
        if (o == null) {
            for (Node<E> x = first; x != null; x = x.next) {
                if (x.item == null) {
                    unlink(x);
                    return true;
                }
            }
        } else {
            for (Node<E> x = first; x != null; x = x.next) {
                if (o.equals(x.item)) {
                    unlink(x);
                    return true;
                }
            }
        }
        return false;
    }

    public boolean addAll(Collection<? extends E> c) { /*...*/ }

    public boolean addAll(int index, Collection<? extends E> c) { /*...*/ }
 
    public void clear() {
        // Clearing all of the links between nodes is "unnecessary", but:
        // - helps a generational GC if the discarded nodes inhabit
        //   more than one generation
        // - is sure to free memory even if there is a reachable Iterator
        for (Node<E> x = first; x != null; ) {
            Node<E> next = x.next;
            x.item = null;
            x.next = null;
            x.prev = null;
            x = next;
        }
        first = last = null;
        size = 0;
        modCount++;
    }


    // Positional Access Operations

    public E get(int index) {
        checkElementIndex(index);
        return node(index).item;
    }

    public E set(int index, E element) { /*...*/ }

    public void add(int index, E element) {
        checkPositionIndex(index);

        if (index == size)
            linkLast(element);
        else
            linkBefore(element, node(index));
    }

    public E remove(int index) {
        checkElementIndex(index);
        return unlink(node(index));
    }

    /**
     * Tells if the argument is the index of an existing element.
     */
    private boolean isElementIndex(int index) {
        return index >= 0 && index < size;
    }

    /**
     * Tells if the argument is the index of a valid position for an
     * iterator or an add operation.
     */
    private boolean isPositionIndex(int index) {
        return index >= 0 && index <= size;
    }

    /**
     * Returns the (non-null) Node at the specified element index.
     */
    Node<E> node(int index) {
        // assert isElementIndex(index);
        // 根据 index 判断结点在链表的前半段还是后半段
        if (index < (size >> 1)) {
            Node<E> x = first;
            for (int i = 0; i < index; i++)
                x = x.next;
            return x;
        } else {
            Node<E> x = last;
            for (int i = size - 1; i > index; i--)
                x = x.prev;
            return x;
        }
    }

    // Search Operations

    public int indexOf(Object o) {
        int index = 0;
        if (o == null) {
            for (Node<E> x = first; x != null; x = x.next) {
                if (x.item == null)
                    return index;
                index++;
            }
        } else {
            for (Node<E> x = first; x != null; x = x.next) {
                if (o.equals(x.item))
                    return index;
                index++;
            }
        }
        return -1;
    }

    public int lastIndexOf(Object o) { /*...*/ }

    // Queue operations.

    public E peek() {
        final Node<E> f = first;
        return (f == null) ? null : f.item;
    }

    public E element() {
        return getFirst();
    }

    public E poll() {
        final Node<E> f = first;
        return (f == null) ? null : unlinkFirst(f);
    }

    public E remove() {
        return removeFirst();
    }

    public boolean offer(E e) {
        return add(e);
    }

    // Deque operations
    public boolean offerFirst(E e) { /*...*/ }

    public boolean offerLast(E e) { /*...*/ }

    public E peekFirst() { /*...*/ }

    public E peekLast() { /*...*/ }

    public E pollFirst() { /*...*/ }

    public E pollLast() { /*...*/ }

    public void push(E e) {
        addFirst(e);
    }

    public E pop() {
        return removeFirst();
    }

    public boolean removeFirstOccurrence(Object o) {
        return remove(o);
    }

    public boolean removeLastOccurrence(Object o) { /*...*/ }

    public ListIterator<E> listIterator(int index) {
        checkPositionIndex(index);
        return new ListItr(index);
    }

    private class ListItr implements ListIterator<E> {
        private Node<E> lastReturned;
        private Node<E> next;
        private int nextIndex;
        private int expectedModCount = modCount;

        ListItr(int index) {
            // assert isPositionIndex(index);
            next = (index == size) ? null : node(index);
            nextIndex = index;
        }

        public boolean hasNext() {
            return nextIndex < size;
        }

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

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

        public boolean hasPrevious() {
            return nextIndex > 0;
        }

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

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

        public int nextIndex() {
            return nextIndex;
        }

        public int previousIndex() {
            return nextIndex - 1;
        }

        public void remove() {
            checkForComodification();
            if (lastReturned == null)
                throw new IllegalStateException();

            Node<E> lastNext = lastReturned.next;
            unlink(lastReturned);
            if (next == lastReturned)
                next = lastNext;
            else
                nextIndex--;
            lastReturned = null;
            expectedModCount++;
        }

        public void set(E e) {
            if (lastReturned == null)
                throw new IllegalStateException();
            checkForComodification();
            lastReturned.item = e;
        }

        public void add(E e) {
            checkForComodification();
            lastReturned = null;
            if (next == null)
                linkLast(e);
            else
                linkBefore(e, next);
            nextIndex++;
            expectedModCount++;
        }

        public void forEachRemaining(Consumer<? super E> action) {
            Objects.requireNonNull(action);
            while (modCount == expectedModCount && nextIndex < size) {
                action.accept(next.item);
                lastReturned = next;
                next = next.next;
                nextIndex++;
            }
            checkForComodification();
        }

        final void checkForComodification() {
            if (modCount != expectedModCount)
                throw new ConcurrentModificationException();
        }
    }

    private static class Node<E> {
        E item;
        Node<E> next;
        Node<E> prev;

        Node(Node<E> prev, E element, Node<E> next) {
            this.item = element;
            this.next = next;
            this.prev = prev;
        }
    }

    public Iterator<E> descendingIterator() {
        return new DescendingIterator();
    }

    /**
     * Adapter to provide descending iterators via ListItr.previous
     */
    private class DescendingIterator implements Iterator<E> {
        private final ListItr itr = new ListItr(size());
        public boolean hasNext() {
            return itr.hasPrevious();
        }
        public E next() {
            return itr.previous();
        }
        public void remove() {
            itr.remove();
        }
    }
}

LinkedList implements the List and Deque interfaces, and can be used as a list, queue, and two-way queue.
Three properties in LinkedList inner class Node: item, prev, next.
LinkedList internal classes ListItr, and DescendingIterator. DescendingIterator uses the adapter pattern to convert ListItr into a reverse iterator.

Java source code learning (JDK 11)-java.lang.Collection

Collection

AbstractCollection

AbstractSet

HashSet

LinkedHashSet

TreeSet

AbstractList

ArrayList

Vector

Stack

AbstractSequentialList

linkdedlist

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