HashCode

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layout: default title: hashCode()

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If you override equals you also need to override the hashCode method.

public int hashCode() { /* ... */ }

hashCode returns an int whose value may not have any real meaning, but which has an important relationship with equals:

• If two objects are .equals() to each other, then they must both return the same int when their hashCode methods are called.

This will become more important when you start using collection classes like HashMap.

Usually you will implement equals and hashCode together, and any fields compared in equals are also used to calculate the hashCode integer.

• This way objects with different state have different hashcodes.

The default hashCode inherited from Object generally produces a unique integer for each distinct object (usually by deriving it from the memory address of the object.)

As much as possible your hashCode implementation should return unique integers for all non-equivalent objects of your class.

A common practice is to start with some arbitrary prime number, then successively multiply it by any numeric instance fields (converting float and double to int first), as well as the hashcodes of any object fields.

public class Product {
  int id;
  String name;

  public int hashCode() {
    final int prime = 31;
    int result = 1;
    result = prime * result + id;
    result = prime * result + ((name == null) ? 0 : name.hashCode());
    return result;
  }
  // ...
}

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