equals()

Because the default hashCode() gives an answer based on the instance's identity, it is incorrect to implement equals() without implementing a corresponding hashCode() method. e.g.

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public class Person {     private String name;     private Date birth;     public boolean equals(Object other) {         if (other == this) return true;         if (other == null) return false;         if (getClass() != other.getClass()) return false;         Person person = (Person)other;         return (             (name == person.name ||              (name != null && name.equals(person.name))) &&             (birth == person.birth ||              (birth != null && birth.equals(person.birth)))             );     } }