Detailed code explanation of how to use List

The principle of variable arrays in Java is to continuously create new arrays and add the original array to the new array. The usage of Java List is explained in detail below. Friends who need it can refer to it

The principle of variable arrays in Java is to continuously create new arrays and add the original array to the new array. The usage of Java List is explained in detail below.

•  List: The elements are in order (you can take them out how you save them, the order will not be messed up), and the elements can be repeated (there is a 3 on the subscript 1 and a subscript 2 There can also be a 3) because the collection system has an index

• ## ArrayList: The underlying data structure uses an array structure (the array length is variable and can be extended by 50% ) (Characteristics are that queries are fast, but additions and deletions are slow) Threads are not synchronized

•  LinkedList: The underlying data structure is a linked list structure (Characteristics are that queries are slow, additions and deletions are fast)

•  Vector: The bottom layer is an array data structure thread synchronization (the array length is variable and 100% extended) (no matter query or addition or deletion is very slow, replaced by ArrayList)

List: a unique method. All methods that can operate the corner mark are unique to the system.

boolean add(int index, E element)
boolean addAll(index,Collection)
public static void List_add(){
   ArrayList a1 = new ArrayList();
   a1.add("java");
   a1.add("php");//List集合中的元素可以重复
  a1.add(".net");
   System.out.println("原集合："+a1);
   a1.add(1, "Flash");
   a1.add(0, "ps");  
   System.out.println(a1);
  ArrayList a2 = new ArrayList();
  a2.add("javascript");
  a2.add("3dMax");
  a2.add("IBM");
  a1.addAll(0, a2);
  System.out.println(a1);
}

Delete the element at the specified position

boolean remove(int index)
public static void List_remove(){
   ArrayList a1 = new ArrayList();
   a1.add("javascript");
   a1.add("php");
   a1.add("flash");
   System.out.println("原集合："+a1);
   a1.remove(0);
   System.out.println(a1);
}

Modify the element with the specified index set(int index, E element) Returns the modified element

public static void List_set() {
   ArrayList a1 = new ArrayList();
   a1.add("javascript");
   a1.add("php");
   a1.add(".net");
   System.out.println("原集合："+a1);
   a1.set(1, "falsh");
   System.out.println(a1);
}

Get

get(int index) Returns the element at the specified position in the list

subList(int fromIndex, int toIndex) Returns some elements between the specified fromIndex (inclusive) and toIndex (exclusive) in the list.

public static void List_get() {
   ArrayList a1 = new ArrayList();
   a1.add("java");
   a1.add("php");
   a1.add("flash");

System.out.println(a1.get(0));//获取指定角标的元素，有了该方法就可以遍历该集合中的所有元素
   System.out.println(a1.subList(1, 3));//获取集合中某一部分的元素,包含头不包含尾
}

Iterator unique to List collection: ListIterator (is a sub-interface of Iterator

)

Note:

When iterating, you cannot operate the elements in the collection through the methods of the collection

Object because a ConcurrentModificationException (concurrency exception) will occur. Therefore, when using an iterator, you can only use the iterator method. Because the Iterator method is limited, the element can only be judged, taken out, and deleted. If you want other operations such as adding, modifying, etc., you need to use its sub-interface, ListIterator. This interface can only be used through the listIterator of the List collection. Method to get

public class ListIteratorDemo {
   public static void main(String[] args) {
     ArrayList a1 = new ArrayList();
     a1.add("java01");
     a1.add("java02");
     a1.add("java03");
     a1.add("java04");
     System.out.println("原集合是："+a1);
   /*在迭代过程中准备添加或者删除元素
   Iterator it = al.iterator();
   while (it.hasNext()){
   　　Object obj = it.next();
   　　if (obj.equals("java02"))
   　　//al.add("java008");//会出现并发异常，因为迭代器正在操作集合，不能再用集合的方法操作集合了
   　　it.remove();//将java02的引用从集合中删除了
   　　System.out.println("obj:"+obj);
    }
    */  
  //只有List的listIterator有增，删，改，查这些功能，因为只有List有索引
    ListIterator li = a1.listIterator();
     while (li.hasNext()){
     if(li.next().equals("java02"))
     //li.add("java009");
     li.set("java006");
    }  
  }
}

Vector: Enumeration is a unique way to extract Vector, which is very similar to iterator (in fact, enumeration and iteration are the same) and has been replaced by iterator

 public class VectorDemo {
   public static void main(String[] args) {
     Vector v = new Vector();
     v.add("java01");
     v.add("java02");
     v.add("java03");
     v.add("java04");
     for(Enumeration en = v.elements();en.hasMoreElements();){
      System.out.println(en.nextElement());
    }
  }
}

LinkedList:

Special methods:

addFirst(); Add elements at the head

addLast(); Add elements at the tail

getFirst(); getLast() ;

Get elements but do not delete elements. If there are no elements in the collection, NoSuchElementException will occur

removeFirst(); removeLast();

Get the element but delete the element. If there are no elements in the collection, NoSuchElementException will occur

An alternative method appeared in JDK1.6

offerFirst(); offerLast();
peekFirst(); peekLast(); 获取元素，但是元素不被删除。如果集合中没有元素，会返回null
pollFirst(); pollLast(); 获取元素，但是元素被删除。如果集合中没有元素，会返回null
 public class LinkedListDemo {
   public static void main(String[] args) {
     LinkedList link = new LinkedList();
     link.add("java01");
     link.add("java02");
     link.add("java03");
     link.add("java04");
     while(!link.isEmpty()){
      System.out.println((link.removeLast()));
    }
  }
}

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