Java学习-hashcode综合练习

1.自定义字符串的hashcode

2.自定义MyHashMap

 3.内容查找性能比较

其中：IHashMap接口、Entry类如下

package Collection;

public interface IHashMap {
    public void put(String key,Object value);
    public Object get(String key);
        
}

package Collection;

public class Entry {
    public Object key;
    public Object value;
    public Entry(Object key,Object value){    //构造方法
        this.key=key;
        this.value=value;
    }
    public String toString(){
        return "[key=" + key + ", value=" + value + "]";
    }
    
}

给出代码：

package Collection;

import java.util.ArrayList;
import java.util.HashSet;
import java.util.Iterator;
import java.util.LinkedList;
import java.util.List;

import charactor.Hero;

public class myHashMap implements IHashMap {
    public static int hashcode(String str) {
        if (str.length() == 0)
            return 0;
        char tmpCharArr[] = str.toCharArray();
        int cnt = 0;
        for (int i = 0; i < tmpCharArr.length; i++)
            cnt += (int) tmpCharArr[i];
        cnt = cnt * 23;
        if (cnt < 0)
            cnt = cnt * (-1);
        if (cnt > 1999)
            cnt = cnt % 2000;
        return cnt;

        // //测试hashcode,大小为5
        // if(str.length()==0)
        // return 0;
        // int cnt=Integer.parseInt(str);
        // if(cnt>=5)
        // return cnt%5;
        // return cnt;
        //
    }

    public static char ranChar() {
        while (true) {
            int n = (int) Math.round(Math.random() * 128);
            char c = (char) n;
            if (Character.isDigit(c) || Character.isLetter(c))
                return c;
        }
    }

    public static List<String> ranArr(int num) { // 随机生成长度是2-10的不等的num个字符串
        List<String> arr = new ArrayList(); // 最后返回arr
        HashSet<String> hs = new HashSet(); // 保证随机生成的字符串互不相等
        int i = 0;
        while (i < num) {
            int randLength = (int) (Math.round(Math.random() * 8) + 2);// 随机生成长度是2-10
            String tmpStr = "";
            for (int j = 0; j < randLength; j++) {
                tmpStr += ranChar();
            }
            if (hs.add(tmpStr) == true) // 保证随机生成的字符串互不相等
                i++;
        }
        for (String s : hs) { // HashSet转成List便于以后的操作
            arr.add(s);
        }
        return arr;
    }

    Object[] hashArr = new Object[2000]; // 自己定义的Hash表，由2000个对象组成

    @Override
    public void put(String key, Object value) {
        // TODO Auto-generated method stub
        int index = hashcode(key);
        // 下面两行用于应对hashArr[index]为空时，填入LinkedList<Entry>类型
        LinkedList<Entry> tmpLinkedList = new LinkedList();
        Entry E = new Entry(key, value);
        tmpLinkedList.add(E);

        if (hashArr[index] == null) {
            hashArr[index] = tmpLinkedList;
        } else {// hashArr[index]非空，说明它本身是LinkedList<Entry>类型，直接add即可
            LinkedList<Entry> ll = (LinkedList<Entry>) hashArr[index];
            ll.add(E);
        }
    }

    @Override
    public Object get(String key) {
        // TODO Auto-generated method stub
        int index = hashcode(key);
        if (hashArr[index] == null)
            return null;
        else {
            LinkedList<Entry> tmpList = (LinkedList<Entry>) hashArr[index];
            for (int i = 0; i < tmpList.size(); i++) {
                if (tmpList.get(i).key.equals(key)) { // 找到了对应的key--value
                    return tmpList.get(i).value; // 返回key对应的value
                } else {
                    return null;
                }
            }
        }
        return null;
    }

    public Object get2(String key) { // 用于测试myHashMap性能
        // TODO Auto-generated method stub
        int index = hashcode(key);
        if (hashArr[index] == null)
            return null;
        else {
            LinkedList<Entry> tmpList = (LinkedList<Entry>) hashArr[index];
            int cnt = 0;//"hero-5555"的数量
            //"hero-5555"以外的hero也可能hash到和"hero-5555"相同的地方，只好for循环找出来
            for (int i = 0; i < tmpList.size(); i++) {
                if (tmpList.get(i).key.equals(key)) { // 找到了对应的key-value
                    cnt++;
                }
            }
            return cnt;
        }
    }
    public static void testHashCode(){
        List<String> rndArr=ranArr(100);
        List <Integer>rndArrHashCode=new ArrayList();
        for(int i=0;i<100;i++){
            rndArrHashCode.add(hashcode(rndArr.get(i)));
        }
        System.out.println("HashCode后的值为：");
        System.out.println(rndArrHashCode);
    }

    public static void testEfficiency(int n) {        //n是测试规模
        ArrayList<Hero> heros = new ArrayList<>();
        myHashMap mp = new myHashMap();
        for (int i = 0; i < n; i++) {
            int rnd = (int) Math.round(Math.random() * 9000 + 1000);
            heros.add(new Hero("hero-" + rnd)); // 办法一
            mp.put("hero-" + rnd, rnd); // 办法二
        }

        // 办法一：for循环查找
        long st1 = System.currentTimeMillis();
        int cnt1 = 0; // 统计"hero-5555"个数
        for (int i = 0; i < n; i++) {
            if (heros.get(i).name.equals("hero-5555"))
                cnt1++;
        }
        long et1 = System.currentTimeMillis();
        System.out.printf("for循环耗时：%d ms
共找到%d个
", (et1 - st1), cnt1);

        // 办法二：myHashMap查找

        long st2 = System.currentTimeMillis();
        int cnt2 = 0; // 统计"hero-5555"个数
        cnt2 = (int) mp.get2("hero-5555");
        long et2 = System.currentTimeMillis();
        System.out.printf("myHashMap耗时：%d ms
共找到%d个", (et2 - st2), cnt2);
    }
    
    public static void main(String[] args) {
        testHashCode();
        testEfficiency(300000);
    }
}

效果图：