Detailed explanation of hash table

1、 Basic introduction of hash table

Hash table (Hash table, Also called hash table ) According to the key code value (Key value) Data structures that are accessed directly . in other words , It passes through Key values are mapped to tables To access records , To speed up the search . This The mapping function is called hash number, and the array storing records is called hash table

2、 Hashtable （ hash -Google） On the computer

1) Look at a real need ,google A computer problem of the company ：
2) There's a company , When new employees come to report , Ask to add the employee's information to d, Gender , Age , address ), When you enter the employee's id when , Ask to find all the information of the employee
3) requirement ： Don't use databases , Try to save memory , The faster the better => Hashtable （ hash ）

3、 Hashtable （ hash -Google） Thinking analysis of computer problems

4、 Code implementation

package com.qf.hash;

import java.util.Scanner;

public class HashTabDemo {
    
    public static void main(String[] args) {
    
        HashTab hashTab=new HashTab(8);
        boolean loop=true;
        String sc="";
        while (loop){
    
            System.out.println("add  Add employees ");
            System.out.println("list  Show employees ");
            System.out.println("find  Find employees ");
            System.out.println("exit  sign out ");
            Scanner systemPut=new Scanner(System.in);
            sc=systemPut.next();
            switch (sc){
    
                case "add":
                    System.out.println(" Please enter employee number ：");
                    Scanner addNum=new Scanner(System.in);
                    int id = addNum.nextInt();
                    System.out.println(" Please enter the employee's name ：");
                    Scanner nameScan=new Scanner(System.in);
                    String name = nameScan.next();
                    Emp emp=new Emp(id,name);
                    hashTab.add(emp);
                    break;
                case "list":
                    hashTab.list();
                    break;
                case "find":
                    System.out.println(" Please enter employee number ：");
                    Scanner find=new Scanner(System.in);
                    int findId = find.nextInt();
                    hashTab.find(findId);
                    break;
                case "exit":
                    loop=false;
                    System.out.println(" Exit loop ");
                    break;
                default :
                    break;
            }
        }
    }
}
class HashTab{
    
    public EmpLinkedList[] linkedListArray;
    public  int maxSize;
    public HashTab(int maxSize){
    
        this.maxSize=maxSize;
        linkedListArray=new EmpLinkedList[maxSize];
        for (int i = 0; i < linkedListArray.length; i++) {
    
            linkedListArray[i]=new EmpLinkedList();
        }

    }
    public void add(Emp emp){
    
        int id=emp.id;
        int location=id%maxSize;
        EmpLinkedList linkedList=linkedListArray[location];
        linkedList.add(emp);
    }

    public void list(){
    
        for (int i = 0; i < linkedListArray.length; i++) {
    
            linkedListArray[i].list(i);
        }
    }
    public void find(int id){
    
        int location=id%maxSize;
        EmpLinkedList linkedList=linkedListArray[location];
        linkedList.find(id);
    }
}
class EmpLinkedList{
    
    public Emp head;
    public EmpLinkedList(){
    }

    public void add(Emp emp){
    
        if (head==null){
    
            head=emp;
        }else{
    
            Emp temp=head;
            while (true){
    
                if (temp.next==null){
    
                    break;
                }
                temp=temp.next;
            }
            temp.next=emp;
        }
    }

    public void list(int i){
    
        if (head==null){
    
            System.out.println(" The first "+i+" The length of the linked list is empty ");
        }else {
    
            Emp temp=head;
            while (true){
    
                System.out.println(" The first "+i+" Linked list employee id by ："+temp.id+" The name is ："+temp.name);
                if (temp.next==null){
    
                    break;
                }
                temp=temp.next;
            }
        }
    }

    public void find(int id){
    
        if (head==null){
    
            System.out.println(" The length of the linked list is empty , Failed to find the employee ");
        }else{
    
            Emp temp=head;
            boolean isFind=false;
            while (true){
    
                if (temp.id==id){
    
                    isFind=true;
                    System.out.println(" The employee's name is ："+temp.name);
                    break;
                }
                if (temp.next==null){
    
                    break;
                }
                temp=temp.next;
            }
            if (!isFind){
    
                System.out.println(" Failed to find the employee ");
            }
        }
    }
}
class Emp{
    
    public int id;
    public String name;
    public Emp next;

    public Emp(int id, String name) {
    
        this.id = id;
        this.name = name;
    }

    @Override
    public String toString() {
    
        return "Emp{" +
                "id=" + id +
                ", name='" + name + '\'' +
                '}';
    }
}