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If two objects return same hashCode, doesn't it mean that they are equal? Or we need equals to prevent collisions?
And can I implement equals by comparing hashCodes?
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In order to use our own class objects as keys in collections like HashMap, Hashtable etc.. , we should override both methods ( hashCode() and equals() ) by having an awareness on internal working of collection. Otherwise, it leads to wrong results which we are not expected.
If you don't override hashcode() then the default implementation in Object class will be used by collections. This implementation gives different values for different objects, even if they are equal according to the equals() method.
The equals() and hashcode() are the two important methods provided by the Object class for comparing objects. Since the Object class is the parent class for all Java objects, hence all objects inherit the default implementation of these two methods.
Only Override HashCode, Use the default Equals: Only the references to the same object will return true. In other words, those objects you expected to be equal will not be equal by calling the equals method. Only Override Equals, Use the default HashCode: There might be duplicates in the HashMap or HashSet.
equals() is used to compare the two objects by some business logic and returns a boolean value. Use == operator when you want to compare the values of two primitive data types or you want to compare two references are same or not. Use . equals() method when you want to compare two objects by functionality.
If an object's hashcode is not the same as another object's hashcode, there is no reason to execute the equals() method: you just know the two objects are not the same. On the other hand, if the hashcode is the same, then you must execute the equals() method to determine whether the values and fields are the same.
If two objects have the same hashCode then they are NOT necessarily equal. Otherwise you will have discovered the perfect hash function. But the opposite is true - if the objects are equal, then they must have the same hashCode.
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