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();
    }
}