...
| Code Block | ||
|---|---|---|
| ||
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;
public XCard(int number, String type) {
super(number);
this.type = type;
}
public boolean equals(Object o) {
if (!(o instanceof Card))
return false;
//normal Card, do not compare type
if (!(o instanceof XCard))
return o.equals(this);
//It is an XCard, compare type as well
XCard xc = (XCard)o;
return super.equals(o) && xc.type == 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 true
System.out.println(p2.equals(p3)); //returns true
System.out.println(p1.equals(p3)); //returns false, violating transitivity
}
}
|
Compliant Solution
| Wiki Markup |
|---|
"There is simply no way to extend an instantiable class and add an aspect while preserving the equals contract." This implies that composition must be preferred over inheritance in this case. This is done by giving the {{XCard}} class a private {{card}} field and providing a a public {{viewCard}} method. |
\[[Bloch 08|AA. Java References#Bloch 08]\] |
| Code Block | ||
|---|---|---|
| ||
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 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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