1. Java
equals, hashCode, toString
2011 4 2
twitter: @zaki50

2. • Effective Java 2
• ISBN:978-4-89471-499-1
• 3,780 ( )
• 8, 9, 10

3. • equals(Object)
• equals(Object)
hashCode()
• toString()

4. Equals

5. Equals ==
• == (
null)
• equals

6. Map put
get

7. override
• ( : Thread)
•
( :enum)
• ( : Random)
•
( :HashSet(AbstractSet ))
• equals
• AssertionError

8. override
equals

9. Object equals
•
• public boolean equals(Object obj) {
return (this == obj);
}

10. equals
• x.equals(x) == true x != null && y != null
&& z != null
• x.equals(null) == false
• x.equals(y) == y.equals(x)
• x.equals(y) && y.equals(z) => x.equals(z)
• setter

11. private final int id;
private final String name;
@Override
public boolean equals(Object obj) {
if (this == obj)
return true;
if (obj == null)
return false;
if (getClass() != obj.getClass())
return false;
Sample other = (Sample) obj;
if (id != other.id)
return false;
if (name == null) {
if (other.name != null)
equals(Object)
return false;
} else if (!name.equals(other.name))
return false;
return true;
}

12. • equals
equals
•
(java.net.URL ...)

13. hashCode

14. override
• equals(Object)
hashCode()

16. hashCode
• equals
• x.equals(y) x.hashCode() ==
y.hashCode()
• !x.equals(y)
x.hashCode() != y.hashCode()
x != null && y != null

17. private final int userId;
private final String name;
@Override
public int hashCode() {
final int prime = 31;
int result = 1;
result = prime * result + userId;
result = prime * result + ((name == null)
? 0 : name.hashCode());
return result;
}
hashCode()

18. ToString

19. ToString
•
•
•

20. private final int id;
private final String name;
@Override
public String toString() {
return "GoodClass [userId=" + userId + ", name=" + name + "]";
}

22. • IDE
• commons-lang EqualsBuilder/
HashCodeBuilder/ToStringBuilder

23. IDE

24. @Override
public int hashCode() {
final int prime = 31;
int result = 1;
result = prime * result + id;
result = prime * result + ((name == null) ? 0 : name.hashCode());
return result;
}
@Override
public boolean equals(Object obj) {
if (this == obj)
return true;
if (obj == null)
return false;
if (getClass() != obj.getClass())
return false;
Sample other = (Sample) obj;
if (id != other.id)
return false;
if (name == null) {
if (other.name != null)
return false;
} else if (!name.equals(other.name))
return false;
return true;
}

25. commons-lang
• EqualsBuilder/HashCodeBuilder/
ToStringBuilder
• reflection*
•

26. • equals(Object)
• equals(Object)
hashCode()
• toString()