Set Hashcode contract violation example

gistfile1.txt

import java.util.Set;
import java.util.HashSet;
import java.util.stream.Collectors;

public class HelloWorld{

     public static void main(String []args){
        Set<Obj> set = new HashSet<>();
        Obj a = new Obj("a");
        Obj b = new Obj("b");
        set.add(a);
        set.add(b);
        System.out.println("Equal: " + a.equals(b)); // false
        System.out.println("Set contains a: " + set.contains(a)); // true
        System.out.println("Set contains b: " + set.contains(b)); // true
        
        b.setName("a");
        System.out.println("Equal: " + a.equals(b)); // true
        System.out.println("Set contains a: " + set.contains(a)); // true
        System.out.println("Set contains b: " + set.contains(b)); // true <-- false positive; undefined behaviour since Set contract was broken.
        
        b.setName("c");
        System.out.println("Equal: " + a.equals(b)); // false
        System.out.println("Set contains a: " + set.contains(a)); // true
        System.out.println("Set contains b: " + set.contains(b)); // false <-- set contract broken
       
         System.out.println(set.stream().map(Obj::toString).collect(Collectors.joining(", "))); // a, c
     }
     
     static class Obj {
         private String name;
         
         public Obj(String name) {
             this.name = name;
         }
        
         public int hashCode() {
             return name.hashCode();
         }
         
         public void setName(String name) {
             this.name = name;
         }
         
         @Override
         public boolean equals(Object other) {
             if (other == this) {
                 return true;
             }
             if (!(other instanceof Obj)) {
                 return false;
             }
             Obj o = (Obj) other;
             return this.name == o.name;
         }
         
         @Override
         public String toString() {
             return name;
         }
     }
}