java集合类（二）List学习

     接上篇  java集合类（一）

1. List接口继承了Collection接口和Iterable接口，即同样含有Collection和 Iterable的特性，还有方法，其基本方法有：

    1）有关添加： boolean add(E e):添加元素   void add(int index,E element):在特定位置添加元素        

    boolean addAll(Collection c):添加集合中所有的元素    boolean addAll(int index,Collection c):在特定位置添加一组元素

    2）有关清除：void clear(), E remove(int index),boolean remove(Object o),boolean remove(Collection c)

    3）有关检验：boolean contains(Object o),boolean containAll(Collection c),boolean equals(Object o),boolean isEmpty()

    4）有关获取：E get(int index),int hashCode(),int indexOf(Object o),int lastIndexOf(Object o),int size(),Lisy subList(int fromindex,int toindex)

    5）有关设定：E set(int index, E element),boolean retainAll(Collection c)

    6）有关打印：Object[] toArray(),    T[] toArray(T[] a)    

    7）有关迭代：Iterator iterator(),ListIterator listIterator(),ListIterator listIterator(int index)

    8）其他：int hashcode()

       List接口的实现类有很多，如：AbstractList, AbstractSequentialList, ArrayList, AttributeList, CopyOnWriteArrayList, LinkedList, RoleList,

      RoleUnresolvedList,  Stack, Vector，下面介绍List的几个基本实现类的用法：

• About ArrayList:

    All Implemented Interfaces:  Serializable, Cloneable, Iterable, Collection, List, RandomAccess extends AbstractList

    Direct Known Subclasses:  AttributeList, RoleList, RoleUnresolvedList

    实例代码：

class iphone{} 
	
public void arraylist(){
	//1)Constructs an empty list with an initial capacity of ten
	ArrayList al = new ArrayList();
	/*
	 * 2)Constructs a list containing the elements of the specified 
	 * collection, in the order they are returned by the collection's iterator
	*/
	ArrayList alist = new ArrayList(Arrays.asList(1,2,3));
	//3)Constructs an empty list with the specified initial capacity
	ArrayList alist1 = new ArrayList(10);
	al.add(new iphone());
	al.add(1);
	al.add("a");
	al.add(alist);
	System.out.println(al.hashCode()); //int hashcode()
		
	/*
	 * usage of methods that aren't mentioned within "List basic methods"
	 */
	//ensure that list can hold at least the number of elements specified by the minimum capacity argument
	alist1.ensureCapacity(30);
	//Trims the capacity of this ArrayList instance to be the list's current size
	alist.trimToSize();
        
        //Returns a shallow copy of this ArrayList instance(The elements themselves are not copied)
        Object o = al.clone();
}

• About LinkedList：　　

     All Implemented Interfaces:Serializable, Cloneable, Iterable, Collection, Deque, List, Queue  extends AbstractSequentialList

     从上面可以知道，由于LinkedList实现的接口涉及到队列，所以它也会新增一些有关队列独有操作方法，还有pop(),push()等，下面举例：

public void linkedlist(){
	LinkedList linklist = new LinkedList();
	ArrayList a = new ArrayList(Arrays.asList(1,2,3));
	LinkedList linklist2 = new LinkedList(a);
		
	Iterator i = linklist2.iterator();
	while(i.hasNext()){
		System.out.println(i.next());   //1  2  3
	}
	Iterator ri = linklist2.descendingIterator();
	while(ri.hasNext()){
		System.out.println(ri.next());   //3  2  1
	}
		
	linklist.addFirst(1);
	linklist.addLast("e");
	System.out.println(linklist);  //[1,e]  
	/*linklist.getFirst();
	linklist.getLast();*/
		
	//Retrieves, but does not remove, the head (first element) of this list
	System.out.println(linklist.element());  //1
		
	linklist.offer("x");
	System.out.println(linklist);//[1,e,x]
	linklist.offerFirst(0);
	linklist.offerLast("y");
	System.out.println(linklist);//[0,1,e,x,y]
		
	//peek..()--->[Retrieves, but does not remove]
	System.out.println(linklist.peek());//0
	System.out.println(linklist.peekFirst());//0 ,list is empty,return "null"
	System.out.println(linklist.peekLast());//y,list is empty, return "null"
		
	//poll..()--->[Retrieves and removes]
	System.out.println(linklist.poll()); //0
	System.out.println(linklist.pollFirst());//1
	System.out.println(linklist.pollLast()); //y
	System.out.println(linklist);//[e,x]
		
	/*linklist.removeFirst();    //return first element
	linklist.removeFirstOccurrence(Object);   //boolean
	linklist.removeLast();   //return last element
	linklist.removeLastOccurrence(Object);  //boolean
	*/ 
		
	System.out.println(linklist2); //[1  2  3]
	//Pops an element from the stack represented by this list
	System.out.println(linklist2.pop()); //1
	//Pushes an element onto the stack represented by this list
	linklist2.push("m"); 
	System.out.println(linklist2);//[m,2,3]
	}

• ArrayList 和LinkedList的比较：

    　1.ArrayList是基于动态数组，LinkedList基于链表
       2.对于随机访问get和set，ArrayList觉得优于LinkedList，因为LinkedList要移动指针
       3.对于新增和删除操作add和remove，LinedList比较占优势，因为ArrayList要移动数据

       验证：

public void comparearraylistwithlinklist(){
	SimpleDateFormat f = new SimpleDateFormat("yyyy-MM-dd  hh:mm:ss");
	System.out.println(f.format(new Date()));
	    
	ArrayList a = new ArrayList();
	long starttime = new Date().getTime();
	for(int i = 0; i<10000; i++){
		a.add(i);
	}
	long finishtime = new Date().getTime();
	System.out.println(finishtime-starttime);
		
	LinkedList l = new LinkedList();
	long lstarttime = new Date().getTime();
	for(int i = 0; i<10000; i++){
		l.add(i);
	}
	long lfinishtime = new Date().getTime();
	System.out.println(lfinishtime-lstarttime);
}

　　输出：

      

  额外说明：以上验证代码是基于较大量数据的，输出也是不稳定的，即答案也不能确定，可能是我用错测试方法，也可能是因为数据量不够大，也可能是因为getTime（）获得的是毫秒，程序可能需要更精确的时间单位，这样才有办法比较。另外，如果对于单个数据的插入或删除，是不是LinkedList还优于ArrayList呢？答案也很明显是不一定的，读者可以按照上面的实例验证一下

由于此博文可能有点长了，其他List的学习见“java集合类（三）List学习（续）”，尽请期待！！

 

###    学习从来都是一个过程，对对错错对对...若文中有错误，还望读者批评指出      ###

 

转载于:https://www.cnblogs.com/allenpengyu/p/3460453.html