hashtable_eg.txt

一个关于哈希表的例子,里面有哈希表的几点重要的介绍

//Hashtable简易数据库程序
import java.io.*;
import java.util.*;
class str14
{
  InputStreamReader stdin=new InputStreamReader(System.in);
  BufferedReader bufin=new BufferedReader(stdin);
  public static void main(String args[]) throws IOException
  {
    new str14();//run the application
  }
  public str14() throws IOException//constructor
  {
    Hashtable htable=new Hashtable(20,0.75F);
    dataBase(htable);
  }
  public str14(Hashtable htable) throws IOException //override the constructor
  {
    dataBase(htable);
  }
  public void dataBase(Hashtable htable) throws IOException
  {
    int count=htable.size();//get the ammount of the data in htable
    int value,id,num;
    String key,enter,data;
    Enumeration keys=htable.keys();//get all the key in htable
    Enumeration elements=htable.elements();//get all the elements in htable
    System.out.println("\nHashtable  简易数据库程序\n");
    System.out.println("(1)输入数据");
    System.out.println("(2)请除所有数据");
    System.out.println("(3)显示单笔数据");
    System.out.println("(4)删除单笔数据");
    System.out.println("(5)显示所有数据");
    System.out.println("(6)结束程序");
    System.out.print("请输入您的选择:");
    value=select();//call select(),return the function number
    switch(value)//the function
    {
    case 1:
      System.out.print("\n请输入一笔数据:");//need data input
      data=bufin.readLine();
      count++;
      key=String.valueOf(count);
      htable.put(key,data);//store it to the htable
      System.out.print("\n输入完成，按任意键继续...");
      enter=bufin.readLine();
      new str14(htable);//reenter
      break;
    case 2://clear all data from the htable
      htable.clear();
      System.out.print("\n已删除了所有数据... 按任意键继续...");
      enter=bufin.readLine();
      new str14(htable);//reenter
      break;
    case 3:
      System.out.print("\n请输入要显示的数据编号:");
      id=getid(count);//call getid()
      key=String.valueOf(id);
      Object select=htable.get(key);//fetch the data from the htable
      data=select.toString();
      System.out.print("\n编号"+"\t\t"+"内容");
      System.out.println("\n "+key+"\t\t"+data);//display the data
      System.out.print("\n按任意键继续...");
      enter=bufin.readLine();
      new str14(htable);//reenter
      break;
    case 4:
      System.out.print("请输入要删除的数据编号:");
      id=getid(count);
      key=String.valueOf(id);
      htable.remove(key);//remove data
      count--;
      num=count;
      System.out.print("已删除了所选择的数据...按任意见继续...");
      Hashtable htable1=new Hashtable(20,0.75F);//create new htable named htable1
      elements=htable.elements();//fetch all the data from htable
      while(elements.hasMoreElements())
      {
        key=String.valueOf(num);//a new value
        data=(String)elements.nextElement();//fetch  content of the data
        htable1.put(key,data);//store it to htable1
        num--;
      }
      htable.clear();
      enter=bufin.readLine();
      new str14(htable1);
      break;
    case 5:
      String[] sortkey=new String[count];//create a new sort array
      String[] sortdata=new String[count];
      num=count;
      elements=htable.elements();
      keys=htable.keys();
      while(elements.hasMoreElements())
      {
        key=(String)keys.nextElement();//fetch value of key
        data=(String)elements.nextElement();//fetch value of data
        sortkey[num-1]=key;//store the value of key to sortkey array
        sortdata[num-1]=data;//store the value of data to sortdata array
        num--;
      }
      System.out.println("\n编号"+"\t\t"+"内容");
      for(int i=0;i<count;i++)//display all content after sorting
        System.out.println(" "+sortkey[i]+"\t\t"+sortdata[i]);
      System.out.print("\n目前共有"+count+"笔数据");
      System.out.print("\n\n按任意键继续...");
      enter=bufin.readLine();
      new str14(htable);
      break;
    default:
    }
  }
  public int select() throws IOException//method of getting a function selector
  {
    String input;
    int value=0;
    input=bufin.readLine();//read a input from keyboard
    try
    {
      value=Integer.parseInt(input);//convert a string to a int value
    }catch(NumberFormatException e)//can't vonverted
    {
      System.out.print("请输入选项1~6:");
      value=select();
    }
    if(value>6||value<1)//if exceed then print a message and reenter
    {
      System.out.print("请输入选项1~6:");
      value=select();
    }
    return value;//return a value
  }
  public int getid(int count)throws IOException//a method of return the number of  data
  {
    String input;
    int value=0;
    input=bufin.readLine();//read a user input string from keyboard
    try
    {
      value=Integer.parseInt(input);//convert the string to a int 
    }catch(NumberFormatException e)//if can't convert to a integer then reenter
    {
      System.out.print("请输入数据编号:");
      value=getid(count);
    }
    if(value>count)//the input value is out of bound
    {
      System.out.print("无此编号的数据,请重新输入:");
      getid(count);
    }
    return value;//return a value
  }
}