// equalshashcode/CountedString.java

// (c)2021 MindView LLC: see Copyright.txt

// We make no guarantees that this code is fit for any purpose.

// Visit http://OnJava8.com for more book information.

// Creating a good hashCode()

import java.util.*;

public class CountedString {

private static List<String> created =

new ArrayList<>();

private String s;

private int id = 0;

public CountedString(String str) {

s = str;

created.add(s);

// id is the total number of instances

// of this String used by CountedString:

for(String s2 : created)

if(s2.equals(s))

id++;

}

@Override public String toString() {

return "String: " + s + " id: " + id +

" hashCode(): " + hashCode();

}

@Override public int hashCode() {

// The very simple approach:

// return s.hashCode() * id;

// Using Joshua Bloch's recipe:

int result = 17;

result = 37 * result + s.hashCode();

result = 37 * result + id;

return result;

}

@Override public boolean equals(Object o) {

return o instanceof CountedString &&

Objects.equals(s, ((CountedString)o).s) &&

Objects.equals(id, ((CountedString)o).id);

}

public static void main(String[] args) {

Map<CountedString,Integer> map = new HashMap<>();

CountedString[] cs = new CountedString[5];

for(int i = 0; i < cs.length; i++) {

cs[i] = new CountedString("hi");

map.put(cs[i], i); // Autobox int to Integer

}

System.out.println(map);

for(CountedString cstring : cs) {

System.out.println("Looking up " + cstring);

System.out.println(map.get(cstring));

}

}

}

/* Output:

{String: hi id: 4 hashCode(): 146450=3, String: hi id:

5 hashCode(): 146451=4, String: hi id: 2 hashCode():

146448=1, String: hi id: 3 hashCode(): 146449=2,

String: hi id: 1 hashCode(): 146447=0}

Looking up String: hi id: 1 hashCode(): 146447

0

Looking up String: hi id: 2 hashCode(): 146448

1

Looking up String: hi id: 3 hashCode(): 146449

2

Looking up String: hi id: 4 hashCode(): 146450

3

Looking up String: hi id: 5 hashCode(): 146451

4

*/