JDK源码学习笔记——Iterable/Iterator实现原理

Collection接口继承java.lang.Iterable接口，集合类重写了iterator()方法

public interface Iterable<T> {
    Iterator<T> iterator();

    default void forEach(Consumer<? super T> action) {
        Objects.requireNonNull(action);
        for (T t : this) {
            action.accept(t);
        }
    }

    default Spliterator<T> spliterator() {
        return Spliterators.spliteratorUnknownSize(iterator(), 0);
    }
}

public interface Iterator<E> {
    boolean hasNext();
    E next();
    default void remove() {
        throw new UnsupportedOperationException("remove");
    }
    default void forEachRemaining(Consumer<? super E> action) {
        Objects.requireNonNull(action);
        while (hasNext())
            action.accept(next());
    }
}

原理：

一开始迭代器在所有元素的左边，调用next()之后，迭代器移到第一个和第二个元素之间，next()方法返回迭代器刚刚经过的元素。
hasNext()若返回True，则表明接下来还有元素，迭代器不在尾部。
remove()方法必须和next方法一起使用，功能是去除刚刚next方法返回的元素。

自己实现iterator遍历（参考JDK集合类）：

import java.util.Iterator;

public class ForEachAPIDemo {
    public static void main(String[] args) throws Exception {
        Students students = new Students(10);
        for (Student student : students) {
            System.out.println(student.getSid() + ":" + student.getName());
        }
    }
}

// 支持for each迭代循环的学生集合类
class Students implements Iterable<Student> {
    // 存储所有学生类的数组
    private Student[] students;

    // 该构造函数可以生成指定大小的学生类变量数组，并初始化该学生类变量数组
    public Students(int size) {
        students = new Student[size];
        for (int i = 0; i < size; i++) {
            students[i] = new Student(String.valueOf(i), "学生" + String.valueOf(i));
        }
    }

    @Override
    public Iterator<Student> iterator() {
        return new StudentIterator();
    }

    // 实现Iterator接口的私有内部类，外界无法直接访问
    private class StudentIterator implements Iterator<Student> {
        // 当前迭代元素的下标
        private int index = 0;

        // 判断是否还有下一个元素，如果迭代到最后一个元素就返回false
        public boolean hasNext() {
            return index != students.length;
        }

        @Override
        public Student next() {
            return students[index++];
        }

        // 这里不支持，抛出不支持操作异常
        public void remove() {
            throw new UnsupportedOperationException();
        }
    }
}

class Student {
    private String sid;
    private String name;

    public Student(String sid, String name) {
        setSid(sid);
        setName(name);
    }

    public String getSid() {
        return sid;
    }

    public void setSid(String sid) {
        this.sid = sid;
    }

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    @Override
    public String toString() {
        return "Student{" +
                "sid='" + sid + '\'' +
                ", name='" + name + '\'' +
                '}';
    }
}

foreach原理：

foreach语法最终被编译器转为了对Iterator.next()的调用

package test;

import java.util.List;

/**
 * Created by vino on 2016/5/6.
 */
public class TestForeach {
    List<Integer> integers;
    public void testForeach(){
        for(Integer i : integers){

        }
    }
}

// 字节码
public void testForeach();
    descriptor: ()V
    flags: ACC_PUBLIC
    Code:
      stack=1, locals=3, args_size=1
         0: aload_0
         1: getfield      #2                  // Field integers:Ljava/util/List;
         4: invokeinterface #3,  1            // InterfaceMethod java/util/List.iterator:()Ljava/util/Iterator;
         9: astore_1
        10: aload_1
        11: invokeinterface #4,  1            // InterfaceMethod java/util/Iterator.hasNext:()Z
        16: ifeq          32
        19: aload_1
        20: invokeinterface #5,  1            // InterfaceMethod java/util/Iterator.next:()Ljava/lang/Object;
        25: checkcast     #6                  // class java/lang/Integer
        28: astore_2
        29: goto          10
        32: return
      LineNumberTable:
        line 11: 0
        line 13: 29
        line 14: 32
      LocalVariableTable:
        Start  Length  Slot  Name   Signature
      0     2     i   Ljava/lang/Integer;
     33     0  this   Ltest/TestForeach;
}

Iterator对Collection 或 Map 进行迭代操作过程中是不允许修改 Collection / Map 的内容（add/remove）   允许set

可以使用 Iterator 本身的方法 remove() 来删除对象

原因：java中的fail-fast(快速失败)机制

撸代码解释（ArrayList为例）：

1.ArrayList的属性modCount：add()  remove()时都会modCount++；

2.new Itr(implements Iterator)时属性expectedModCount=modCount，且next()是要检查expectedModCount ?= modCount；

3.调用ArrayList的add()  remove()时改变了modCount的值，下一次next()时检查expectedModCount != modCount，throw new ConcurrentModificationException()；

4.Iterator 本身的方法 remove()删除元素之后会将modCount重新赋值给expectedModCount。

    // ArrayList:
    public E remove(int index) {
        rangeCheck(index);

        modCount++;
        E oldValue = elementData(index);

        int numMoved = size - index - 1;
        if (numMoved > 0)
            System.arraycopy(elementData, index+1, elementData, index,
                             numMoved);
        elementData[--size] = null; // clear to let GC do its work

        return oldValue;
    }

    // Itr:
    private class Itr implements Iterator<E> {
        int cursor;       // index of next element to return
        int lastRet = -1; // index of last element returned; -1 if no such
        int expectedModCount = modCount;

        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();
            }
        }

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

参考资料：

为什么Iterator的remove方法可保证从源集合中安全地删除对象，而在迭代期间不能直接删除集合内元素