Java的equals方法使用方法

在标准Java库中包含150多个equals方法的实现，这里给出一个比较完美的实现方法。

1）显示参数命名为otherObject，稍后需要将它转换成另一个叫做other的变量。
2）检测this与otherObject是否引用同一个对象：
if(this == otherObject) return true;
这条语句只是一个优化。实际上这是一种经常使用的形式。
3）检测otherObject是否为null，如果为null，返回false。这项检测很必要。
if(otherObejct == null) return false;
比较this与otherObject是否为属于同一个类。如果equals的语义在每个子类中有所改变，就使用getClass检测：
if(getClass() != otherObject.getClass())  return false;
如果所有的子类都拥有统一的语义，就使用instanceof检测：
if(!(otherObejct instanceof ClassName))  return false;
4）将otherObject转换为相应的类类型变量：
ClassName other = (ClassName)otherObject;
5）现在开始对所有需要比较的域进行比较了。使用==比较基本类型域，使用equals比较对象域。如果所有的域都匹配，就返回true，否则返回false。
return field == other.field&&field.equals(other.field)&&...;
如果在子类中重新定义equals，就要在其中包含调用super.equals(other)。

import java.util.*;

public class EqualsTest
{
	public static void main(String[] args)
	{
		Employee alice1 = new Employee("Alice Adams", 75000, 1987, 12, 15);
		Employee alice2 = alice1;
		Employee alice3 = new Employee("Alice Adams", 75000, 1987, 12, 15);
		Employee bob = new Employee("Bob Brandson", 50000, 1989, 10, 1);
		
		System.out.println("alice1 == alice2: " + (alice1 == alice2));
		System.out.println("alice1 == alice3: " + (alice1 == alice3));
		System.out.println("alice1.equals(alice3): " + alice1.equals(alice3));
		System.out.println("alice1.equals(bob): " + alice1.equals(bob));
		System.out.println("bob.toString(): " + bob);
		
		Manager carl = new Manager("Carl Cracker", 80000, 1987, 12, 15);
		Manager boss = new Manager("Carl Cracker", 80000, 1987, 12, 15);
		boss.setBonus(5000);
		System.out.println("boss.toString(): "+ boss);
		System.out.println("carl.equals(boss): " + carl.equals(boss));
		System.out.println("alice1.hashCode(): " + alice1.hashCode());
		System.out.println("alice3.hashCode(): " + alice3.hashCode());
		System.out.println("bob.hashCode(): " + bob.hashCode());
		System.out.println("carl.hashCode(): " + carl.hashCode());
	}
}

class Employee
{
	public Employee(String n, double s, int year, int month, int day)
	{
		name = n;
		salary = s;
		GregorianCalendar calendar = new GregorianCalendar(year, month-1, day);
		hireDay = calendar.getTime();
	}
	
	public String getName()
	{
		return name;
	}
	
	public double getSalary()
	{
		return salary;
	}
	
	public Date getHireDay()
	{
		return hireDay;
	}
	
	public void raiseSalary(double byPercent)
	{
		double raise = salary*byPercent/100;
		salary += raise;
	}
	
	public boolean equals(Object otherObject)
	{
		//a quick test to see if the objects are identical
		if(this == otherObject)
			return true;
		//must return false if the explicit parameter is null
		if(otherObject == null)
			return false;
		//if the classes don't match, they can't be equal
		if(getClass() != otherObject.getClass())
			return false;
		//now we know otherObject is a non-null Employee
		Employee other = (Employee)otherObject;
		//test whether the fields have identical values
		return name.equals(other.name)
			&& salary == other.salary
			&& hireDay.equals(other.hireDay);
	}
	
	public int hashCode()
	{
		return 7*name.hashCode() + 11*new Double(salary).hashCode()
			+ 13*hireDay.hashCode();
	}
	
	public String toString()
	{
		return getClass().getName() + "[name=" + name
			+ ",salary=" + salary + ",hireDay=" + hireDay
			+ "]";
	}
	
	private String name;
	private double salary;
	private Date hireDay;
}

class Manager extends Employee
{
	public Manager(String n, double s, int year, int month, int day)
	{
		super(n, s, year, month, day);
		bonus = 0;
	}
	
	public double getSalary()
	{
		double baseSalary = super.getSalary();
		return baseSalary + bonus;
	}
	
	public void setBonus(double b)
	{
		bonus = b;
	}
	
	public boolean equals(Object otherObject)
	{
		if(!super.equals(otherObject))
			return false;
		Manager other = (Manager)otherObject;
		//super.equals checked that this and other belong to the same class
		return bonus == other.bonus;
	}
	
	public int hashCode()
	{
		return super.hashCode() + 17*new Double(bonus).hashCode();
	}
	
	public String toString()
	{
		return super.toString() + "[bonus=" + bonus + "]";
	}
	
	private double bonus;
}

--------------------Configuration: <Default>--------------------
alice1 == alice2: true
alice1 == alice3: false
alice1.equals(alice3): true
alice1.equals(bob): false
bob.toString(): Employee[name=Bob Brandson,salary=50000.0,hireDay=Sun Oct 01 00:00:00 CST 1989]
boss.toString(): Manager[name=Carl Cracker,salary=80000.0,hireDay=Tue Dec 15 00:00:00 CST 1987][bonus=5000.0]
carl.equals(boss): false
alice1.hashCode(): 377780067
alice3.hashCode(): 377780067
bob.hashCode(): 955285015
carl.hashCode(): 386513600

Process completed.