Equals比较对象

最新推荐文章于 2023-07-18 22:00:00 发布

权基龙

最新推荐文章于 2023-07-18 22:00:00 发布

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分类专栏： java基础文章标签： equals 继承 super

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本文通过一个Java示例程序详细解析了如何使用equals方法来比较对象内容的相等性，以及hashCode方法的正确实现方式。同时展示了不同情况下对象引用与内容比较的区别。

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package test;

import java.util.Date;
import java.util.GregorianCalendar;

public class EqualTest{
	
	public static void main(String[] args){
		Employee a1 = new Employee("adams", 75000, 1987, 12, 15);
		Employee a2 = a1;
		Employee a3 = new Employee("adams", 75000, 1987, 12, 15);
		Employee bob = new Employee("bob", 50000, 1989, 10, 1);
		
		System.out.println("a1==a2:" + (a1==a2));
		System.out.println("a1==a3:"+(a1==a3));
		System.out.println("a1.equals(a3):"+a1.equals(a3));
		System.out.println("a1.equals(bob):"+a1.equals(bob));
		System.out.println("bob.toString():"+bob);
		
		Manager car1 = new Manager("Car Cracker",80000,1987,12,15);
		Manager boss = new Manager("Car Cracker",80000,1987,12,15);
		boss.setBonus(5000);
		System.out.println("boss.toString():" + boss);
		System.out.println("car1.equals(boss):"+ car1.equals(boss));
		System.out.println("a1.hashCode():" + a1.hashCode());
		System.out.println("a3.hashCode():" + a3.hashCode());
		System.out.println("bob.hashCode():"+ bob.hashCode());
		System.out.println("car1.hashCode():"+ car1.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();
	}
	
	private String name;
	private Double salary;
	private Date hireDay;
	
	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){
		if(this == otherObject) return true;
		
		if(otherObject == null) return false;
		
		if(getClass() != otherObject.getClass()) return false;
		
		Employee other = (Employee) otherObject;
		
		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 +"]";
	}
}

class Manager extends Employee{
	
	private double bonus;
	
	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;
		return bonus == other.bonus;
	}
	
	public int hashCode(){
		return super.hashCode() + 17 * new Double(bonus).hashCode();
	}
	
	public String toString(){
		return super.toString() + "[bonus=" + bonus + "]";
	}

}

结果为：

a1==a2:true
a1==a3:false
a1.equals(a3):false
a1.equals(bob):false
bob.toString():test.Employee[name=bob,salary=50000.0,hireDay=Sun Oct 01 00:00:00 CST 1989]
boss.toString():test.Manager[name=Car Cracker,salary=80000.0,hireDay=Tue Dec 15 00:00:00 CST 1987][bonus=5000.0]
car1.equals(boss):false
a1.hashCode():-1671370621
a3.hashCode():-1671370621
bob.hashCode():53370192
car1.hashCode():-1609448924

个人小结：

总结：equals方法是用来判断两个对象是否是相等的，如果连个对象指向同一块存储区域中，方法返回true；否则方法返回false；