Equals e HashCode

[Mylene Morato]

Hi !

I have a doubt in this code.

The source is Iquisition.

public class Teste
{
   int x; Teste(int input) { x = input; }
   public static void main( String args[] )
   {
      System.out.println( new Teste(3).equals(new Teste(3)) );
      
   }
   public boolean equals( Object o)
   {
	  return ( (Teste)o ).x == x;
     
   }

    }
 

The result is true.

But the class should override the hashCode() method to result true??
I think that hashCode() of the class Object provides different numbers of hashCode to each object .

Thanks!

[Nitish Bangera]

well as far as i see it, you are only checking for equals here and the override is valid and you are calling the valid overridden method and within it the operations are checking for the values of the instance variable. So the equals will return true for meaningful equivalents.But yes this will have different hashcodes. Try to check that by calling the hashcode method on the reference variable. Well the contract says that you must override it but if you don't there is no harm but it will result in a bad design that's all.

[Jesper de Jong]

There is an equals() method which compares two Teste objects by checking if their member variable x contains the same value. Then you call equals() to compare two Teste objects which both have x = 3. Ofcourse the result will be true.

You are simply calling the equals() method, which works just like any other method. There is no magic involved with hash codes here.

Hash codes are used by certain collection classes, such as HashSet and HashMap. If you would want to put Teste objects in a HashSet, or use Teste objects as keys in a HashMap, then class Teste would need to implement the hashCode() method so that two Teste objects which are equal have the same hash code. If class Teste does not have a correct hashCode() method, then you will get strange results when putting Teste objects in a hash code based collection.

[Satyajeet Kadam]

Two objects are equal but it's giving different Hascodes. Can you explain me in how can implent hashcode in below example so that i can use them in hashMap for searching.

 
public class Teste
{
   int x;
   Teste(int input)
   {
       x = input;
   }
   public static void main( String args[] )
   {

      Teste obj1 =new Teste(3);
      Teste obj2 =new Teste(3);
       System.out.println( obj1.equals(obj2));

      System.out.println(obj1.hashCode());
      System.out.println(obj2.hashCode());

   }
   public boolean equals( Object o)
   {
      return ( (Teste)o ).x == x;

   }

[Jesper de Jong]

To make instances of a class useable as keys in a HashMap, you must make sure that they obey the hash code contract:

When two objects are equal, their hash code value must be the same

When two objects are not equal, it doesn't matter (their hash code values might be the same, or different)

You must implement your hashCode() and equals() in such a way that this holds.