数组，只能声明已知长度的，哪怕是二维也是必须声明行数
所以引入ArrayList，HashMap（键值对形式），对可变长度的数据进行存储

import java.util.*;

public class Main {
    public static void main(String[] args) {
        // TODO Auto-generated method stub
        //常规数组声明
        int Max_Size= 20;
        int num[] = new int[Max_Size];      //方式一
        num[0] = 10;
        num[1] = 11;
        int[] num1 = {1,3,4,5};     //方式二
//      System.out.println(num1[0]);
//      System.out.println(num[19]);

        //List创建,可变长度数组，并且遍历
         //遍历 ArrayList 方式一
        List<String> list = new ArrayList<String>();
        list.add("Hello");
        list.add(1, "World");
        for(String item : list){
            System.out.println(item);
        }
        
        System.out.println("List转数组");
        //遍历 ArrayList 方式二
        String[] list_ =  new String[list.size()];
        list.toArray(list_);    
        for(String item : list_){
            System.out.println(item);
        }
        //遍历 ArrayList 方式三
        System.out.println("迭代器");
        Iterator<String> ite = list.iterator(); 
        while(ite.hasNext()){
            System.out.println(ite.next());
        }
        
        //Map创建，遍历
        Map<String,String> map = new HashMap<String,String>();
        map.put("1", "Hello");
        map.put("2", "Map");
        
        System.out.println("Map.keySet");
        for(String key : map.keySet()){
            System.out.println("key=" + key + " value=" + map.get(key));
        }
        
        System.out.println("Map.entrySet iterator");
        Iterator<Map.Entry<String, String>> it = map.entrySet().iterator();
        while(it.hasNext()){
            Map.Entry<String, String> entry = it.next();
            System.out.println("key=" + entry.getKey() + " value=" + entry.getValue());
        }
        System.out.println("Map.entrySet");
        for(Map.Entry<String, String> entry : map.entrySet()){
            System.out.println("key=" + entry.getKey() + " value=" + entry.getValue());
        }   
        System.out.println("Map.values");
        for (String value : map.values()) {
            System.out.println("value=" + value);
        }
    }
}

Hello
World
List转数组
Hello
World
迭代器
Hello
World
Map.keySet
key=1 value=Hello
key=2 value=Map
Map.entrySet iterator
key=1 value=Hello
key=2 value=Map
Map.entrySet
key=1 value=Hello
key=2 value=Map
Map.values
value=Hello
value=Map