重載Object類的equals函數

在java中不能由用戶重載 “==”操作符，只能重載Object類的equals函數。這裏列出了重載時的一般流程和注意事項：(引自Core Java 2)

import java.util.*; class Employee{ public Employee(String n, double s, int year, int month, int day){ name = n; salary = s; calender = new GregorianCalendar(year, month-1, day); hireDay = calender.getTime(); } public String getName() { return name; } public double getSalary() { return salary; } public Date getHireday() { return hireDay; } public void raiseSalary(double byPercent) { salary *= (1 + byPercent/100); } public boolean equals(Employee other){ if( this == other ) return true; if( other == null) return false; //在子類可以繼承時，使用getClass()方法： //否則使用final修飾，且使用instanceof if( getClass() != other.getClass()) return false; // if( !(other instanceof Employee)) // return false; return name.equals(other.name) && salary == other.salary && hireDay.equals(other.hireDay); } private String name; private double salary; private Date hireDay; static private GregorianCalendar calender; } 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 base = super.getSalary(); return base + bonus; } public void SetBonus(double b) { bonus = b; } public boolean equals(Employee otherEmployee){ if(super.equals(otherEmployee)) return true; Manager other = (Manager)otherEmployee; return bonus == other.bonus; } private double bonus; } class Saler extends Employee{ public Saler(String n, double s, int year, int month, int day) { super(n, s, year, month, day); scores = 0; } public double getSalary() { double base = super.getSalary(); return base + scores*0.1; } public void SetScores(double s) { scores = s; } public boolean equals(Employee otherEmployee){ if(super.equals(otherEmployee)) return true; Saler other = (Saler)otherEmployee; return scores == other.scores; } private double scores; } public class EmployeeTest { public static void main(String[] args) { Manager boss = new Manager("Carl Cracker", 18000, 2001, 9, 29); boss.SetBonus(5000); Employee[] staff = new Employee[5]; staff[0] = boss; staff[1] = new Employee("Harry Hacker", 8000, 2010, 9, 38); staff[2] = new Employee("Tommy Tester", 7000, 2010, 13, 2); staff[3] = new Saler("Harry Hacker", 8000, 2010, 9, 38); staff[4] = new Saler("Tommy Tester", 7000, 2010, 13, 2); for(Employee e : staff) System.out.println("/nName: " + e.getName() + "/nSalary: " + e.getSalary() + "/nHireDay: " + e.getHireday()); if(staff[1].equals(staff[3])) System.out.println(staff[1].getName() + "==" + staff[3].getName()); if(staff[2].equals(staff[4])) System.out.println(staff[2].getName() + "==" + staff[4].getName()); } }  

在這裏主要關心父類（Employee）和兩個子類（Manager和Saler）的equals函數，可見在父類中定義equals函數主要有5步(已在代碼中標出)，在子類中則有兩步。一般這些步驟最好都不要省略。

這裏有一點需要注意：即在step3有兩個方案，這兩個方案到底選擇哪個有一個準則可供參考。

 

> 若子類需要自己的equals函數，爲了滿足“相等”的對稱性，必須使用getClass();

> 若父類已經完全決定了equals的定義，使用instanceof。且對equals使用final修飾

 

另外，在Java中要求若x.equals(y)成立，則x.hashCode() == y.hashCode()，Object中默認的hashCode返回的是對象的內存地址，不同的對象一般不同，所以在定義了equals函數後不要忘記同時定義hashCode函數。常用的方案如下：

//針對以上的Employee類，可以定義如下的hashCode() public int hashCode() { return 7*name.hashCode() + 11* new Double(salary).hashCode() + 13*hireDay.hashCode(); }