集合应用场景1:迭代器

Posted 2021-03-04 macro-renzhansheng

迭代器底层原理解析:本质还是数组+多个指示器（相对寻址）：cursor,lastCursor

/**
 * The number of times this list has been <i>structurally modified</i>.
 * Structural modifications are those that change the size of the
 * list, or otherwise perturb it in such a fashion that iterations in
 * progress may yield incorrect results.

*/

主要是： Iterator 或者 ListIteraotr 迭代器

Use of this field by subclasses is optional：

---？单线程基本不会出现：

---结构发生改变后，迭代器需要重新获取。

Collection collection = new ArrayList();
collection.add("he");
collection.add("ha");
collection.add("乐观");
collection.add("向上");
collection.add("勇猛");
System.out.println("colletion:"+collection);

Iterator :ArrayList.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;

+hasNext(out boolean){return cursor != size}

+next(out:E)

{

cursor = i + 1;
return (E) elementData[lastRet = i];

}

+remove()

{

ArrayList.this.remove(lastRet);
cursor = lastRet;
lastRet = -1;
expectedModCount = modCount;

}

//测试 iterator 接口
Iterator iterator = collection.iterator();
while (iterator.hasNext()){
System.out.print("删除："+iterator.next());
iterator.remove();
System.out.println(",结果："+collection);
}
System.out.println(collection);
System.out.println(iterator.hasNext());
//iterator.next(); //iterator – modcount gua关联？
//iterator.remove();
collection.add("坚毅");
Iterator iterator1 = collection.iterator();
System.out.println(iterator1.next());

ListIterator接口定义

+void set(E e);

/*Replaces the last element returned by {@link #next} or
* {@link #previous} with the specified element (optional operation).
* This call can be made only if neither {@link #remove} nor {@link
* #add} have been called after the last call to {@code next} or
* {@code previous}.*/

//LinkedList 实现
public void set(E e) {
    if (lastReturned == null)
        throw new IllegalStateException();
    checkForComodification();
    lastReturned.item = e;
}
//ArrayList
public void set(E e) {
    if (lastRet < 0)
        throw new IllegalStateException();
    checkForComodification();

    try {
        ArrayList.this.set(lastRet, e);
    } catch (IndexOutOfBoundsException ex) {
        throw new ConcurrentModificationException();
    }
}

+void add(E e);

/*

*Inserts the specified element into the list (optional operation).
* The element is inserted immediately before the element that
* would be returned by {@link #next}, if any, and after the element
* that would be returned by {@link #previous}, if any.  

*/

//LinkedList

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

//ArrayList

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();
        }
    }
}

ListIterator:ArrayList.ListIterator extends Itr implents Listerator

+previous(out:E)
//lastRet = cursor
+set
{
ArrayList.this.set(lastRet, e);
}
+add
{
int i = cursor;
ArrayList.this.add(i, e);
cursor = i + 1;
lastRet = -1;
expectedModCount = modCount;
}
//
public boolean hasPrevious() {
    return cursor != 0;
}
public int nextIndex() {
    return cursor;
}
public int previousIndex() {
    return cursor - 1;
}

SplitIterator？？？坑代填

优先级队列

/*//优先级队列：任务调度
//特点：内部使用了一种堆的数据结构————自我调整的二叉树
//      当向队列中删除/添加元素的时候，可以快速地找到队列中最小的元素，并且移动到树的根部
//          {不用对所有元素进行排序}
//对优先级队列进行遍历地时候，无序！！

//1：快速去除队列中最小的元素：快速地找到队列中最小的元素，并且移动到树的根部

PriorityQueue<String> pq = new PriorityQueue();
pq.add("1");
pq.add("9");
pq.add("3");
pq.add("5");
//数组或者实现了 Iterable接口地类可以用于 for each 语法
for (String s:pq){
    System.out.println(s);
}
//
System.out.println("优先级队列获取：");
System.out.println(pq.remove());
System.out.println(pq.remove());
System.out.println(pq.remove());
System.out.println(pq.remove());

//2 --内部实现 Comparable接口   \构造传入 Comparator接口

siftUp(),通过调用该方法，实现堆的结构（二叉树）{代填的坑}

HashMap  &&  HashSet

Collections工具类

典型内部类应用:

1.UnmodifiableList  &  UnmodifiableRandomAccessList   [add() emove() 等修改结构的方法内部：实现抛异常操作]

2.etcs…..(待填)

Arrays类中内部类：ArrayList,内部属性 集合 的实现是数组，实质操作的是同一个引用地址。

String[] strings = {"hello","world"};
System.out.println("java 8 Lambda表达式");
Arrays.asList(strings).forEach(numb-> System.out.println(numb));
List list = Arrays.asList(strings);
//list.add("macro");
//System.out.println(list.toString());
strings[0] = "macro";
System.out.println(list.toString());

Consumer anction 应用

Lambda 表达式本质？？

抛一个问题：1

//底层原理还不是很清楚！？——泛型应用问题

//Arrays.asList(array);采用内部类，将array赋值给内部数组，但是基本类型是不能应用泛型。

int[] numb = {1,2,5,7,3,5,0,3,2};
System.out.println(Arrays.toString(numb));
Arrays.sort(numb);
System.out.println(Arrays.toString(numb));

//asList方法，如果参数是一个基本数据类型
    //集合中不能存放基本数据类型
List list = Arrays.asList(numb);
System.out.println(list.get(0));
/*Integer[] numb1 = (Integer[]) list.get(0);//ClassCastException
System.out.println(numb1);*/

Integer[] numbs = {1,2,6,3,3,9,4,0,5};
List<Integer> list1 = Arrays.asList(numbs);
System.out.println("数组转列表:"+list1);

泛型方法：

public <T> T[] toArray(T[] a) {
    if (a.length < size)
        a = (T[])java.lang.reflect.Array.newInstance(
                            a.getClass().getComponentType(), size);
    int i = 0;
    Object[] result = a;
    for (Node<E> x = first; x != null; x = x.next)
        result[i++] = x.item;

    if (a.length > size)
        a[size] = null;

    return a;
}

所以这些：标签接口，RandomAccess等跟JVM最终的执行策略是关联的。

//高优雅写法

static boolean eq(Object o1, Object o2) {
    return o1==null ? o2==null : o1.equals(o2);
}