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
1 package Collection;
2
3 public interface IHashMap {
4 public void put(String key,Object value);
5 public Object get(String key);
6
7 }
1 package Collection;
2
3 public class Entry {
4 public Object key;
5 public Object value;
6 public Entry(Object key,Object value){ //构造方法
7 this.key=key;
8 this.value=value;
9 }
10 public String toString(){
11 return "[key=" + key + ", value=" + value + "]";
12 }
13
14 }
给出代码：
1 package Collection;
2
3 import java.util.ArrayList;
4 import java.util.HashSet;
5 import java.util.Iterator;
6 import java.util.LinkedList;
7 import java.util.List;
8
9 import charactor.Hero;
10
11 public class myHashMap implements IHashMap {
12 public static int hashcode(String str) {
13 if (str.length() == 0)
14 return 0;
15 char tmpCharArr[] = str.toCharArray();
16 int cnt = 0;
17 for (int i = 0; i < tmpCharArr.length; i++)
18 cnt += (int) tmpCharArr[i];
19 cnt = cnt * 23;
20 if (cnt < 0)
21 cnt = cnt * (-1);
22 if (cnt > 1999)
23 cnt = cnt % 2000;
24 return cnt;
25
26 // //测试hashcode,大小为5
27 // if(str.length()==0)
28 // return 0;
29 // int cnt=Integer.parseInt(str);
30 // if(cnt>=5)
31 // return cnt%5;
32 // return cnt;
33 //
34 }
35
36 public static char ranChar() {
37 while (true) {
38 int n = (int) Math.round(Math.random() * 128);
39 char c = (char) n;
40 if (Character.isDigit(c) || Character.isLetter(c))
41 return c;
42 }
43 }
44
45 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
46 List<String> arr = new ArrayList(); // 最后返回arr
47 HashSet<String> hs = new HashSet(); // 保证随机生成的字符串互不相等
48 int i = 0;
49 while (i < num) {
50 int randLength = (int) (Math.round(Math.random() * 8) + 2);// 随机生成长度是2-10
51 String tmpStr = "";
52 for (int j = 0; j < randLength; j++) {
53 tmpStr += ranChar();
54 }
55 if (hs.add(tmpStr) == true) // 保证随机生成的字符串互不相等
56 i++;
57 }
58 for (String s : hs) { // HashSet转成List便于以后的操作
59 arr.add(s);
60 }
61 return arr;
62 }
63
64 Object[] hashArr = new Object[2000]; // 自己定义的Hash表，由2000个对象组成
65
66 @Override
67 public void put(String key, Object value) {
68 // TODO Auto-generated method stub
69 int index = hashcode(key);
70 // 下面两行用于应对hashArr[index]为空时，填入LinkedList<Entry>类型
71 LinkedList<Entry> tmpLinkedList = new LinkedList();
72 Entry E = new Entry(key, value);
73 tmpLinkedList.add(E);
74
75 if (hashArr[index] == null) {
76 hashArr[index] = tmpLinkedList;
77 } else {// hashArr[index]非空，说明它本身是LinkedList<Entry>类型，直接add即可
78 LinkedList<Entry> ll = (LinkedList<Entry>) hashArr[index];
79 ll.add(E);
80 }
81 }
82
83 @Override
84 public Object get(String key) {
85 // TODO Auto-generated method stub
86 int index = hashcode(key);
87 if (hashArr[index] == null)
88 return null;
89 else {
90 LinkedList<Entry> tmpList = (LinkedList<Entry>) hashArr[index];
91 for (int i = 0; i < tmpList.size(); i++) {
92 if (tmpList.get(i).key.equals(key)) { // 找到了对应的key--value
93 return tmpList.get(i).value; // 返回key对应的value
94 } else {
95 return null;
96 }
97 }
98 }
99 return null;
100 }
101
102 public Object get2(String key) { // 用于测试myHashMap性能
103 // TODO Auto-generated method stub
104 int index = hashcode(key);
105 if (hashArr[index] == null)
106 return null;
107 else {
108 LinkedList<Entry> tmpList = (LinkedList<Entry>) hashArr[index];
109 int cnt = 0;//"hero-5555"的数量
110 //"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
111 for (int i = 0; i < tmpList.size(); i++) {
112 if (tmpList.get(i).key.equals(key)) { // 找到了对应的key-value
113 cnt++;
114 }
115 }
116 return cnt;
117 }
118 }
119 public static void testHashCode(){
120 List<String> rndArr=ranArr(100);
121 List <Integer>rndArrHashCode=new ArrayList();
122 for(int i=0;i<100;i++){
123 rndArrHashCode.add(hashcode(rndArr.get(i)));
124 }
125 System.out.println("HashCode后的值为：");
126 System.out.println(rndArrHashCode);
127 }
128
129 public static void testEfficiency(int n) { //n是测试规模
130 ArrayList<Hero> heros = new ArrayList<>();
131 myHashMap mp = new myHashMap();
132 for (int i = 0; i < n; i++) {
133 int rnd = (int) Math.round(Math.random() * 9000 + 1000);
134 heros.add(new Hero("hero-" + rnd)); // 办法一
135 mp.put("hero-" + rnd, rnd); // 办法二
136 }
137
138 // 办法一：for循环查找
139 long st1 = System.currentTimeMillis();
140 int cnt1 = 0; // 统计"hero-5555"个数
141 for (int i = 0; i < n; i++) {
142 if (heros.get(i).name.equals("hero-5555"))
143 cnt1++;
144 }
145 long et1 = System.currentTimeMillis();
146 System.out.printf("for循环耗时：%d ms\n共找到%d个\n", (et1 - st1), cnt1);
147
148 // 办法二：myHashMap查找
149
150 long st2 = System.currentTimeMillis();
151 int cnt2 = 0; // 统计"hero-5555"个数
152 cnt2 = (int) mp.get2("hero-5555");
153 long et2 = System.currentTimeMillis();
154 System.out.printf("myHashMap耗时：%d ms\n共找到%d个", (et2 - st2), cnt2);
155 }
156
157 public static void main(String[] args) {
158 testHashCode();
159 testEfficiency(300000);
160 }
161 }
效果图：
来源：https://www.cnblogs.com/gilgamesh-hjb/p/12229186.html