hash - Java: Duplicate objects getting added to set? -

if run below code output 2 means set contains 2 elements. think set should contain 1 since both objects equal based on hashcode() value .equals() method. seems obvious mistake in understanding ?

package hello;  import java.util.hashset; import java.util.set;  public class test {      public static void main(string[] args) throws exception {         set<alpha> s = new hashset<alpha>();         alpha a1 = new alpha();         alpha a2 = new alpha();         s.add(a1);         s.add(a2);         system.out.println(s.size());     } }     class alpha {     int = 10;      public int hashcode() {         return a;     }      public boolean equals(object obj) {         return (obj instanceof alpha && ((alpha) obj).a == this.a);     }      public string tostring() {         return "alpha : " + a;     } } 

your hashcode method not override object class's hashcode method , equals method breaks contract since doesn't agree hashcode results, , can have objects "equal" have different hashcodes.

remember: should use @override annotation when overriding methods catch , similar errors.

@override  // ** don't forget annotation public int hashcode() { // *** note capitalization of "c"   return a; } 

also, want improve code formatting, when posting code here our review. able better understand code , if conforms standards (that's why standards exist). try keep indentations consistent code lines in same block indented @ same level, , want sure base level code, including imports, outer class declarations , end curly brace, flush left:

import java.util.hashset; import java.util.set;  public class test {     public static void main(string[] args) throws exception {       set<alpha> s = new hashset<alpha>();       alpha a1 = new alpha();       alpha a2 = new alpha();       s.add(a1);       s.add(a2);       system.out.println(s.size());    } }  class alpha {    int = 10;     @override    public int hashcode() {       return a;    }     public string tostring() {       return "alpha : " + a;    }     @override    public boolean equals(object obj) {       if (this == obj)          return true;       if (obj == null)          return false;       if (getclass() != obj.getclass())          return false;       alpha other = (alpha) obj;       if (a != other.a)          return false;       return true;    } } 

for beautiful review on this, please read: overriding equals , hashcode in java