One 、ArrayList Use ( A little )

Two 、 Basic concept of container

( One )、Collection Is the basic interface of a collection class

The main method :

public interface Collection<E>{
boolean add(E element);// Add elements to the collection ,E For generics
Iterator<E> iterator();// Back to implementation Iterator Object of the interface
}

About :Iterator After that, I will explain .

( Two )、 Realized Collection Subclasses of :

List: Insert save elements in order .                  Set: There can't be duplicate elements in this container                     Queue: Determine the order of objects according to the rules of the queue

( 3、 ... and )、Map Interface

Definition : One makes right “ Key value pair ” object , Use key lookup value

notes :Map This is a separate interface , Do not belong to Collection, So map None of the ending classes belong to Collection

( Four )、 Use the container according to the appeal analysis

1、ArrayList Go up to Collection, And pass Collection Add processing data .( Technology applied : The parent class reference points to the subclass object , Call the method of the subclass )

public class UpToCollection {
public static void main(String [] args){
ArrayList<Integer> arrayList = new ArrayList();
for(int i=0; i<5; ++i){
arrayList.add(i);
}
Collection<Integer> collection = arrayList;
collection.add(5);
// It's used here foreach loop , As long as it's done Iterator Interface can be used foreach
//Collection No, List Of get Method , Need to use Iterator Let's do the traversal
for(Integer i: collection){
System.out.println(i);
}
}
}

2、 Add a set of elements

public class AddElementToCollection {
public static void main(String[]args){ Collection<Integer> collection = new ArrayList<Integer>(Arrays.asList(1,2,3,4,5,6));
// Knowledge point 1: utilize Arrays This static class , Will array , Or a column of numbers converted to List
// Knowledge point 2:ArrayList Can be nested List establish
/* Knowledge point 3: Why not? Collection<Integer> collection =
* Arrays.asList(1,2,3,4,5,6);, Found no problem creating , But when it comes to the following
* collection.addAll(Arrays.asList(datas)); It's a mistake
* reason : because Arrays.asList(1,2,3,4,5,6) Created List It's a fixed length List
* So it's impossible to execute add() Methodical .
*/
Integer [] datas = {7,8,9,10};
/* Knowledge point 4: You have to use Integer As a class , Out of commission int, Otherwise, an error will be reported
* Because before Collection<Integer> collection, Generics have been defined as Integer
* So you can't use reference types , Get what the input generics are , You need to create an array of classes
*/
collection.addAll(Arrays.asList(datas));
for(Integer i: collection){
System.out.println(i);
}
// Above is the passage of Collection Go to List Add a set of data to ( Array , Or custom data ), Here's another way Collections.addAll(collection, 11,12,13,14,15,16);
Collections.addAll(collection, datas);
// notes : This is a Collections The class is not Collection class
}
}

AddElementToCollection

Collection How to add elements   advantage : The method is efficient .   shortcoming : But you need to create a Collection class , As an object

Collections How to add elements advantage : convenient , No need to create Collection class .

3、Collection Role of type : Unified type add , Delete ...( Uniform traversal is made up of Iterator affirmatory )

public class CollectionEffect {
// Unify all kinds of containers , Just one way to add , Delete
public static void main(String[] args){
print(fill(new ArrayList<String>()));
print(fill(new HashSet<String>()));
print(fill(new HashMap<String,String>()));
}
//Collection Collection of interfaces
public static Collection<String> fill(Collection<String> c){
c.add("dog");
c.add("cat");
c.add("pig");
return c;
}
// heavy load fill Method , Provide Map Interface
public static Map<String,String> fill(Map<String,String> map){
map.put("dog", "pig");
map.put("cat","small"); return map;
} public static void print(Collection<String> c){ } public static void print(Map<String,String> map){ }
}

CollectionEffect

3、 ... and 、 iterator (Iterator)

effect : Uniform traversal of various containers

Interface method :

public interface Iterator<E>{
E next();// Get next element
boolean hasNext();// See if the next element exists
void remove();// Remove elements
}

Iterator

Be careful :1. In the use of next() Method before using hasNext();

2.next() and remove() They exist in pairs .

Use :

public class UseIterator {
public static void main(String[]args){
ArrayList<Integer> arrayList = new ArrayList();
for(int i=0; i<5; ++i){
arrayList.add(i);
}
// obtain Iterator, The method is from Collection Interface inherited from
Iterator<Integer> iterator = arrayList.iterator();
// Use
while(iterator.hasNext()){
Integer x = iterator.next();
System.out.println(x);
iterator.remove();
}
}
}

UseIterator

Uniform traversal : modify CollectionEffect.java Medium print Method ,

//... Continued above 
public static void print(Iterator<String> iterator){
while(iterator.hasNext()){
System.out.println(iterator.next());
}
}

CollectionEffect

Four 、 A specific collection

①、ArrayList: Arrays that can grow and shrink dynamically , Easy to search , Not conducive to insertion and deletion .   Detailed explanation : A little .

②、LinkedList: A sequence of linked lists , Easy to insert , And delete , It's not good for finding .LinkedList It also provides a way to use it as stack and queue or double ended queue . Lead to LinkedList Of Iterator Unlike other collections , So we'll introduce it later .

Give Way LinkedList Realization Stack( Already there. Stack This class , however LinkedList Can produce better Stack)

public class LinkedListStack<T> {
private LinkedList<T> mLinkedList;
public LinkedListStack(){
mLinkedList = new LinkedList<T>();
}
// Push
public void push(T view){
mLinkedList.add(view);
}
// Get stack top element
public T peek(){
return mLinkedList.getFirst();
}
// Remove the top element
public T pop(){
return mLinkedList.removeFirst();
} public boolean isEmpty(){
return mLinkedList.isEmpty();
}
public String toString(){
return mLinkedList.toString();
}
}

LinkedListStack

Others are not available for the time being .

LinkedList The iterator ListIterator

1、 It can only be used for a variety of List Class method .

2、 effect :Iterator Can only move forward ,ListIterator It can move in two directions .

public class UseListIterator {
public static void main(String[] args){
LinkedList<Integer> linkedList = new LinkedList<Integer>();
for(int i=0; i<10; ++i){
linkedList.add(i);
}
// obtain ListIterator
ListIterator<Integer> listIterator = linkedList.listIterator();
//linkedList.listIterator(3); Give Way Iterator Start at the third position
while(listIterator.hasNext()){
int i = listIterator.next();
System.out.println(i);
// New characteristics 1: The ability to modify data at the current location
listIterator.set(5);
// characteristic 2: Be able to move forward , And get the data
listIterator.hasPrevious();
listIterator.previous();
}
}
}

UseListIterator

③、Set: Don't save duplicate elements .

Hash set (Hasing): Reading data is not in the order in which it was stored ( Stochastic ranking ), Can quickly find the required objects .

principle : The hash table calculates an integer for each object , Called hash code . Hash codes are integers generated by object instance fields ( No matter the algorithm )、

notes : If it's a custom class , Is responsible for the implementation of this class hashCode, Self realized hashCode The method should be the same as equals Method compatible ( That is, if there is a return true Then all should return to true)

stay JAVA Hash table is realized by linked list array ( There's an array , Each array holds a linked list ), Each position in this array is called “ bucket ”.

The role of the barrel : Solve the problem of the same hash code 、 When the hash code is the same, put the data into the bucket's linked list . After querying the bucket, traverse the linked list of the bucket .

Little knowledge : When you insert too much data , It may lead to the barrel being full , So when the bucket is occupied more than 75% When , It's going to rehash ( Create a new table to increase the number of buckets , Move the data from the old table to the new table ).

Tree set (Tree): Is an ordered set , You can insert elements into a collection in any order , But when I take it out , Each value is automatically implemented in the sorted order .( Add each element to the tree , They're all in the right place )

ask :Tree How to know how you want each element to be arranged ?

rewrite Comparator Interface , And passed in to TreeSet<E> in .

public class UseTreeSet {
public static void main(String[]args){
// by TreeSet Add logic for comparison
Comparator<Integer> compatator = new Comparator(){ @Override
public int compare(Object o1, Object o2) {
// TODO Auto-generated method stub
// Here's the logic
return 0;
} };
TreeSet<Integer> treeSet = new TreeSet(compatator);
}
}

UseTreeSet

So : To use TreeSet The step is ①、 Create a class and override equal() and hashCode()②、 establish Comparator Interface , And rewrite coparator() Method ③、 take Comparator Object to TreeSet in

queue (Queen):ArrayDeque and LinkedList Realization . And both classes implement two terminal queues ( It can be added and deleted at the beginning and end of the linked list ),LinkedList Realized Queue Interface ArrayDeque Realized Deque Interface . There's also a special queue “ Priority queue (Priority queue)”, Insert any , Read in priority order .( Priority is related to Tree equally , By rewriting Coparator Interface to sort , If you don't rewrite it Coparator Then we sort them naturally —— Call object equal() Method ).

The mapping table (Map): Through the key you set (key) Find the corresponding value (value). because Map Do not belong to Collection that , How to traverse Map Of key, or Map Of value, also key And value At the same time through

1、 Traversing key information

HashMap<String,String> map = new HashMap();
map.put("123","asda");
map.put("234","zxczxc");
map.put("asd", "zxcdd");
// Traverse Map All the keys in
Set<String> set = map.keySet();// By acquiring Set Map all the keys in the table
// notes : The Set No HashSet Neither TreeSet, It just works Set The class of the interface
for (String str : set){
System.out.println(str);
}

Traversing key information

2、 Traverse the information of the value

// Connect 
// Traverse all values
Collection<String> collection = map.values();// obtain Collection Get value
for (String str : collection){
System.out.println(str);
}

Traversal value

3、 All over

        // All over 
for (Map.Entry<String,String> mapData : map.entrySet()){
String key = mapData.getKey();
String value = mapData.getValue();
}

5、 ... and 、 Customize Collection and Iterator

1、 By inheritance Collection Interface implementation method   shortcoming :Collection There are too many methods to override , It takes a lot of energy

2、 therefore JAVA Provides AbstactCollection Method , Completed most of Collection Method   shortcoming : When a class has inherited other classes , Must be rewritten Collection.

3、 Rewrite directly Iterator Interface .

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