1. List collection
ArrayList is a dynamic array
LinkedList storage structure based on linked list
Features:
1.ArrayList query is fast, addition and deletion are slow
2.LinkedList query is slow, addition and deletion are fast
List collections are ordered and elements can be repeated
List<String> list = new ArrayList<>();
list.add("a");
list.add("d");
list.add("y");
list.add("m");
System.out.println(list);
System.out.println(list.get(2));
System.out.println("list集合的遍历");
for(int i = 0; i< list.size(); i++){
String s = list.get(i);
System.out.println(s);
}
for(String s : list){
System.out.println(s);
}2. Set collection
The Set collection cannot directly obtain an element, but can only traverse the elements through an iterator.
Popularly understood as a primary school student's schoolbag, elements are thrown directly into it without any order, so the elements cannot be repeated.
The elements stored in Set are non-repeatable and the elements are unordered.
Set<String> set = new HashSet<>();
set.add("z");
set.add("t");
set.add("y");
set.add("s");
System.out.println(set);
System.out.println("set集合的遍历");
Iterator<String> iterator = set.iterator();
while (iterator.hasNext()){
String a = iterator.next();
System.out.println(a);
}3. Map collection
Data structure based on key-value pair storage
In Map, keys are not allowed to be repeated, but values can be repeated.
Map will store all keys in a set collection
Map<String,Object> map = new HashMap<>();
map.put("name","张三");
map.put("age",18);
map.put("sex","男");
map.put("address","海南");
System.out.println(map);
System.out.println("Map的遍历");
Set<String> keySet = map.keySet();//获取map中所有Key值的set集合
Iterator<String> ite = keySet.iterator();
while (ite.hasNext()){
String key = ite.next();//获取key的值
Object value = map.get(key);
System.out.println(key+"--->"+value);
}