/**
* Iterator用于遍歷Collection下的集合，Collection下的每個集合底層實現不一樣，意味著遍歷邏輯也不一樣，
* 所以Java的設計者將Iterator設計成了接口，讓Collection下的每個集合實現Iterator
*/
public interface Iterator<E> {//判斷是否有可迭代的元素boolean hasNext();//返回下一個元素E next();//刪除（默認方法） -- 報錯default void remove() {throw new UnsupportedOperationException("remove");}
}

public abstract class AbstractList<E> extends AbstractCollection<E> implements List<E> {//外部操作數（記錄集合添加、刪除的次數）protected transient int modCount = 0;//6
}

public class ArrayList<E> extends AbstractList<E> implements List<E>{//數據容器 - ["aaa","ccc","ddd","eee",null,null,null,null,null,null]transient Object[] elementData;//數據個數（指針）private int size;//4//e - "eee"public boolean add(E e) {ensureCapacityInternal(size + 1);  // 判斷是否擴容elementData[size++] = e;return true;}private void ensureCapacityInternal(int minCapacity) {if (elementData == DEFAULTCAPACITY_EMPTY_ELEMENTDATA) {minCapacity = Math.max(DEFAULT_CAPACITY, minCapacity);}ensureExplicitCapacity(minCapacity);}private void ensureExplicitCapacity(int minCapacity) {modCount++;if (minCapacity - elementData.length > 0)grow(minCapacity);}//o - "bbb"public boolean remove(Object o) {if (o == null) {for (int index = 0; index < size; index++)if (elementData[index] == null) {fastRemove(index);return true;}} else {//線性查詢（從頭遍歷到size的位置）for (int index = 0; index < size; index++)if (o.equals(elementData[index])) {//刪除元素（傳入下標）fastRemove(index);return true;}}return false;}//index - 1private void fastRemove(int index) {modCount++;int numMoved = size - index - 1;//計算移動次數if (numMoved > 0)System.arraycopy(elementData, index+1, elementData, index, numMoved);elementData[--size] = null; // clear to let GC do its work}public Iterator<E> iterator() {return new Itr();}//Itr是成員內部類，因為Itr中使用到了外部類(ArrayList)的成員屬性（modCount、size）private class Itr implements Iterator<E> {int cursor;       // 游標 - 4int lastRet = -1; // 當前元素的下標 - 3int expectedModCount = modCount;//內部操作數 - 6public boolean hasNext() {return cursor != size;//4 != 4}@SuppressWarnings("unchecked")public E next() {checkForComodification();//判斷外部操作數和內部操作數是否相同int i = cursor;//i = 3if (i >= size)throw new NoSuchElementException();//elementData - ["aaa","ccc","ddd","eee",null,null,null,null,null,null]Object[] elementData = ArrayList.this.elementData;if (i >= elementData.length)throw new ConcurrentModificationException();cursor = i + 1;//cursor - 4return (E) elementData[lastRet = i];}final void checkForComodification() {if (modCount != expectedModCount)throw new ConcurrentModificationException();}public void remove() {if (lastRet < 0)throw new IllegalStateException();checkForComodification();//判斷外部操作數和內部操作數是否相同try {//Itr依賴于ArrayList對象的remove()去刪除元素ArrayList.this.remove(lastRet);cursor = lastRet;lastRet = -1;//重新將外部操作數賦值給內部操作數，保證內外部操作數一致不會出現臟數據expectedModCount = modCount;} catch (IndexOutOfBoundsException ex) {throw new ConcurrentModificationException();}}@Override@SuppressWarnings("unchecked")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 trafficcursor = i;lastRet = i - 1;checkForComodification();}}
}

ArrayList<String> list = new ArrayList<>();list.add("aaa");
list.add("bbb");
list.add("ccc");
list.add("ddd");
list.add("eee");list.remove("bbb");Iterator<String> it = list.iterator();
while(it.hasNext()){String element = it.next();System.out.println(element);
}

注意：看源碼找場景