补补基础知识 hashcode

hashcode是专为HashTable、HashMap和HashSet使用的

Apache Commons Lang的包生成hashcode，equals方法
不过也可用eclipse的方法

如果不重写hashcode方法，以下set集合中会保留2个值。

public class EqualsTest
{
public static void main(String[] args)
{
Employee e1 = new Employee();
Employee e2 = new Employee();

e1.setId(100);
e2.setId(100);

//Prints 'true'
System.out.println(e1.equals(e2));

Set<Employee> employees = new HashSet<Employee>();
employees.add(e1);
employees.add(e2);
//Prints two objects
System.out.println(employees);
}


import org.apache.commons.lang3.builder.EqualsBuilder;
import org.apache.commons.lang3.builder.HashCodeBuilder;
public class Employee
{
private Integer id;
private String firstname;
private String lastName;
private String department;
public Integer getId() {
return id;
}
public void setId(Integer id) {
this.id = id;
}
public String getFirstname() {
return firstname;
}
public void setFirstname(String firstname) {
this.firstname = firstname;
}
public String getLastName() {
return lastName;
}
public void setLastName(String lastName) {
this.lastName = lastName;
}
public String getDepartment() {
return department;
}
public void setDepartment(String department) {
this.department = department;
}
@Override
public int hashCode()
{
final int PRIME = 31;
return new HashCodeBuilder(getId()%2==0?getId()+1:getId(), PRIME).
toHashCode();
}
@Override
public boolean equals(Object o) {
if (o == null)
return false;
if (o == this)
return true;
if (o.getClass() != getClass())
return false;
Employee e = (Employee) o;
return new EqualsBuilder().
append(getId(), e.getId()).
isEquals();
}
}