迭代器-白红宇

Iterable 接口

Iterable 接口，表示可迭代的能力。实现接口，可用foreach 进行迭代。 foreach迭代编译器会转换为iterator迭代

public interface Iterable
    
      {    // 返回一个迭代器    Iterator
     
       iterator();        default void forEach(Consumer
       action) {        Objects.requireNonNull(action);        for (T t : this) {            action.accept(t);        }    }        default Spliterator
      
        spliterator() {        return Spliterators.spliteratorUnknownSize(iterator(), 0);    }}复制代码

Iterator

public interface Iterator
    
      {    boolean hasNext();    E next();    default void remove() {        throw new UnsupportedOperationException("remove");    }    default void forEachRemaining(Consumer
      action) {        Objects.requireNonNull(action);        while (hasNext())            action.accept(next());    }}复制代码

使用

ArrayList
    
      intList = new ArrayList<>();intList.add(123);intList.add(456);Iterator
     
       iterator = intList.iterator();while (iterator.hasNext()){    log.info(iterator.next());}复制代码

ListIterator

public interface ListIterator
    
      extends Iterator
     
       {    boolean hasNext();    E next();    boolean hasPrevious();    E previous();    int nextIndex();    int previousIndex();    void remove();    void set(E e);    void add(E e);}复制代码

迭代时插入和删除

只能用 iterator 对象的 add 和 remove 方法

// 向前遍历ListIterator
    
      iterator = intList.listIterator(intList.size());while (iterator.hasPrevious()){    if (iterator.previous()<200) {    	iterator.remove();    }}log.info(intList);复制代码

实现

public class ArrayList
    
      extends AbstractList
     
              implements List
      
       , RandomAccess, Cloneable, java.io.Serializable {        public Iterator
       
         iterator() {        return new Itr();    }        // 内部类    private class Itr implements Iterator
        
          {        int cursor;       // index of next element to return        int lastRet = -1; // index of last element returned; -1 if no such        int expectedModCount = modCount;            public boolean hasNext() {            return cursor != size;        }            @SuppressWarnings("unchecked")        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];        }            public void remove() {            if (lastRet < 0)                throw new IllegalStateException();            checkForComodification();                try {                ArrayList.this.remove(lastRet);                cursor = lastRet;                lastRet = -1;                expectedModCount = modCount;            } catch (IndexOutOfBoundsException ex) {                throw new ConcurrentModificationException();            }        }            @Override        @SuppressWarnings("unchecked")        public void forEachRemaining(Consumer
          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();        }            final void checkForComodification() {            if (modCount != expectedModCount)                throw new ConcurrentModificationException();        }    }        private class ListItr extends Itr implements ListIterator
         
           { ListItr(int index) { super(); cursor = index; } public boolean hasPrevious() { return cursor != 0; } public int nextIndex() { return cursor; } public int previousIndex() { return cursor - 1; } @SuppressWarnings("unchecked") 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]; } public void set(E e) { if (lastRet < 0) throw new IllegalStateException(); checkForComodification(); try { ArrayList.this.set(lastRet, e); } catch (IndexOutOfBoundsException ex) { throw new ConcurrentModificationException(); } } public void add(E e) { checkForComodification(); try { int i = cursor; ArrayList.this.add(i, e); cursor = i + 1; lastRet = -1; expectedModCount = modCount; } catch (IndexOutOfBoundsException ex) { throw new ConcurrentModificationException(); } } }}复制代码

: 迭代器表示的是一种关注点分离的思想，将数据的实际组织方式和数据的迭代遍历相分离，是一种常见的设计模式。