Simple to understand: a collection of

001. overview

Java is a collection vessel which is located under the java.util package, including: Collection (interface), the Map (the interface), a set of tools.

Collection structure 002. FIG.

003.List Interface

0031. Features

• Ordered, allowing duplicate set of elements

0032. The two main implementation class:

• ArrayList: Based on the underlying array, the memory addresses are contiguous
• LinkedList: Based on the underlying doubly linked list of memory addresses are not continuous

0033.ArrayList

• Element may be one or more null;
• Default initial capacity of 10, when the array size is insufficient, expand the capacity of 1.5 times;

List<String> list = new ArrayList<>();
list.add("hua");
list.add("ming");
list.add("fan");
list.add(null);
list.add("hua");
list.add(null);
System.out.println(list); 

//输出
[hua, ming, fan, null, hua, null]
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0034.LinkedList

• Element may be one or more null
• While achieving List and Deque interfaces

0035.ArrayList and LinkedList commonly used in several ways:

void add(E element)             添加元素
void add(int i,E element)       指定位置添加元素
E get(int i)                    获取元素
E set(int i,E element)          替换元素
E remove(int i)                 移除元素
int size()                      元素数量
boolean isEmpty()               集合中没有元素，返回true
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0036. traversal three ways

//举以下例子
public static void main(String[] args) {
    List<Integer> list = new ArrayList<>();
    for(int i=0;i<10;i++){
    	list.add(i);
    }
    System.out.println(list);
    
    //迭代器遍历
    Iterator<Integer> it = list.iterator();
    while(it.hasNext()){
    	System.out.print(it.next() +" ");
    }
    
    System.out.println();
    //遍历：增强型for循环
    for(Integer n:list){
    	System.out.print(n+" ");
    }
    
    System.out.println();
    //遍历：for
    for(int i=0;i<list.size();i++){
    	System.out.print(list.get(i)+" ");
    }	
}
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0037. scenarios

• ArrayList is more suitable for query operations;
• LinkedList more suitable insert and delete operations;

004.Set Interface

0041. Features

• Disordered, does not allow the collection of repeating elements

0042. implementation class

• HashSet: Based on the underlying HashMap achieve
• TreeSet: Based on the underlying Binary Tree

0043.HashSet

• Only one null elements

Set<String> sh = new HashSet<>();
sh.add("fan");
sh.add(null);
sh.add("hua");
sh.add("ming");
sh.add("hua");//被过滤掉
sh.add(null); //被过滤掉
System.out.println(sh);//[null, fan, hua, ming]
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005.Map Interface

0051. Features

• It has a collection of mappings
• Key data for (key-value) is stored

0052. The two main implementation class

• HashMap: + list array based underlayer (and before the JDK7), an array of red-black tree + + list (JDK8)
• HashTable: Based on the underlying hash table

0053.HashMap

• Unique, value may not be unique
• Null keys and allow a null value
• Entry object is randomly oriented
• 16 default initial capacity, the default load factor 0.75

0054.HashMap common method

Object put(Object key,Object value)  添加
Object get(Object key)               查询
Object remove(Object key)            移除
int size()                           长度
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0055. Code

Map<Integer,String> map = new HashMap<>();
map.put(1, "wang");
map.put(2, "li");
map.put(3, "hua");
map.put(4, null);
map.put(5, null);
map.put(2, "fan"); //直接覆盖前面相同的key的value
map.put(null, null);//key value 可以为空
System.out.println(map);//{null=null, 1=wang, 2=fan, 3=hua, 4=null, 5=null}
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0056. traversal way

//方式一：遍历
for(Integer i:map.keySet()){
    System.out.println("key:"+i+" values:"+map.get(i));
}

//方式二：遍历
Iterator<Entry<Integer,String>> list = map.entrySet().iterator();
while(list.hasNext()){
    Entry<Integer,String>entry = list.next();
    System.out.println("key:"+entry.getKey()+" values:"+entry.getValue());
}
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006. Tools

• Collections, Arrays: class set of tools to help provide order the collection elements, such as search and thread-safe operations;
• Comparable, Comparator: comparing the object is generally achieved for sorting the two slightly different;