An object is characterized by its identity (location in memory) and state (actual data). While the '==' operator compares only the identities of two objects (to check if both the references are actually the same object), the equals method defined in java.lang.Object can compare the state as well, when customized by overriding it.
The equals method only applies to Objects, not primitives. Also, immutable objects do not need to override equals. There is no need to override equals if checking logical equality is not useful. Enum types are an example. If a subclass inherits an implementation of equals from a superclass and does not need additional functionality, one can forgo overriding equals().
The general usage contract for equals() has been put forth verbatim from the Java specification:
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public class Card {
private final int number;
public Card(int number) {
this.number = number;
}
public boolean equals(Object o) {
if (!(o instanceof Card))
return false;
Card c = (Card)o;
return c.number == number;
}
}
class XCard extends Card {
private String type;
private Card card;
public XCard(int number, String type) {
super(number);
this.type = type;
}
public Card viewCard() {
return card;
}
public boolean equals(Object o) {
if (!(o instanceof XCard))
return false;
XCard cp = (XCard)o;
return cp.card.equals(card) && cp.type.equals(type);
}
public static void main(String[] args) {
XCard p1 = new XCard(1, "type1");
Card p2 = new Card(1);
XCard p3 = new XCard(1, "type2");
System.out.println(p1.equals(p2)); //returns false
System.out.println(p2.equals(p3)); //returns false
System.out.println(p1.equals(p3)); //returns false
}
}
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Exceptions
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Risk Assessment
Violating the general contract when overriding the equals() method can lead to unexpected results.
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