74 lines
2.0 KiB
Java
74 lines
2.0 KiB
Java
// understandingcollections/CountedString.java
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// (c)2016 MindView LLC: see Copyright.txt
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// We make no guarantees that this code is fit for any purpose.
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// Visit http://OnJava8.com for more book information.
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// Creating a good hashCode()
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import java.util.*;
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public class CountedString {
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private static List<String> created =
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new ArrayList<>();
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private String s;
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private int id = 0;
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public CountedString(String str) {
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s = str;
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created.add(s);
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// id is the total number of instances
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// of this String used by CountedString:
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for(String s2 : created)
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if(s2.equals(s))
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id++;
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}
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@Override
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public String toString() {
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return "String: " + s + " id: " + id +
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" hashCode(): " + hashCode();
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}
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@Override
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public int hashCode() {
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// The very simple approach:
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// return s.hashCode() * id;
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// Using Joshua Bloch's recipe:
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int result = 17;
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result = 37 * result + s.hashCode();
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result = 37 * result + id;
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return result;
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}
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@Override
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public boolean equals(Object o) {
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return o instanceof CountedString &&
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s.equals(((CountedString)o).s) &&
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id == ((CountedString)o).id;
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}
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public static void main(String[] args) {
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Map<CountedString,Integer> map =
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new HashMap<>();
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CountedString[] cs = new CountedString[5];
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for(int i = 0; i < cs.length; i++) {
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cs[i] = new CountedString("hi");
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map.put(cs[i], i); // Autobox int to Integer
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}
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System.out.println(map);
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for(CountedString cstring : cs) {
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System.out.println("Looking up " + cstring);
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System.out.println(map.get(cstring));
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}
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}
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}
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/* Output:
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{String: hi id: 4 hashCode(): 146450=3, String: hi id: 5
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hashCode(): 146451=4, String: hi id: 2 hashCode():
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146448=1, String: hi id: 3 hashCode(): 146449=2, String: hi
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id: 1 hashCode(): 146447=0}
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Looking up String: hi id: 1 hashCode(): 146447
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0
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Looking up String: hi id: 2 hashCode(): 146448
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1
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Looking up String: hi id: 3 hashCode(): 146449
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2
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Looking up String: hi id: 4 hashCode(): 146450
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3
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Looking up String: hi id: 5 hashCode(): 146451
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4
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*/
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