array
Three ways to create an array
int[] array1 = new int[5]; int[] array2 = new int[]{1,2,3,4,5}; int[] array3 = {1,2,3,4,5}- array properties
- Once created, the length of the array never changes
Arrays are all static methods that are utility classes for arrays
Located in: java.util
Throws NullPointerException if array reference is null
Common method
sort(int a) //对数组进行排序 fill(int[] a,int val) //将制动内容填充到数组中 equals(int[] a,int[] b) //判断两个数组是否相等 toString(int[] a) //输出数组信息 deepToString(int[] a) //输出多维数组信息 deepEquals(int[] a,int[] b) //判断多维数组是否相等container class
graph TD
a[java.lang.object]-->b[Map接口]
b-->c[HashMap]
b-->d[TreeMap]
a-->e[Collection接口]
e-->f[Set子接口]
e-->g[List子接口]
f-->h[HashSet]
f-->i[TreeSet]
g-->j[ArrayList]
g-->k[LinkedList]Compared with sets, it is mainly reflected in efficiency, type, and the ability to save basic types
- Sets are simple linear arrangements, so they are fast
- But not flexible, the size is fixed once created
- Collections can be saved directly, basic data types
The container class saves the object type, and the basic data type needs to be converted into a wrapper class before it can be saved (completed by automatic packing and unboxing)
The bottom layer of ArrayList is implemented with arrays, which are called dynamic arrays. They are not thread-synchronized and thread-unsafe Object types.
Vector Automatically growing array of objects, thread-safe, generally similar to the Object type of arrayslist
```
ArrayList a = new ArrayList():
a.add("ss");
System.out.println(a.get(0));
get(int index) //Get the value at the specified index
````
HashSet can only get values through iterators
- Call the hash function, the storage method is disordered and not repeated, and the bottom layer is implemented with HashMap
- The hash function is to obtain the hash value by calculation, and use the hash value as the storage location
Advantages: fast
TreeSet can only get values through iterators
Orderly and not repeated, the bottom layer is implemented with TreeMap
The Map interface is similar to the dictionary in c# python
```Java
Map d = new HashMap();
d.put("a",2); //Add key-value pair
System.out.println(d.get("a")//Get value by key) ;
### HashMap 值是无序的
* 底层由数组构建
* 影响性能的主要参数是: 底层容量 加载因子
* 底层容量 底层数组的大小
* 加载因子 当满足什么条件的时候自动扩容
* #### 遍历
* 第一种转换为entrySet;//效率高O(1)Map d = new HashMap();
d.put("a",2);
d.put(1,333);
Iterator e = d.entrySet().iterator(); //把Map类型转换为entry集合的迭代器
while (e.hasNext()){
Map.Entry a = (Map.Entry) e.next(); //转换为Map类型
a.getKey(); //获取键
a.getValue(); //获取值
}* 第二种转换为keySet;//效率低O(2)Map d = new HashMap();
d.put("a",2);
d.put(1,333);
Iterator a = d.keySet().iterator(); //把键集合转换为迭代器
while (a.hasNext()){
Object c = a.next(); //获取键
Object e = d.get(c); //通过获得的键获取值
}### TreeMap 基于红黑树实现,值是有序的,能够保持固定的顺序
## Conllections
* 是服务于Conllection的工具类
* 关系类似于 Arrays 对 数组
* ## Java对集合的遍历
* **Iterator** 遍历迭代,最常见的迭代方式
* **ListIterator** : **Iterator** 的子接口,专门用于迭代List中的元素
* **Enumeration** 一个旧的接口类似于 Iterator
* **foreach** 可以便利数组和集合 JDK 5 之后增加的
**Iterator**常用方法Iterator
boolean hasNext(); //Is it still possible to iterate
E next() //returns the next element of type E
void remove() //Remove the last returned element from the iterator's object
**ListIterator**常用方法void add(E e) //Insert the specified element into the list
boolean hasNext() //Is it still possible to iterate
E next() //returns the next element of the iteration
boolean hasPrevious() //if iterates backwards, returns whether there are any elements before
int nextIndex() //returns the index value of the next element
E previous() //return to the previous element
void set(E e) //Replace the last element of forward, or reverse with the specified element
```