JavaCollection 源码阅读1

从最核心的Collection 接口开始看；

AbstractCollection 是实现了Collection 的框架类。 AbstractList 是继承与AbstractCollection的， 所以AbstractCollection类是关键。

Collection类最基础的功能是增删查改：都是通过Iterator实现的。 举个例子： Add:AbstractCollection没有实现

Remove:

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;
}

Contain

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;
}

ToArray:

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;
}

所以Collection中最核心的东西就是：Iterator和Size

Iterator[hasnext、next、remove、foreachremaining]

所以List以及Set 最核心的，就是对Collection的Iterator和Size Override

List 类除了Iterator外， 还增加了ListIterator实现了双向的迭代

来看一个：Arraylist:

精简后版本：

public Iterator<E> iterator() {
    return new Itr();
}

private class Itr implements Iterator<E> {
    int cursor;       // 游标
    int lastRet = -1; // 当前游标的上一个，用于remove

    Itr() {}

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

    public E next() {
        checkForComodification();
        Object[] elementData = ArrayList.this.elementData;
        //从此处可以看出 ArrayList是基于数组实现的
        cursor = i + 1;
        return (E) elementData[lastRet = i];
    }

    public void remove() {
        if (lastRet < 0)
            throw new IllegalStateException();
        try {
            ArrayList.this.remove(lastRet);
            cursor = lastRet;
            lastRet = -1;
        } catch (IndexOutOfBoundsException ex) {
            throw new ConcurrentModificationException();
        }
    }

    public void forEachRemaining(Consumer<? super E> consumer) {
        Objects.requireNonNull(consumer);
        final int size = ArrayList.this.size;
        int i = cursor;
        if (i >= size) {
            return;
        }
        final Object[] elementData = ArrayList.this.elementData;
        if (i >= elementData.length) {
            throw new ConcurrentModificationException();
        }
        while (i != size && modCount == expectedModCount) {
            consumer.accept((E) elementData[i++]);
        }
        // update once at end of iteration to reduce heap write traffic
        cursor = i;
        lastRet = i - 1;
        checkForComodification();
    }
}

HashSet是基于 Hashmap 实现的

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

/**
 * Returns the number of elements in this set (its cardinality).
 *
 * @return the number of elements in this set (its cardinality)
 */
public int size() {
    return map.size();
}